Tag Archives: Atmospheric and Oceanic Physics

The two-time energy spectrum of weak magnetohydrodynamic turbulence [CL]

Posted on by

http://arxiv.org/abs/2004.03518


In this work a weak-turbulence closure is used for the first time to determine the structure of the two-time power spectrum of weak magnetohydrodynamic (MHD) turbulence from the nonlinear equations describing the dynamics. The two-time energy spectrum is a fundamental quantity in turbulence theory from which most statistical properties of a homogeneous turbulent system can be derived. A closely related quantity, obtained via a spatial Fourier transform, is the two-point, two-time correlation function describing the space-time correlations arising from the underlying dynamics of the turbulent fluctuations. Both quantities are central in fundamental turbulence theories as well as in the analysis of turbulence experiments and simulations. However, a first-principle derivation of these quantities has remained elusive due to the statistical closure problem, in which dynamical equations for correlations at order $n$ depend on correlations of order $n+1$. The recent launch of the \emph{Parker Solar Probe} (PSP), which will explore the near-Sun region where the solar wind is born, has renewed the interest in the scientific community to understand the structure, and possible universal properties of space-time correlations. The weak MHD turbulence regime that we consider in this work allows for a natural asymptotic closure of the two-time spectrum, which may be applicable to other weak turbulence regimes found in fluids and plasmas. An integro-differential equation for the scale-dependent temporal correlation function is derived for incompressible Alfv\’enic fluctuations whose nonlinear dynamics is described by the Reduced MHD equations.

Read this paper on arXiv…

J. Perez, A. Azelis and S. Bourouaine
Wed, 8 Apr 20
20/72

Comments: Accepted for publication in Physical Review Research

Design of the HARMONI Pyramid WFS module [IMA]

Posted on by

http://arxiv.org/abs/2003.07228


Current designs for all three extremely large telescopes show the overwhelming adoption of the pyramid wavefront sensor (P-WFS) as the WFS of choice for adaptive optics (AO) systems sensing on natural guide stars (NGS) or extended objects. The key advantages of the P-WFS over the Shack-Hartmann are known and are mainly provided by the improved sensitivity (fainter NGS) and reduced sensitivity to spatial aliasing. However, robustness and tolerances of the P-WFS for the ELTs are not currently well understood. In this paper, we present simulation results for the single-conjugate AO mode of HARMONI, a visible and near-infrared integral field spectrograph for the European Extremely Large Telescope. We first explore the wavefront sensing issues related to the telescope itself; namely the island effect (i.e. differential piston) and M1 segments phasing errors. We present mitigation strategies to the island effect and their performance. We then focus on some performance optimisation aspects of the AO design to explore the impact of the RTC latency and the optical gain issues, which will in particular affect the high-contrast mode of HARMONI. Finally, we investigate the influence of the quality of glass pyramid prism itself, and of optical aberrations on the final AO performance. By relaxing the tolerances on the fabrication of the prism, we are able to reduce hardware costs and simplify integration. We show the importance of calibration (i.e. updating the control matrix) to capture any displacement of the telescope pupil and rotation of the support structure for M4. We also show the importance of the number of pixels used for wavefront sensing to relax tolerances of the pyramid prism. Finally, we present a detailed optical design of the pyramid prism, central element of the P-WFS.

Read this paper on arXiv…

N. Schwartz, J. Sauvage, E. Renault, et. al.
Tue, 17 Mar 20
35/63

Comments: 12 pages, 17 figures, AO4ELT6 Proceedings, Adaptive Optics for Extremely Large Telescopes 6, Conference Proceeding, Qu\’ebec City, Canada, Spain, June 9-14, 2019

Volatile evolution and atmospheres of Trans-Neptunian Objects [EPA]

Posted on by

http://arxiv.org/abs/2003.04955


At 30-50 K, the temperatures typical for surfaces in the Kuiper Belt (e.g. Stern & Trafton 2008), only seven species have sublimation pressures higher than 1 nbar (Fray & Schmitt 2009): Ne, N$_2$, CO, Ar, O$_2$, CH$_4$, and Kr. Of these, N$_2$, CO, and CH$_4$ have been detected or inferred on the surfaces of Trans-Neptunian Objects (TNOs). The presence of tenuous atmospheres above these volatile ices depends on the sublimation pressures, which are very sensitive to the composition, temperatures, and mixing states of the volatile ices. Therefore, the retention of volatiles on a TNO is related to its formation environment and thermal history. The surface volatiles may be transported via seasonally varying atmospheres and their condensation might be responsible for the high surface albedos of some of these bodies. The most sensitive searches for tenuous atmospheres are made by the method of stellar occultation, which have been vital for the study of the atmospheres of Triton and Pluto, and has to-date placed upper limits on the atmospheres of 11 other bodies. The recent release of the Gaia astrometric catalog has led to a “golden age” in the ability to predict TNO occultations in order to increase the observational data base.

Read this paper on arXiv…

L. Young, F. Braga-Ribas and R. Johnson
Thu, 12 Mar 20
13/49

Comments: N/A

Provenance of the Cross Sign of 806 in the Anglo-Saxon Chronicle: A possible Lunar Halo over Continental Europe? [CL]

Posted on by

http://arxiv.org/abs/2003.04192


While graphical records of astronomical/meteorological events before telescopic observations are of particular interest, they have frequently undergone multiple copying and may have been modified from the original. Here, we analyze a graphical record of the cross-sign of 806 CE in the Anglo-Saxon Chronicle, which has been considered one of the earliest datable halo drawings in British records, whereas another cross-sign in 776 CE has been associated with the aurora. However, philological studies have revealed the later 806 event is derived from Continental annals. Here, records and drawings for the 806 event have been philologically traced back to mid-9th Century Continental manuscripts and the probable observational site identified as the area of Sens in northern France. The possible lunar halos at that time have been comprehensively examined by numerical ray tracing. Combined with calculations of twilight sky brightness, they identify a visibility window supporting monastic observation. Cruciform halos are shown to be fainter and rarer than brighter and more commonplace lunar halos. Physically credible cloud ice crystal variations can reproduce all the manuscript renditions. The manuscript records prove less than desirable detail but what is presented is fully consistent with a lunar halo interpretation. Finally, the possible societal impacts of such celestial events have been mentioned in the context of contemporary coins in Anglo-Saxon England and the Carolingian Empire. These analyses show that we need to trace their provenance back as far as possible, to best reconstruct the original event, even if graphical records are available for given astronomical/meteorological events.

Read this paper on arXiv…

Y. Uchikawa, L. Cowley, H. Hayakawa, et. al.
Tue, 10 Mar 20
46/63

Comments: 23 pages, 7 figures, and 1 table; accepted for publication in the HGSS; Figures 1 and 3 is available only in the record version

Filtering techniques to enhance optical turbulence forecast performances at short time scales [IMA]

Posted on by

http://arxiv.org/abs/2002.12658


The efficiency of the management of top-class ground-based astronomical facilities supported by Adaptive Optics (AO) relies on our ability to forecast the optical turbulence (OT) and a set of relevant atmospheric parameters. Indeed, in spite of the fact that the AO is able to achieve, at present, excellent levels of wavefront corrections (a Strehl Ratio up to 90% in H band), its performances strongly depend on the atmospheric conditions. Knowing in advance the turbulence conditions allows an optimization of the AO use. It has already been proven that it is possible to provide reliable forecasts of the optical turbulence (CN2 profiles and integrated astroclimatic parameters such as seeing, isoplanantic angle, wavefront coherence time, …) for the next night. In this paper we prove that it is possible to improve the forecast performances on shorter time scales (order of one or two hours) with consistent gains (order of 2 to 8) using filtering techniques. This has permitted us to achieve forecasts accuracies never obtained before and reach a fundamental milestone for the astronomical applications. The time scale of one or two hours is the most critical one for an efficient management of the ground-based telescopes supported by AO. Results shown here open, therefore, to an important revolution in the field. We implemented this method in the operational forecast system of the Large Binocular Telescope, named ALTA Center that is, at our knowledge, the first operational system providing forecasts of turbulence and atmospheric parameters at short time scales to support science operations.

Read this paper on arXiv…

E. Masciadri, G. Martelloni and A. Turchi
Mon, 2 Mar 20
25/62

Comments: 7 figures, 10 tables

Comment on "Collision and radiative processes in emission of atmospheric carbon dioxide'' [EPA]

Posted on by

http://arxiv.org/abs/2002.10601


Recently, Smirnov published a paper [1] which dismisses the role of increasing concentrations of anthropogenic CO$_2$ on global warming of planet Earth. We show that these conclusions are the consequence of two flaws in Smirnov’s theoretical model which neglect the effects of the increased concentrations of CO$_2$ on the absorption of Earth’s blackbody radiation in the 12–15$\mu$m region. The influence of doubling the concentration of CO$_2$ in the atmosphere on the surface temperature is not $\Delta$T=0.02K, or even $\Delta$T=0.4K if only one of the two mistakes in Smirnov’s analysis is corrected. The correct value lies within $\Delta$T=1.1-1.3K as outlined by other authors analysis using simplified, yet more theoretically consistent models.

Read this paper on arXiv…

M. Silva and J. Vargas
Wed, 26 Feb 20
31/72

Comments: 12 pages, 5 figures

Modelling the 3D Climate of Venus with OASIS [EPA]

Posted on by

http://arxiv.org/abs/2002.09506


Flexible 3D models to explore the vast diversity of terrestrial planets and interpret observational data are still in their early stages. In this work, we present OASIS: a novel and flexible 3D virtual planet laboratory. With OASIS we envision a platform that couples self-consistently seven individual modules representing the main physical and chemical processes that shape planetary environments. Additionally, OASIS is capable of producing simulated spectra from different instruments and observational techniques. In this work we focus on the benchmark test of coupling four of the physical modules: fluid dynamics, radiation, turbulence and surface/soil. To test the OASIS platform, we produced 3D simulations of the Venus climate and its atmospheric circulation and study how the modeled atmosphere changes with various cloud covers, atmospheric heat capacity, and surface friction. 3D simulations of Venus are challenging because they require long integration times with a computationally expensive radiative transfer code. By comparing OASIS results with observational data, we verify that the new model is able to successfully simulate Venus. With simulated spectra produced directly from the 3D simulations, we explore the capabilities of future missions, like LUVOIR, to observe Venus analogs located at a distance of 10 pc. With OASIS, we have taken the first steps to build a sophisticated and very flexible platform capable of studying the environment of terrestrial planets, which will be an essential tool to characterize observed terrestrial planets and plan future observations.

Read this paper on arXiv…

J. Mendonça and L. Buchhave
Tue, 25 Feb 20
34/76

Comments: Submitted to MNRAS, revised version after first referee report

Cavity ring-down spectroscopy of CO$_2$ near $λ$ = 2.06 $μ$m: Accurate transition intensities for the Orbiting Carbon Observatory-2 (OCO-2) "strong band" [CL]

Posted on by

http://arxiv.org/abs/2002.09584


The $\lambda$ = 2.06 $\mu$m absorption band of $^{12}$C$^{16}$O$_2$ is widely used for the remote sensing of atmospheric carbon dioxide, making it relevant to many important top-down measurements of carbon flux. The forward models used in the retrieval algorithms employed in these measurements require increasingly accurate line intensity and line shape data from which absorption cross-sections can be computed. To overcome accuracy limitations of existing line lists, we used frequency-stabilized cavity ring-down spectroscopy to measure 39 transitions in the $^{12}$C$^{16}$O$_2$ absorption band. We estimate the relative combined standard uncertainty for the measured intensities to be $u_r$ = 0.09 %, which is dominated by several type B (systematic) contributions each with $u_r$ = 0.04 %. We predicted the $J$-dependence of the measured intensities using two theoretical models: a one-dimensional quantum mechanical model with Herman-Wallis rotation-vibration corrections, and a line-by-line ab initio model [Zak et al. JQSRT 2016;177:31-42]. For the second approach, we fit only a single factor to rescale the theoretical integrated band intensity to be consistent with the measured intensities. We find that the latter approach yields an equally adequate representation of the fitted $J$-dependent intensity data and provides the most physically general representation of the results. Our recommended value for the integrated band intensity equal to 7.180$\times$10$^{-21}$ cm molecule$^{-1}$ $\pm$ 6$\times$10$^{-24}$ cm molecule$^{-1}$ is based on the rescaled ab initio model and corresponds to a fitted scale factor of 1.0065 $\pm$ 0.0002. Comparisons of literature intensity values to our results reveal systematic deviations ranging from $-$1.1 % to 0.4 %.

Read this paper on arXiv…

H. Fleurbaey, H. Yi, E. Adkins, et. al.
Tue, 25 Feb 20
66/76

Comments: 30 pages, 9 figures, 4 tables

Datasets of ionospheric parameters provided by SCINDA GNSS receiver from Lisbon airport area [CL]

Posted on by

http://arxiv.org/abs/2002.08883


Here we present datasets provided by a SCINDA GNSS receiver installed in Lisbon airport area from November of 2014 to July of 2019. The installed equipment is a NovAtel EURO4 with a JAVAD Choke-Ring antenna. The data are in the archived format and include the general messages on quality of records (.msg), random number generator files (.rng), raw observables as the signal-to-noise (S/N) ratios, pseudoranges and phases (.obs), receiver position information (.psn), ionosphere scintillations monitor (.ism) and ionospheric parameters: total electron content, TEC, rate of change of TEC, ROTI, and the scintillation index S4 (.scn). The presented data cover the full 2015 year. The raw data are of 1-minute resolution and available for each of receiver-satellite pairs. The processing and the analysis of the ionosphere scintillation datasets can be done using a dedicated “SCINDA-Iono” toolbox for MATLAB developed by T. Barlyaeva (2019) and available online via MathWorks File Exchange system. The toolbox allows to calculate 1-hour means for ionospheric parameters both for each of available receiver-satellite pairs and averaged over all available satellites during the analyzed hour. Here we present also the processed data for the following months in 2015: March, June, October, and December. The months were selected as containing most significant geomagnetic events of 2015. The 1-hour means for other months can be obtained from the raw data using, e.g. the aforementioned toolbox. The provided datasets can be of interest for the GNSS and ionosphere scientific communities.

Read this paper on arXiv…

T. Barlyaeva, T. Barata and A. Morozova
Fri, 21 Feb 20
9/67

Comments: arXiv admin note: text overlap with arXiv:1910.04044

Stabilization of dayside surface liquid water via tropopause cold trapping on arid slowly rotating tidally locked planets [EPA]

Posted on by

http://arxiv.org/abs/2002.08600


Terrestrial-type exoplanets orbiting nearby red dwarf stars (M-dwarfs) are among the best targets for atmospheric characterization and biosignature searches in the near future. Recent evolutionary studies have suggested that terrestrial planets in the habitable zone of M-dwarfs are probably tidally locked and have limited surface water inventories as a result of their host stars’ high early luminosities. Several previous climate simulations of such planets have indicated that their remaining water would be transported to the planet’s permanent nightside and become trapped as surface ice, leaving the dayside devoid of water. Here we use a three-dimensional general circulation model with a water cycle and accurate radiative transfer scheme to investigate the surface water evolution on slowly rotating tidally locked terrestrial planets with limited surface water inventories. We show that there is a competition for water trapping between the nightside surface and the substellar tropopause in this type of climate system. Although under some conditions the surface water remains trapped on the nightside as an ice sheet, in other cases liquid water stabilizes in a circular area in the substellar region as a wetland. Planets with 1 bar N$_2$ and atmospheric CO$_2$ levels greater than 0.1 bar retain stable dayside liquid water, even with very small surface water inventories. Our results reveal the diversity of possible climate states on terrestrial-type exoplanets and highlight the importance of surface liquid water detection techniques for future characterization efforts.

Read this paper on arXiv…

F. Ding and R. Wordsworth
Fri, 21 Feb 20
34/67

Comments: accepted for publication in ApJL

Atmospheric Temperature Effect in secondary cosmic rays observed with a two square meter ground-based detector [IMA]

Posted on by

http://arxiv.org/abs/2002.04277


A high resolution 2 m$^2$ tracking detector, based on timing Resistive Plate Chamber (tRPC) cells, has been installed at the Faculty of Physics of the University of Santiago de Compostela (Spain) in order to improve our understanding of the cosmic rays arriving at the Earth’s surface. Following a short commisioning of the detector, a study of the atmospheric temperature effect of the secondary cosmic ray component was carried out. A method based on Principal Component Analysis (PCA) has been implemented in order to obtain the distribution of temperature coefficients, $W_T(h)$, using as input the measured rate of nearly vertical cosmic ray tracks, showing good agreement with the theoretical expectation. The method succesfully removes the correlation present between the different atmospheric layers, that would be dominant otherwise. We briefly describe the initial calibration and pressure correction procedures, essential to isolate the temperature effect. Overall, the measured cosmic ray rate displays the expected anticorrelation with the effective atmospheric temperature, through the coefficient $\alpha_T= -0.279 \pm 0.051 $ \%/K. Rates follow the seasonal variations, and unusual short-term events are clearly identified too.

Read this paper on arXiv…

I. Riádigos, D. García-Castro, D. González-Díaz, et. al.
Wed, 12 Feb 20
14/58

Comments: 18 pages, 10 figures, Submitted to Earth and Space Science

A photometric mapping of the night sky brightness of the Maltese islands [IMA]

Posted on by

http://arxiv.org/abs/2002.04435


Over the years, the Maltese Islands have seen a marked rise in the prevalence of artificial lighting at night. The most evident type of light pollution arising from this evolution in anthropogenic night-time lighting is artificial skyglow via partial back-scattering in the atmosphere, leading to an increase in the Night Sky Brightness (NSB). The importance of understanding and quantifying the geographical distribution of the NSB is underscored by the adverse impact of light pollution on various spheres, from astronomical observation to ecology and human health. For the first time, we present a detailed map of the NSB over the Maltese archipelago carried out with Unihedron Sky Quality Meters. We show that the vast majority of the area of the Maltese Islands is heavily light polluted, with 87% of the area registering a NSB $<$ 20.39~mag${\rm SQM}$/arcsec$^2$ (Bortle Class 5 or higher) and 37.3% $<$ 19.09~mag${\rm SQM}$/arcsec$^2$ (Bortle Class 6 or higher), with the Milky Way being visible for only 12.8% of the area (adopting a visibility threshold $>$ 20.4 – 21.29~mag$_{\rm SQM}$/arcsec$^2$; Bortle Class 4). Coastal Dark Sky Heritage Areas on the island of Gozo retain generally darker skies than the rest of the islands, but light pollution originating further inland is encroaching upon and adversely affecting these sites. The methodology presented in this study can be adopted for continued future studies in Malta as well as for other regions.

Read this paper on arXiv…

J. Caruana, R. Vella, D. Spiteri, et. al.
Wed, 12 Feb 20
23/58

Comments: 29 pages, 6 figures, 11 supplementary figures, accepted for publication in Journal of Environmental Management

Impact of gravity waves on the middle atmosphere of Mars: a non-orographic gravity wave parameterization based on Global Climate modeling and MCS observations [EPA]

Posted on by

http://arxiv.org/abs/2002.00723


The impact of gravity waves (GW) on diurnal tides and the global circulation in the middle/upper atmosphere of Mars is investigated using a General Circulation Model (GCM). We have implemented a stochastic parameterization of non-orographic GW into the Laboratoire de M\’et\’eorologie Dynamique (LMD) Mars GCM (LMD-MGCM) following an innovative approach. The source is assumed to be located above typical convective cells ($\sim$ 250 Pa) and the effect of GW on the circulation and predicted thermal structure above 1 Pa ($\sim$ 50 km) is analyzed. We focus on the comparison between model simulations and observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter during Martian Year 29. MCS data provide the only systematic measurements of the Martian mesosphere up to 80 km to date. The primary effect of GW is to damp the thermal tides by reducing the diurnal oscillation of the meridional and zonal winds. The GW drag reaches magnitudes of the order of 1 m/s/sol above 10$^{-2}$ Pa in the northern hemisphere winter solstice and produces major changes in the zonal wind field (from tens to hundreds of m/s), while the impact on the temperature field is relatively moderate (10-20K). It suggests that GW induced alteration of the meridional flow is the main responsible for the simulated temperature variation. The results also show that with the GW scheme included, the maximum day-night temperature difference due to the diurnal tide is around 10K, and the peak of the tide is shifted toward lower altitudes, in better agreement with MCS observations.

Read this paper on arXiv…

G. Gilli, F. Forget, A. Spiga, et. al.
Tue, 4 Feb 20
5/52

Comments: JGR (Planets), accepted

Spatial Analysis of Seasonal Precipitation over Iran: Co-Variation with Climate Indices [CL]

Posted on by

http://arxiv.org/abs/2001.09757


Temporary changes in precipitation may lead to sustained and severe drought or massive floods in different parts of the world. Knowing variation in precipitation can effectively help the water resources decision-makers in water resources management. Large-scale circulation drivers have a considerable impact on precipitation in different parts of the world. In this research, the impact of El Ni\~no-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO) on seasonal precipitation over Iran was investigated. For this purpose, 103 synoptic stations with at least 30 years of data were utilized. The Spearman correlation coefficient between the indices in the previous 12 months with seasonal precipitation was calculated, and the meaningful correlations were extracted. Then the month in which each of these indices has the highest correlation with seasonal precipitation was determined. Finally, the overall amount of increase or decrease in seasonal precipitation due to each of these indices was calculated. Results indicate the Southern Oscillation Index (SOI), NAO, and PDO have the most impact on seasonal precipitation, respectively. Also, these indices have the highest impact on the precipitation in winter, autumn, spring, and summer, respectively. SOI has a diverse impact on winter precipitation compared to the PDO and NAO, while in the other seasons, each index has its special impact on seasonal precipitation. Generally, all indices in different phases may decrease the seasonal precipitation up to 100%. However, the seasonal precipitation may increase more than 100% in different seasons due to the impact of these indices. The results of this study can be used effectively in water resources management and especially in dam operation.

Read this paper on arXiv…

M. Dehghani, S. Salehi, A. Mosavi, et. al.
Tue, 28 Jan 20
12/63

Comments: 25 pages, 10 figures

Gamma Ray Flashes Produced by Lightning Observed at Ground Level by TETRA-II [CL]

Posted on by

http://arxiv.org/abs/2001.09759


In its first 2 years of operation, the ground-based Terrestrial gamma ray flash and Energetic Thunderstorm Rooftop Array(TETRA)-II array of gamma ray detectors has recorded 22 bursts of gamma rays of millisecond-scale duration associated with lightning. In this study, we present the TETRA-II observations detected at the three TETRA-II ground-level sites in Louisiana, Puerto Rico, and Panama together with the simultaneous radio frequency signals from the VAISALA Global Lightning Data set, VAISALA National Lightning Detection Network, Earth Networks Total Lightning Network, and World Wide Lightning Location Network. The relative timing between the gamma ray events and the lightning activity is a key parameter for understanding the production mechanism(s) of the bursts. The gamma ray time profiles and their correlation with radio sferics suggest that the gamma ray events are initiated by lightning leader activity and are produced near the last stage of lightning leader channel development prior to the lightning return stroke.

Read this paper on arXiv…

D. Pleshinger, S. Alnussirat, J. Arias, et. al.
Tue, 28 Jan 20
60/63

Comments: 10 pages, 9 figures

Turbulence in stratified atmospheres: implications for the intracluster medium [GA]

Posted on by

http://arxiv.org/abs/2001.06494


The gas motions in the intracluster medium (ICM) are governed by stratified turbulence. Stratified turbulence is fundamentally different from Kolmogorov (isotropic, homogeneous) turbulence; kinetic energy not only cascades from large to small scales, but it is also converted into buoyancy potential energy. To understand the density and velocity fluctuations in the ICM, we conduct high-resolution ($1024^2\times 1536$ grid points) hydrodynamical simulations of subsonic turbulence (with rms Mach number $\mathcal{M}\approx 0.25$) and different levels of stratification, quantified by the Richardson number $\mathrm{Ri}$, from $\mathrm{Ri}=0$ (no stratification) to $\mathrm{Ri}=13$ (strong stratification). We quantify the density, pressure and velocity fields for varying stratification because observational studies often use surface brightness fluctuations to infer the turbulent gas velocities of the ICM. We find that the standard deviation of the logarithmic density fluctuations ($\sigma_s$), where $s=\ln(\rho/\left<\rho(z)\right>)$, increases with $\mathrm{Ri}$. For weakly stratified subsonic turbulence ($\mathrm{Ri}\lesssim10$, $\mathcal{M}<1$), we derive a new $\sigma_s$–$\mathcal{M}$–$\mathrm{Ri}$ relation, $\sigma_s^2=\ln(1+b^2\mathcal{M}^4+0.09\mathcal{M}^2\mathrm{Ri}H_P/H_S)$, where $b=1/3$–$1$ is the turbulence driving parameter, and $H_P$ and $H_S$ are the pressure and entropy scale heights respectively. We further find that the power spectrum of density fluctuations, $P(\rho_k/\left<\rho\right>)$, increases in magnitude with increasing $\mathrm{Ri}$, whereas the velocity power spectrum is invariant. Thus, the ratio between density and velocity power spectra strongly depends on $\mathrm{Ri}$. Pressure fluctuations, on the other hand, are independent of stratification and only depend on $\mathcal{M}$.

Read this paper on arXiv…

R. Mohapatra, C. Federrath and P. Sharma
Wed, 22 Jan 20
70/116

Comments: 15 pages, 12 figures, submitted to MNRAS, comments are welcome. For simulation movies see: this https URL

Global climate modeling of Saturn's atmosphere. Part IV: stratospheric equatorial oscillation [CL]

Posted on by

http://arxiv.org/abs/2001.07009


The Composite InfraRed Spectrometer (CIRS) on board Cassini revealed an equatorial oscillation of stratospheric temperature, reminiscent of the Earth’s Quasi-Biennial Oscillation (QBO), as well as anomalously high temperatures under Saturn’s rings. To better understand these predominant features of Saturn’s atmospheric circulation in the stratosphere, we have extended towards higher altitudes the DYNAMICO-Saturn global climate model (GCM), already used in a previous publication to study the tropospheric dynamics, jets formation and planetary-scale waves activity. Firstly, we study the higher model top impact on the tropospheric zonal jets and kinetic energy distribution. Raising the model top prevents energy and enstrophy accumulation at tropopause levels. The reference GCM simulation with 1/2$^{\circ}$ latitude/longitude resolution and a raised model top exhibits a QBO-like oscillation produced by resolved planetary-scale waves. However, the period is more irregular and the downward propagation faster than observations. Furthermore, compared to the CIRS observation retrievals, the modeled QBO-like oscillation underestimates by half both the amplitude of temperature anomalies at the equator and the vertical characteristic length of this equatorial oscillation. This QBO-like oscillation is mainly driven by westward-propagating waves; a significant lack of eastward wave-forcing explains a fluctuating eastward phase of the QBO-like oscillation. At 20$^{\circ}$N and 20$^{\circ}$S latitudes, the DYNAMICO-Saturn GCM exhibits several strong seasonal eastward jets, alternatively in the northern and southern hemisphere. These jets are correlated with the rings’ shadowing. Using a GCM simulation without rings’ shadowing, we show its impact on Saturn’s stratospheric dynamics. Both residual-mean circulation and eddy forcing are impacted by rings’ shadowing.

Read this paper on arXiv…

D. Bardet, A. Spiga, S. Guerlet, et. al.
Wed, 22 Jan 20
96/116

Comments: 57 pages, 22 figures, submitted to Icarus (under review)

Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets [EPA]

Posted on by

http://arxiv.org/abs/1912.11377


Tidally locked terrestrial planets around low-mass stars are the prime targets for future atmospheric characterizations of potentially habitable systems, especially the three nearby ones–Proxima b, TRAPPIST-1e, and LHS 1140b. Previous studies suggest that if these planets have surface ocean they would be in an eyeball-like climate state: ice-free in the vicinity of the substellar point and ice-covered in the rest regions. However, an important component of the climate system–sea ice dynamics has not been well studied in previous studies. A fundamental question is: would the open ocean be stable against a globally ice-covered snowball state? Here we show that sea-ice drift cools the ocean’s surface when the ice flows to the warmer substellar region and melts through absorbing heat from the ocean and the overlying air. As a result, the open ocean shrinks and can even disappear when atmospheric greenhouse gases are not much more abundant than on Earth, turning the planet into a snowball state. This occurs for both synchronous rotation and spin-orbit resonances (such as 3 to 2). These results suggest that sea-ice drift strongly reduces the open ocean area and can significantly impact the habitability of tidally locked planets.

Read this paper on arXiv…

J. Yang, W. Ji and Y. Zeng
Wed, 25 Dec 19
18/31

Comments: 52 pages, 4 figure in main text, 19 figures and 1 video in SI

Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: a 3D Model Intercomparison [EPA]

Posted on by

http://arxiv.org/abs/1912.11329


Robustly modeling the inner edge of the habitable zone is essential for determining the most promising potentially habitable exoplanets for atmospheric characterization. Global climate models (GCMs) have become the standard tool for calculating this boundary, but divergent results have emerged among the various GCMs. In this study, we perform an inter-comparison of standard GCMs used in the field on a rapidly rotating planet receiving a G-star spectral energy distribution and on a tidally locked planet receiving an M-star spectral energy distribution. Experiments both with and without clouds are examined. We find relatively small difference (within 8 K) in global-mean surface temperature simulation among the models in the G-star case with clouds. In contrast, the global-mean surface temperature simulation in the M-star case is highly divergent (20-30 K). Moreover, even differences in the simulated surface temperature when clouds are turned off are significant. These differences are caused by differences in cloud simulation and/or radiative transfer, as well as complex interactions between atmospheric dynamics and these two processes. For example we find that an increase in atmospheric absorption of shortwave radiation can lead to higher relative humidity at high altitudes globally and, therefore, a significant decrease in planetary radiation emitted to space. This study emphasizes the importance of basing conclusions about planetary climate on simulations from a variety of GCMs and motivates the eventual comparison of GCM results with terrestrial exoplanet observations to improve their performance.

Read this paper on arXiv…

J. Yang, J. Leconte, E. Wolf, et. al.
Wed, 25 Dec 19
21/31

Comments: 34 pages, 17 figures

Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: a 3D Model Intercomparison [EPA]

Posted on by

http://arxiv.org/abs/1912.11329


Robustly modeling the inner edge of the habitable zone is essential for determining the most promising potentially habitable exoplanets for atmospheric characterization. Global climate models (GCMs) have become the standard tool for calculating this boundary, but divergent results have emerged among the various GCMs. In this study, we perform an inter-comparison of standard GCMs used in the field on a rapidly rotating planet receiving a G-star spectral energy distribution and on a tidally locked planet receiving an M-star spectral energy distribution. Experiments both with and without clouds are examined. We find relatively small difference (within 8 K) in global-mean surface temperature simulation among the models in the G-star case with clouds. In contrast, the global-mean surface temperature simulation in the M-star case is highly divergent (20-30 K). Moreover, even differences in the simulated surface temperature when clouds are turned off are significant. These differences are caused by differences in cloud simulation and/or radiative transfer, as well as complex interactions between atmospheric dynamics and these two processes. For example we find that an increase in atmospheric absorption of shortwave radiation can lead to higher relative humidity at high altitudes globally and, therefore, a significant decrease in planetary radiation emitted to space. This study emphasizes the importance of basing conclusions about planetary climate on simulations from a variety of GCMs and motivates the eventual comparison of GCM results with terrestrial exoplanet observations to improve their performance.

Read this paper on arXiv…

J. Yang, J. Leconte, E. Wolf, et. al.
Wed, 25 Dec 19
10/31

Comments: 34 pages, 17 figures

Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets [EPA]

Posted on by

http://arxiv.org/abs/1912.11377


Tidally locked terrestrial planets around low-mass stars are the prime targets for future atmospheric characterizations of potentially habitable systems, especially the three nearby ones–Proxima b, TRAPPIST-1e, and LHS 1140b. Previous studies suggest that if these planets have surface ocean they would be in an eyeball-like climate state: ice-free in the vicinity of the substellar point and ice-covered in the rest regions. However, an important component of the climate system–sea ice dynamics has not been well studied in previous studies. A fundamental question is: would the open ocean be stable against a globally ice-covered snowball state? Here we show that sea-ice drift cools the ocean’s surface when the ice flows to the warmer substellar region and melts through absorbing heat from the ocean and the overlying air. As a result, the open ocean shrinks and can even disappear when atmospheric greenhouse gases are not much more abundant than on Earth, turning the planet into a snowball state. This occurs for both synchronous rotation and spin-orbit resonances (such as 3 to 2). These results suggest that sea-ice drift strongly reduces the open ocean area and can significantly impact the habitability of tidally locked planets.

Read this paper on arXiv…

J. Yang, W. Ji and Y. Zeng
Wed, 25 Dec 19
27/31

Comments: 52 pages, 4 figure in main text, 19 figures and 1 video in SI

Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: a 3D Model Intercomparison [EPA]

Posted on by

http://arxiv.org/abs/1912.11329


Robustly modeling the inner edge of the habitable zone is essential for determining the most promising potentially habitable exoplanets for atmospheric characterization. Global climate models (GCMs) have become the standard tool for calculating this boundary, but divergent results have emerged among the various GCMs. In this study, we perform an inter-comparison of standard GCMs used in the field on a rapidly rotating planet receiving a G-star spectral energy distribution and on a tidally locked planet receiving an M-star spectral energy distribution. Experiments both with and without clouds are examined. We find relatively small difference (within 8 K) in global-mean surface temperature simulation among the models in the G-star case with clouds. In contrast, the global-mean surface temperature simulation in the M-star case is highly divergent (20-30 K). Moreover, even differences in the simulated surface temperature when clouds are turned off are significant. These differences are caused by differences in cloud simulation and/or radiative transfer, as well as complex interactions between atmospheric dynamics and these two processes. For example we find that an increase in atmospheric absorption of shortwave radiation can lead to higher relative humidity at high altitudes globally and, therefore, a significant decrease in planetary radiation emitted to space. This study emphasizes the importance of basing conclusions about planetary climate on simulations from a variety of GCMs and motivates the eventual comparison of GCM results with terrestrial exoplanet observations to improve their performance.

Read this paper on arXiv…

J. Yang, J. Leconte, E. Wolf, et. al.
Wed, 25 Dec 19
3/31

Comments: 34 pages, 17 figures

Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets [EPA]

Posted on by

http://arxiv.org/abs/1912.11377


Tidally locked terrestrial planets around low-mass stars are the prime targets for future atmospheric characterizations of potentially habitable systems, especially the three nearby ones–Proxima b, TRAPPIST-1e, and LHS 1140b. Previous studies suggest that if these planets have surface ocean they would be in an eyeball-like climate state: ice-free in the vicinity of the substellar point and ice-covered in the rest regions. However, an important component of the climate system–sea ice dynamics has not been well studied in previous studies. A fundamental question is: would the open ocean be stable against a globally ice-covered snowball state? Here we show that sea-ice drift cools the ocean’s surface when the ice flows to the warmer substellar region and melts through absorbing heat from the ocean and the overlying air. As a result, the open ocean shrinks and can even disappear when atmospheric greenhouse gases are not much more abundant than on Earth, turning the planet into a snowball state. This occurs for both synchronous rotation and spin-orbit resonances (such as 3 to 2). These results suggest that sea-ice drift strongly reduces the open ocean area and can significantly impact the habitability of tidally locked planets.

Read this paper on arXiv…

J. Yang, W. Ji and Y. Zeng
Wed, 25 Dec 19
17/31

Comments: 52 pages, 4 figure in main text, 19 figures and 1 video in SI

Measurements and semi-empirical calculations of CO2+CH4 and CO2+H2 collision-induced absorptions across a wide range of wavenumbers and temperatures. Application for the prediction of early Mars surface temperature [EPA]

Posted on by

http://arxiv.org/abs/1912.05630


Reducing atmospheres have recently emerged as a promising scenario to warm the surface of early Mars enough to drive the formation of valley networks and other ancient aqueous features that have been detected so far on the surface of Mars. Here we present a series of experiments and calculations to better constrain CO2+CH4 and CO2+H2 collision-induced absorptions (CIAs) as well as their effect on the prediction of early Mars surface temperature. First, we carried out a new set of experimental measurements (using the AILES line of the SOLEIL synchrotron) of both CO2+CH4 and CO2+H2 CIAs. These measurements confirm the previous results of Turbet et al. 2019, Icarus vol. 321, while significantly reducing the experimental uncertainties. Secondly, we fitted a semi-empirical model to these CIAs measurements, allowing us to compute the CO2+CH4 and CO2+H2 CIAs across a broad spectral domain (0-1500cm-1) and for a wide range of temperatures (100-600K). Last, we performed 1-D numerical radiative-convective climate calculations (using the LMD Generic Model) to compute the surface temperature expected on the surface of early Mars for several CO2, CH4 and H2 atmospheric contents, taking into account the radiative effect of these revised CIAs. These calculations demonstrate that thick CO2+H2-dominated atmospheres remain a viable solution for warming the surface of Mars above the melting point of water, but not CO2+CH4-dominated atmospheres. Our calculated CO2+CH4 and CO2+H2 CIA spectra and predicted early Mars surface temperatures are provided to the community for future uses.

Read this paper on arXiv…

M. Turbet, C. Boulet and T. Karman
Fri, 13 Dec 19
73/75

Comments: Submitted for publication to Icarus. CO2+H2 and CO2+CH4 CIAs calculated tables (HITRAN format) – as well as predicted early Mars surface temperatures – are available on demand

Eddy evolution during large dust storms [EPA]

Posted on by

http://arxiv.org/abs/1912.01042


The evolution of eddy kinetic energy during the development of large regional dust storms on Mars is investigated using the Mars Analysis Correction Data Assimilation (MACDA) reanalysis product and the dust storm data derived from Mars Global Surveyor Mars Daily Global Maps. Transient eddies in MACDA are decomposed into different components according to their eddy periods: $P\leq1$ sol, $1<P\leq8$ sols, $8<P\leq60$ sols. This paper primarily focuses on the Mars year 24 pre-solstice “A” storm that starts with many episodes of frontal/flushing dust storms from the northern hemisphere and attains its maximum global mean opacity after dust expansion in the southern hemisphere. During the development of this storm, the dominant eddies in terms of eddy kinetic energy progress from the $1<P\leq8$ sol eddies in the northern mid/high latitudes to the $P\leq1$ sol eddies (dominated by thermal tides) in the southern mid latitudes, and the $8<P\leq60$ sol eddies show a prominent peak with the increased global-mean dust opacity. The peaks of the $1<P\leq8$ sol eddies are found to best correlate with the average area of textured frontal/flushing dust storms within 40$^\circ$N $-$ 60$^\circ$N. The region where the $1<P\leq8$ sol eddies increase the most corresponds to the main flushing channel. The eddy kinetic energy of the $P\leq1$ eddies, dominated by $P$ = 1 and its harmonics, increases with the global mean dust opacity both before and after the winter solstice in Mars year 24. The $8<P\leq60$ sol eddies briefly spike during large, regional dust storms but remain weak if dust storm sequences do not lead to a major dust storm. Zonal wavenumber analysis of eddy kinetic energy shows that the peaks of the $1<P\leq8$ eddies often result from combinations of zonal wavenumbers 1 to 3, while the $P\leq1$ eddies and $8<P\leq60$ sol eddies are each dominated by zonal wavenumber 1.

Read this paper on arXiv…

M. Battalio and H. Wang
Wed, 4 Dec 19
39/58

Comments: N/A

Colour and Tropospheric Cloud Structure of Jupiter from MUSE/VLT: Retrieving a Universal Chromophore [EPA]

Posted on by

http://arxiv.org/abs/1912.00918


Recent work by Sromovsky et al. (2017, Icarus 291, 232-244) suggested that all red colour in Jupiter’s atmosphere could be explained by a single colour-carrying compound, a so-called ‘universal chromophore’. We tested this hypothesis on ground-based spectroscopic observations in the visible and near-infrared (480-930 nm) from the VLT/MUSE instrument between 2014 and 2018, retrieving a chromophore absorption spectrum directly from the North Equatorial Belt, and applying it to model spatial variations in colour, tropospheric cloud and haze structure on Jupiter. We found that we could model both the belts and the Great Red Spot of Jupiter using the same chromophore compound, but that this chromophore must exhibit a steeper blue-absorption gradient than the proposed chromophore of Carlson et al. (2016, Icarus 274, 106-115). We retrieved this chromophore to be located no deeper than 0.2+/-0.1 bars in the Great Red Spot and 0.7+/-0.1 bars elsewhere on Jupiter. However, we also identified some spectral variability between 510 nm and 540 nm that could not be accounted for by a universal chromophore. In addition, we retrieved a thick, global cloud layer at 1.4+/-0.3 bars that was relatively spatially invariant in altitude across Jupiter. We found that this cloud layer was best characterised by a real refractive index close to that of ammonia ice in the belts and the Great Red Spot, and poorly characterised by a real refractive index of 1.6 or greater. This may be the result of ammonia cloud at higher altitude obscuring a deeper cloud layer of unknown composition.

Read this paper on arXiv…

A. Braude, P. Irwin, G. Orton, et. al.
Tue, 3 Dec 19
26/90

Comments: 14 figures + 4 tables, preprint accepted by Icarus on the 29th of November 2019

Revised mass-radius relationships for water-rich terrestrial planets beyond the runaway greenhouse limit [EPA]

Posted on by

http://arxiv.org/abs/1911.08878


Mass-radius relationships for water-rich terrestrial planets are usually calculated assuming most water is present in condensed (either liquid or solid) form. Planet density estimates are then compared to these mass-radius relationships even when these planets are more irradiated than the runaway greenhouse irradiation limit (around 1.06x the Earth irradiation for planets orbiting a Sun-like star), for which water has been shown to be unstable in condensed form and should rather form a thick H2O-dominated atmosphere. Here we use the LMD Generic numerical climate model to derive new mass-radius relationships appropriate for water-rich terrestrial planets located beyond the runaway greenhouse limit, i.e. planets endowed with a steam, water-dominated atmosphere. These new mass-radius relationships significantly differ from those traditionnally used in the literature. For a given water-to-rock mass ratio, these new mass-radius relationships lead to planet bulk densities much lower than calculated when water is assumed to be in condensed form. In other words, using traditional mass-radius relationships for planets that are more irradiated than the runaway greenhouse limit tends to dramatically overestimate their bulk water content. In particular, this result applies to TRAPPIST-1b, c and d, that should not have more (assuming planetary core with a terrestrial composition) than 2, 0.3 and 0.08% of water, respectively. In addition, we show with the example of the TRAPPIST-1 multiplanetary system that the jumps in mass-radius relationships (related to the runaway greenhouse transition) can be used to remove usual composition degeneracies in mass-radius relationships. Finally, we provide an empirical formula for the H2O steam atmosphere thickness that can easily be used to construct mass-radius relationships for water-rich, terrestrial planets located beyond the runaway greenhouse limit.

Read this paper on arXiv…

M. Turbet, E. Bolmont, D. Ehrenreich, et. al.
Thu, 21 Nov 19
6/57

Comments: Submitted for publication to A&A. The abstract is abridged to meet ArXiv size limit

Combining Thermodynamic and Dynamic Perspectives of Tropical Circulation to Constrain the Downdraft Width of the Hadley Cell [CL]

Posted on by

http://arxiv.org/abs/1911.05860


In the Hadley circulation downdraft, to leading order vertical potential temperature advection balances the “effective” heating, comprising the sum of diabatic heating and eddy heat flux divergence, placing a thermodynamic constraint on vertical velocity. Insofar as downdraft-averaged effective heating and static stability do not vary with planetary parameters, neither can vertical velocity — an “omega governor.” Separately, in the eddy-driven (i.e. small-Rossby-number) limit, extratropical eddy stresses also constrain the cell strength dynamically. We combine these thermodynamic and dynamic mechanisms to derive new and identical scalings for the downdraft width and overturning strength with rotation rate. With the omega governor maintaining fixed vertical velocity, the downdraft must narrow or widen in order to attain the overturning strength dictated by the eddy stresses. We evaluate the validity of this new scaling using model simulations over a broad range of rotation rates and model forcing schemes.

Read this paper on arXiv…

J. Mitchell and S. Hill
Fri, 15 Nov 19
25/73

Comments: 13 pages (4200 words) main text, 3 figures 16 pages Supplementary Info, 10 figures

Molecular Simulations for the Spectroscopic Detection of Atmospheric Gases [CL]

Posted on by

http://arxiv.org/abs/1911.04238


Unambiguously identifying molecules in spectra is of fundamental importance for a variety of scientific and industrial uses. Interpreting atmospheric spectra for the remote detection of volatile compounds requires information about the spectrum of each relevant molecule. However, spectral data currently exist for a few hundred molecules and only a fraction of those have complete spectra (e.g. H$_2$O, NH$_3$). Consequently, molecular detections in atmospheric spectra remain vulnerable to false positives, false negatives, and missassignments. There is a key need for spectral data for a broad range of molecules. Given how challenging it is to obtain high-resolution molecular spectra, there is great value in creating intermediate approximate spectra that can provide a starting point for the analysis of atmospheric spectra.
Using a combination of experimental measurements, organic chemistry, and quantum mechanics, RASCALL (Rapid Approximate Spectral Calculations for ALL) is a computational approach that provides approximate spectral data for any given molecule, including thousands of potential atmospheric gases, in seconds. RASCALL 1.0 simulates molecular spectra by interpreting functional group data from experimental and theoretical sources to estimate the position and strength of molecular bands. The RASCALL 1.0 spectra consist of approximate band centers and qualitative intensities. RASCALL can also be used to study spectral patterns between molecules, and to highlight ambiguities in molecular detections. The RASCALL catalogue, and its preliminary version RASCALL 1.0, contains spectral data for more molecules than any other publicly available database, with applications in all fields interested in the detection of molecules in the gas phase. The preliminary catalogue of molecular data and associated documentation are freely available online and will be routinely updated.

Read this paper on arXiv…

C. Sousa-Silva, J. Petkowski and S. Seager
Tue, 12 Nov 19
79/84

Comments: N/A

The Acceleration of Superrotation in Simulated Hot Jupiter Atmospheres [EPA]

Posted on by

http://arxiv.org/abs/1911.03182


Context. Atmospheric superrotating flows at the equator are an almost ubiquitous result of simulations of hot Jupiters, and a theory explaining how this zonally coherent flow reaches an equilibrium has been developed in the literature. However, this understanding relies on the existence of either an initial superrotating or a sheared flow, coupled with a slow evolution such that a linear steady state can be reached. Aims. A consistent physical understanding of superrotation is needed for arbitrary drag and radiative timescales, and the relevance of considering linear steady states needs to be assessed. Methods. We obtain an analytical expression for the structure, frequency and decay rate of propagating waves in hot Jupiter atmospheres around a state at rest in the 2D shallow water beta plane limit. We solve this expression numerically and confirm the robustness of our results with a 3D linear wave algorithm. We then compare with 3D simulations of hot Jupiter atmospheres and study the non linear momentum fluxes. Results. We show that under strong day night heating the dynamics does not transit through a linear steady state when starting from an initial atmosphere in solid body rotation. We further show that non linear effects favour the initial spin up of superrotation and that the acceleration due to the vertical component of the eddy momentum flux is critical to the initial development of superrotation. Conclusions. Overall, we describe the initial phases of the acceleration of superrotation, including consideration of differing radiative and drag timescales, and conclude that eddy-momentum driven superrotating equatorial jets are robust, physical phenomena in simulations of hot Jupiter atmospheres.

Read this paper on arXiv…

F. Debras, N. Mayne, I. Baraffe, et. al.
Mon, 11 Nov 19
46/105

Comments: 28 pages, 20 pages of text – 8 of appendices, 9 figures in text – 6 in appendices

New achievements in optical turbulence forecast systems in operational mode [IMA]

Posted on by

http://arxiv.org/abs/1911.02819


In this contribution, we present the most recent progresses we obtained in the context of a long-term program we undertook since a few years towards the implementation of operational forecast systems (a) on top-class ground-based telescopes assisted by AO systems to support the flexible scheduling of observational scientific programs in night as well in day time and (b) on ground-stations to support free space optical communication. Two topics have been treated and presented in the Conference AO4ELT6:
1. ALTA is an operational forecast system for the OT and all the critical atmospheric parameters affecting the astronomical ground-based observations conceived for the LBT. It operates since 2016 and it is in continuous evolution to match with necessities/requirements of instruments assisted by AO of the LBT (SOUL, SHARK-NIR, SHARK-VIS, LINC-NIRVANA,…). In this contribution, we present a new implemented version of ALTA that, thanks to an auto-regression method making use of numerical forecasts and real-time OT measurements taken in situ, can obtain model performances (for forecasts of atmospherical and astroclimatic parameters) never achieved before on time scales of the order of a few hours.
2. We will go through the main differences between optical turbulence forecast performed with mesoscale and general circulation models (GCM) by clarifying some fundamental concepts and by correcting some erroneous information circulating recently in the literature.

Read this paper on arXiv…

E. Masciadri, A. Turchi and G. Martelloni
Mon, 11 Nov 19
96/105

Comments: 8 figures, Proceedings of AO4ELT6 Conference, Quebec City 9-14 June 2019

Collapse of the general circulation in shortwave-absorbing atmospheres: an idealized model study [EPA]

Posted on by

http://arxiv.org/abs/1911.00629


The response of the general circulation in a dry atmosphere to various atmospheric shortwave absorptivities is investigated in a three-dimensional general circulation model with grey radiation. Shortwave absorption in the atmosphere reduces the incoming radiation reaching the surface but warms the upper atmosphere, significantly shifting the habitable zone toward the star. The strong stratification under high shortwave absorptivity suppresses the Hadley cell in a manner that matches previous Hadley cell scalings. General circulation changes may be observable through cloud coverage and superrotation. The equatorial superrotation in the upper atmosphere strengthens with the shortwave opacity, as predicted based on the gradient wind of the radiative-convective equilibrium profile. There is a sudden drop of equatorial superrotation at very low shortwave opacity. This is because the Hadley cell in those cases are strong enough to fill the entire troposphere with zero momentum air from the surface. A diurnal cycle (westward motion of substellar point relative to the planet) leads to acceleration of the equatorial westerlies in general, through the enhancement of the equatorward eddy momentum transport, but the response is not completely monotonic, perhaps due to the resonance of tropical waves and the diurnal forcing.

Read this paper on arXiv…

W. Kang and R. Wordsworth
Tue, 5 Nov 19
22/72

Comments: N/A

Effects of the Great American Solar Eclipse on the lower ionosphere observed with VLF waves [CL]

Posted on by

http://arxiv.org/abs/1910.14037


The altitude of the ionospheric lower layer (D-region) is highly influenced by the solar UV flux affecting in turn, the propagation of Very Low Frequency (VLF) signals inside the waveguide formed between this layer and the Earth surface. A rapid change of the solar irradiance, as during a solar eclipse, can help to understand the details of the energy transfer of the solar radiation onto the ionospheric D-layer. Using the “Latin American VLF Network” (LAVNet-Mex) receiver station in Mexico City, Mexico, we detected the phase and amplitude changes of the VLF signals transmitted by the NDK station at 25.2 kHz in North Dakota, USA during the August 21, 2017, solar eclipse. As the Sunlight was eclipsed, the rate of ionization in the ionosphere (D-region) was reduced and the effective reflection height increased, causing a considerable drop of the phase and amplitude of the observed VLF waves. The corresponding waveguide path is 3007.15 km long and crossed almost perpendicularly the total eclipse path. Circumstantially, at the time of the total eclipse, a C3 flare took place allowing us to isolate the flare flux from the background flux of a large portion of the disk. In this work, we report the observations and present a new model of the ionospheric effects of the eclipse and flare. The model is based on a detailed setup of the degree of Moon shadow that affects the entire Great Circle Path (GCP). During the eclipse, the maximum phase variation was -63.36$^{\circ}$ at 18:05 UT which, according to our model, accounts for a maximum increase of the reflection height of 9.3 km.

Read this paper on arXiv…

R. Vogrinčič, A. Lara, A. Borgazzi, et. al.
Fri, 1 Nov 19
49/54

Comments: 23 pages, 5 figures, appendix A and B, accepted for publication in Advances in Space Research

Nowcasting the turbulence at the Paranal Observatory [IMA]

Posted on by

http://arxiv.org/abs/1910.13767


At Paranal Observatory, the least predictable parameter affecting the short-term scheduling of astronomical observations is the optical turbulence, especially the seeing, coherence time and ground layer fraction. These are critical variables driving the performance of the instruments of the Very Large Telescope (VLT), especially those fed with adaptive optics systems. Currently, the night astronomer does not have a predictive tool to support him/her in decision-making at night. As most service-mode observations at the VLT last less than two hours, it is critical to be able to predict what will happen in this time frame, to avoid time losses due to sudden changes in the turbulence conditions, and also to enable more aggressive scheduling. We therefore investigate here the possibility to forecast the turbulence conditions over the next two hours. We call this “turbulence nowcasting”, analogously with weather nowcasting, a term already used in meteorology coming from the contraction of “now” and “forecasting”. We present here the results of a study based on historical data of the Paranal Astronomical Site Monitoring combined with ancillary data, in a machine learning framework. We show the strengths and shortcomings of such an approach, and present some perspectives in the context of the Extremely Large Telescope.

Read this paper on arXiv…

J. Milli, R. Gonzalez, P. Fluxa, et. al.
Thu, 31 Oct 19
38/55

Comments: 12 pages, 7 figures, proceeding of the AO4ELT6 conference

Lightning in other planets [EPA]

Posted on by

http://arxiv.org/abs/1910.12722


More than 4000 planet are known that orbit stars other than our Sun. Many harbor a dynamic atmosphere that is cold enough that cloud particles can form in abundance. The diversity of exoplanets leads to differences in cloud coverage depending on global system parameters. Some planets will be fully covered in clouds, some have clouds on the nightside but are largely cloud-free on the dayside. These cloud particles can easily be charged and lightning discharges will occur in cloudy, dynamic exoplanet atmosphere. Lightning supports a Global Electric Circuit (GCE) on Earth and we argue that exoplanet may develop a GCE in particular if parts of the exoplanet atmospheres can remain cloud free.

Read this paper on arXiv…

C. Helling
Tue, 29 Oct 19
50/78

Comments: 7 pages, reviewed proceeding of an invited keynote talk for ‘Electrostatics 2019’, accepted for publication in the Journal of Physics Conference Series

Horizontal shear instabilities in rotating stellar radiation zones: I. Inflectional and inertial instabilities and the effects of thermal diffusion [SSA]

Posted on by

http://arxiv.org/abs/1910.09860


The so-called rotational mixing, which transports angular momentum and chemical elements in stellar radiative zones, is one of the key processes for modern stellar evolution. In the two last decades, the stress has been put on the turbulent transport induced by the vertical shear instability. However, the instabilities of horizontal shears and the strength of the anisotropic turbulent transport they may trigger are still largely unknown. In this paper, we investigate the combined effects of stable stratification, rotation, and thermal diffusion on the instabilities of horizontal shears in the context of stellar radiative zones. The eigenvalue problem describing the instabilities of a flow with a hyperbolic-tangent horizontal shear profile is solved numerically and asymptotically by means of the Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) analysis to provide explicit asymptotic dispersion relations in non-diffusive and highly diffusive limits. Two types of instabilities are identified: the inflectional and the inertial instabilities. The inflectional instability is most unstable at finite streamwise wavenumber and zero vertical wavenumber, independently of the stratification, rotation, and thermal diffusion. It is favored by stable stratification but stabilized by thermal diffusion. The inertial instability is driven by rotation and the WKBJ analysis reveals that the growth rate reaches its maximum in the inviscid limit: $\sqrt{f(1-f)}$ (where $f$ is the dimensionless Coriolis parameter). The inertial instability for finite vertical wavenumber is stabilized as the stratification increases for non-diffusive fluids, while it becomes independent of the stratification and stronger for fluids with high thermal diffusivity. Furthermore, we found a self-similarity of the instabilities based on the rescaled parameter $PeN^{2}$ with the P\’eclet number $Pe$ and the Brunt-V\”ais\”al\”a frequency $N$.

Read this paper on arXiv…

J. Park, V. Prat and S. Mathis
Wed, 23 Oct 19
30/64

Comments: Submitted to A&A

Temporal Variability in Hot Jupiter Atmospheres [EPA]

Posted on by

http://arxiv.org/abs/1910.09523


Hot Jupiters receive intense incident stellar light on their daysides, which drives vigorous atmospheric circulation that attempts to erase their large dayside-to-nightside flux contrasts. Propagating waves and instabilities in hot Jupiter atmospheres can cause emergent properties of the atmosphere to be time-variable. In this work, we study such weather in hot Jupiter atmospheres using idealized cloud-free general circulation models with double-grey radiative transfer. We find that hot Jupiter atmospheres can be time-variable at the $\sim 0.1-1\%$ level in globally averaged temperature and at the $\sim 1-10\%$ level in globally averaged wind speeds. As a result, we find that observable quantities are also time variable: the secondary eclipse depth can be variable at the $\lesssim 2\%$ level, the phase curve amplitude can change by $\lesssim 1\%$, the phase curve offset can shift by $\lesssim 5^{\circ}$, and terminator-averaged wind speeds can vary by $\lesssim 2~ \mathrm{km}~\mathrm{s}^{-1}$. Additionally, we calculate how the eastern and western limb-averaged wind speeds vary with incident stellar flux and the strength of an imposed drag that parameterizes Lorentz forces in partially ionized atmospheres. We find that the eastern limb is blueshifted in models over a wide range of equilibrium temperature and drag strength, while the western limb is only redshifted if equilibrium temperatures are $\lesssim1500~\mathrm{K}$ and drag is weak. In general, our results show that the amplitude of time-variability increases with increasing incident stellar flux and decreasing atmospheric drag strength. We show that this variability may be observationally detectable in the infrared through secondary eclipse observations with JWST, phase curve observations with future space telescopes (e.g., ARIEL), and/or Doppler wind speed measurements with high-resolution spectrographs.

Read this paper on arXiv…

T. Komacek and A. Showman
Tue, 22 Oct 19
81/91

Comments: 17 pages, 12 figures, submitted to AAS Journals

Real-Time Thermospheric Density Estimation Via Two-Line-Element Data Assimilation [CL]

Posted on by

http://arxiv.org/abs/1910.00695


Inaccurate estimates of the thermospheric density are a major source of error in low Earth orbit prediction. To improve orbit prediction, real-time density estimation is required. In this work, we develop a reduced-order dynamic model for the thermospheric density by computing the main spatial modes of the atmosphere and deriving a linear model for the dynamics. The model is then used to estimate the density using two-line element (TLE) data by simultaneously estimating the reduced-order modes and the orbits and ballistic coefficients of several objects using an unscented Kalman filter. Accurate density estimation using the TLEs of 17 objects is demonstrated and validated against CHAMP and GRACE accelerometer-derived densities. Finally, the use of the model for density forecasting is shown.

Read this paper on arXiv…

D. Gondelach and R. Linares
Thu, 3 Oct 19
25/59

Comments: 24 pages, 13 figures, preprint

The Snowball Stratosphere [CL]

Posted on by

http://arxiv.org/abs/1909.12717


According to the Snowball Earth hypothesis, Earth has experienced periods of low-latitude glaciation in its deep past. Prior studies have used general circulation models (GCMs) to examine the effects such an extreme climate state might have on the structure and dynamics of Earth’s troposphere, but the behavior of the stratosphere has not been studied in detail. Understanding the snowball stratosphere is important for developing an accurate account of the Earth’s radiative and chemical properties during these episodes. Here we conduct the first analysis of the stratospheric circulation of the Snowball Earth using ECHAM6 general circulation model simulations. In order to understand the factors contributing to the stratospheric circulation, we extend the Statistical Transformed Eulerian Mean framework. We find that the stratosphere during a snowball with prescribed modern ozone levels exhibits a weaker meridional overturning circulation, reduced wave activity, stronger zonal jets, and is extremely cold relative to modern conditions. Notably, the snowball stratosphere displays no sudden stratospheric warmings. Without ozone, the stratosphere displays slightly weaker circulation, a complete lack of polar vortex, and even colder temperatures. We also explicitly quantify for the first time the cross-tropopause mass exchange rate and stratospheric mixing efficiency during the snowball and show that our values do not change the constraints on CO$_2$ inferred from geochemical proxies during the Marinoan glaciation ($\sim$635 Ma), unless the O$_2$ concentration during the snowball was orders of magnitude less than the CO$_2$ concentration.

Read this paper on arXiv…

R. Graham, T. Shaw and D. Abbot
Mon, 30 Sep 19
25/55

Comments: 28 pages, 11 figures, submitted to JGR-Atmospheres

On physical interpretations of the reference transit radius of gas-giant exoplanets [EPA]

Posted on by

http://arxiv.org/abs/1909.12639


Two theoretical quandaries involving transmission spectra of gas-giant exoplanets are elucidated. When computing the transit radius as a function of wavelength, one needs to specify a reference transit radius corresponding to a reference pressure. Mathematically, the reference transit radius is a constant of integration that originates from evaluating an integral for the transit depth. Physically, its interpretation has been debated in the literature. Jordan & Espinoza (2018) suggested that the optical depth is discontinuous across, and infinite below, the reference transit radius. Betremieux & Swain (2017, 2018) interpreted the spherical surface located at the reference transit radius to represent the boundary associated with an opaque cloud deck. It is demonstrated that continuous functions for the optical depth may be found. The optical depth below and at the reference transit radius need not take on special or divergent values. In the limit of a spatially uniform grey cloud with constant opacity, the transit chord with optical depth on the order of unity mimics the presence of a “cloud top”. While the surface located at the reference pressure may mimic the presence of grey clouds, it is more natural to include the effects of these clouds as part of the opacity function because the cloud opacity may be computed from first principles. It is unclear how this mimicry extends to non-grey clouds comprising small particles.

Read this paper on arXiv…

K. Heng
Mon, 30 Sep 19
53/55

Comments: Accepted by MNRAS. 7 pages, 2 figures

Monitoring transition: expected night sky brightness trends in different photometric bands [IMA]

Posted on by

http://arxiv.org/abs/1909.10909


Several light pollution indicators are commonly used to monitor the effects of the transition from outdoor lighting systems based on traditional gas-discharge lamps to solid-state light sources. In this work we analyze a subset of these indicators, including the artificial zenithal night sky brightness in the visual photopic and scotopic bands, the brightness in the specific photometric band of the widely used Sky Quality Meter (SQM), and the top-of-atmosphere radiance detected by the VIIRS-DNB radiometer onboard the satellite Suomi-NPP. Using a single-scattering approximation in a layered atmosphere we quantitatively show that, depending on the transition scenarios, these indicators may show different, even opposite behaviors. This is mainly due to the combined effects of the changes in the sources’ spectra and angular radiation patterns, the wavelength-dependent atmospheric propagation processes and the differences in the detector spectral sensitivity bands. It is suggested that the possible presence of this differential behavior should be taken into account when evaluating light pollution indicator datasets for assessing the outcomes of public policy decisions regarding the upgrading of outdoor lighting systems.

Read this paper on arXiv…

S. Bará, I. Rigueiro and R. Lima
Wed, 25 Sep 19
57/70

Comments: 28 pages, 7 fig

Weather Research and Forecasting) model and radiative methods for cloud top height retrieval along the EUSO-SPB1 trajectory [CL]

Posted on by

http://arxiv.org/abs/1909.09782


The Extreme Universe Space Observatory-Super Pressure Balloon (EUSO-SPB1) is a pathfinder of the JEM-EUSO program, which aims to observe Ultra High Energy Cosmic Rays (UHECRs) from near-space. It was launched from Wanaka (New Zealand) on April 25, 2017 UTC and was terminated after twelve days of flight in the South Pacific Ocean. A good knowledge of the atmospheric conditions and cloud properties, such as the Cloud Top Height (CTH), is fundamental to correctly reconstruct the energy and geometry of air showers produced by cosmic rays passing through the atmosphere. One of the methods used to retrieve the CTH is based on Numerical Weather Prediction models. In this work, we consider in particular the Weather Research and Forecasting (WRF) model. A first model test is made on the WRF parametrizations for elementary processes, applying a top-bottom directed algorithm based on two quantities: cloud fraction and optical depth. The validated procedure is then applied to the SPB1 trajectory, retrieving the CTH every ten minutes for the days of the flight. A comparison is made with the analyzed data taken from MODerate resolution Imaging Spectroradiometer (MODIS) satellite images, once per day, to understand the reliability of the method. Another way to retrieve the CTH is the so-called radiative method, that allows to calculate the Cloud Top Temperature (CTT). A vertical temperature profile is needed to transform the CTT into CTH. When radiosoundings are not available, WRF can provide vertical temperature profiles. The conversion from the CTT to the CTH is then made.

Read this paper on arXiv…

S. Monte, C. Vigorito, M. Bertaina, et. al.
Tue, 24 Sep 19
21/70

Comments: N/A

Simultaneous Generalized and Low-Layer SCIDAR turbulence profiles at San Pedro Mártir Observatory [IMA]

Posted on by

http://arxiv.org/abs/1909.08140


We present optical turbulence profiles obtained with a Generalized SCIDAR (G-SCIDAR) and a Low Layer SCIDAR (LOLAS) at the Observatorio Astron\’omico Nacional in San Pedro M\’artir (OAN-SPM), Baja California, Mexico, during three observing campaigns in 2013, 2014 and 2015. The G-SCIDAR delivers profiles with moderate altitude-resolution (a few hundred meters) along the entire turbulent section of the atmosphere, while the LOLAS gives high altitude-resolution (on the order of tens of meters) but only within the first few hundred meters. Simultaneous measurements were obtained on 2014 and allowed us to characterize in detail the combined effect of the local orography and wind direction on the turbulence distribution close to the ground. At the beginning of several nights, the LOLAS profiles show that turbulence peaks between 25 and 50 m above the ground, not at ground level as was expected. The G-SCIDAR profiles exhibit a peak within the first kilometer. In 55% and 36% of the nights stable layers are detected between 10 and 15 km and at 3 km, respectively. This distribution is consistent with the results obtained with a G-SCIDAR in 1997 and 2000 observing campaigns. Statistics computed with the 7891 profiles that have been measured at the OAN-SPM with a G-SCIDAR in 1997, 2000, 2014 and 2015 campaigns are presented. The seeing values calculated with each of those profiles have a median of 0.79, first and third quartiles of 0.51 and 1.08 arcsec, which are in close agreement with other long term seeing monitoring performed at the OAN-SPM.

Read this paper on arXiv…

R. Avila, O. Valdés-Hernández, L. Sánchez, et. al.
Thu, 19 Sep 19
42/71

Comments: N/A

Shape and size of large-scale vortices : a universal fluid pattern in geophysical fluid dynamics [CL]

Posted on by

http://arxiv.org/abs/1909.03244


Planetary rotation organizes fluid motions into coherent, long-lived swirls, known as large scale vortices (LSVs). LSVs are ubiquitous in nature, and their shape and size are expected to control their effect on the dynamics and long-term evolution of geophysical and astrophysical fluids. By using high-resolution direct numerical simulations, here we show for the first time that the shape of LSVs in rapidly-rotating mixed convective and stably-stratified fluids, which approximates the two-layer, turbulent-stratified dynamics of many geophysical and astrophysical fluids, is universal and that their size can be predicted. Specifically, we show that LSVs emerge in the convection zone from upscale energy transfers and decay as they penetrate into the stratified layer by thermal wind balance, thus taking the shape of a depth-invariant cylinder in the turbulent layer and of a penetrating half dome in the stable one. Furthermore, we demonstrate that when LSVs penetrate all the way through a stratified layer bounded by a solid boundary, they saturate by boundary friction. We provide a prediction for the penetration depth and maximum radius of LSVs as a function of the LSV vorticity, the stable layer depth and the stratification. Our results suggest that while turbulent vortices can penetrate far into the stratified layers of atmospheres and oceans, they should stay confined within the convective layers of Earth’s liquid core and of the Sun.

Read this paper on arXiv…

L. Couston, D. Lecoanet, B. Favier, et. al.
Tue, 10 Sep 19
59/80

Comments: submitted to Physical Review Research

Comparison of the deep atmospheric dynamics of Jupiter and Saturn in light of the Juno and Cassini gravity measurements [EPA]

Posted on by

http://arxiv.org/abs/1908.09613


The nature and structure of the observed east-west flows on Jupiter and Saturn has been one of the longest-lasting mysteries in planetary science. This mystery has been recently unraveled due to the accurate gravity measurements provided by the Juno mission to Jupiter and the Grand Finale of the Cassini mission to Saturn. These two experiments, which coincidentally happened around the same time, allowed determination of the vertical and meridional profiles of the zonal flows on both planets. This paper reviews the topic of zonal jets on the gas giants in light of the new data from these two experiments. The gravity measurements not only allow the depth of the jets to be constrained, yielding the inference that the jets extend roughly 3000 and 9000 km below the observed clouds on Jupiter and Saturn, respectively, but also provide insights into the mechanisms controlling these zonal flows. Specifically, for both planets this depth corresponds to the depth where electrical conductivity is within an order of magnitude of 1 S/m, implying that the magnetic field likely plays a key role in damping the zonal flows.

Read this paper on arXiv…

Y. Kaspi, E. Galanti, A. Showman, et. al.
Tue, 27 Aug 19
74/85

Comments: Submitted to Space Science Reviews. Part of ISSI special collection on Diversity of Atmospheres

Nonlinear Rossby wave-wave and wave-mean flow theory for long term Solar cycle modulations [SSA]

Posted on by

http://arxiv.org/abs/1908.07056


The Schwabe cycle of solar activity exhibits modulations and frequency fluctuations on slow time scales of centuries and millennia. Plausible physical explanations for the cause of these long-term variations of the solar cycle are still elusive, with possible theories including stochasticity of alpha effect and fluctuations of the differential rotation. It has been suggested recently in the literature that there exists a possible relation between the spatio-temporal structure of Solar cycle and the nonlinear dynamics of magnetohydrodynamic Rossby waves at the solar tachocline, including both wave-wave and wave-mean flow interactions. Here we extend the nonlinear theory of MHD Rossby waves presented in a previous article to take into account long term modulation effects due to a recently discovered mechanism that allows significant energy transfers throughout different wave triads: the precession resonance mechanism. We have found a large number of Rossby-Haurwitz wave triads whose frequency mismatches are compatible with the solar cycle frequency. Consequently, by analyzing the reduced dynamics of two triads coupled by a single mode (five-wave system), we have demonstrated that in the amplitude regime in which precession resonance occurs, the energy transfer throughout the system yields significant long-term modulations on the main $\sim 11$yr period associated with intra-triad energy exchanges. We further show that such modulations display an inverse relationship between the characteristic wave amplitude and the period of intra-triad energy exchanges, which is consistent with the Waldmeier’s law for the solar cycle. In the presence of a constant forcing and dissipation, the five-wave system in the precession resonance regime exhibits irregular amplitude fluctuations with some periods resembling the Grand Minimum states.

Read this paper on arXiv…

B. Raphaldini, A. Teruya, C. Raupp, et. al.
Wed, 21 Aug 19
10/78

Comments: N/A

Detection of Propadiene on Titan [EPA]

Posted on by

http://arxiv.org/abs/1908.07424


The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH${2}$CCH${2}$, an isomer of the well-measured propyne (also called methylacetylene) CH$_{3}$CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule – which has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility (IRTF) in July 2017. We model its emission line near 12 $\mu$m and measure a volume mixing ratio (VMR) of (6.9 $\pm$ 0.8) $\times$10$^{-10}$ at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during April 2017 indicate that the abundance ratio of propyne to propadiene is 8.2$\pm$1.1 at the same altitude. This initial measurement of the molecule in Titan’s stratosphere paves the way towards constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 meter and larger ground based observatories, and future detection on other planetary bodies.

Read this paper on arXiv…

N. Lombardo, C. Nixon, T. Greathouse, et. al.
Wed, 21 Aug 19
73/78

Comments: N/A

The Traditional Approximation of Rotation including the centrifugal acceleration for slightly deformed stars [SSA]

Posted on by

http://arxiv.org/abs/1908.06521


The Traditional Approximation of Rotation (TAR) is a treatment of the dynamical equations of rotating stably stratified fluids where the action of the Coriolis acceleration along the direction of the entropy (and chemicals) stratification is neglected while assuming that the fluid motions are mostly horizontal because of their inhibition in the vertical direction by the buoyancy force. This leads to neglect the horizontal projection of the rotation vector in the equations for the dynamics of gravito-inertial waves (GIWs) that become separable as in the non-rotating case while they are not in the case with the full Coriolis acceleration. This approximation has been broadly applied in stellar (and planetary) astrophysics to study low-frequency GIWs. TAR is built on the assumptions that the star is spherical (i.e. its centrifugal deformation is neglected) and uniformly rotating while an adiabatic treatment of the dynamics of the waves is adopted. However, it has been recently generalised with including the effects of a differential rotation. We aim to do a new generalisation that takes into account the centrifugal acceleration in the case of moderately uniformly rotating deformed stars. As in the case of a differentially rotating spherical star, the problem becomes 2D but can be treated analytically if assuming the Cowling, anelastic and JWKB approximations, which are relevant for low-frequency GIWs. It allows us to derive a generalised Laplace tidal equation for the horizontal eigenfunctions and asymptotic wave periods that can be used to probe the structure and dynamics of rotating deformed stars thanks to asteroseismology. A first numerical exploration of its eigenvalues and horizontal eigenfunctions shows their variation as a function of the pseudo-radius for different rotation rates and frequencies and the development of avoided crossings.

Read this paper on arXiv…

S. Mathis and V. Prat
Tue, 20 Aug 19
71/86

Comments: 12 pages, 9 figures, accepted for publication in Astronomy & Astrophysics, abstract shortened for arXiv

The nature of the diffuse light near cities detected in nighttime satellite imagery [IMA]

Posted on by

http://arxiv.org/abs/1908.05482


Diffuse glow has been observed around brightly lit cities in nighttime satellite imagery since at least the first publication of large scale maps in the late 1990s. In the literature, this has often been assumed to be an error related to the sensor, and referred to as “blooming”, presumably in relation to the effect that can occur when using a CCD to photograph a bright source. Here we show that the effect is not instrumental, but in fact represents a real detection of light scattered by the atmosphere. Data from the Universidad Complutense Madrid sky brightness survey are compared to nighttime imagery from multiple sensors with differing spatial resolutions, and found to be strongly correlated. These results suggest that it should be possible for a future space-based imaging radiometer to monitor changes in the diffuse artificial skyglow of cities.

Read this paper on arXiv…

A. Miguel, C. Kyba, J. Zamorano, et. al.
Fri, 16 Aug 19
45/54

Comments: N/A

Simulation of the 2018 Global Dust Storm on Mars Using the NASA Ames Mars GCM: A Multi-Tracer Approach [EPA]

Posted on by

http://arxiv.org/abs/1908.02453


Global dust storms are the most thermodynamically significant dust events on Mars. They are produced from the combination of multiple local and regional lifting events and maintained by positive radiative-dynamic feedbacks. The most recent of these events, which began in June 2018, was monitored by several spacecraft in orbit and on the surface, but many questions remain regarding its onset, expansion and decay. We model the 2018 global dust storm with the NASA Ames Mars Global Climate Model to better understand the evolution of the storm and how the general circulation and finite surface dust reservoirs impact it. The global dust storm is characterized by the rapid eastward transport of dust in the equatorial regions and subsequent lifting. We highlight the rapid transfer of dust between western and eastern hemispheres reservoirs, which may play an important role in the storm development through the replenishment of surface dust. Both the Hadley cell circulation and the diurnal cycle of atmospheric heating increase in intensity with increasing dustiness. Large dust plumes are predicted during the mature stage of the storm, injecting dust up to 80 km. The water ice cloud condensation level migrates to higher altitudes, leading to the enrichment of water vapor in the upper atmosphere. In our simulations, the intensity of the Hadley cell is significantly stronger than that of non-dusty conditions. This feedback is strongly sensitive to the radiative properties of dust, which depends on the effective size of the lifted dust distribution.

Read this paper on arXiv…

T. Bertrand, R. Wilson, M. Kahre, et. al.
Thu, 8 Aug 19
39/78

Comments: 40 pages, 16 figures, submitted to JGR planets

Scaling Relations for Terrestrial Exoplanet Atmospheres from Baroclinic Criticality [EPA]

Posted on by

http://arxiv.org/abs/1908.02661


The macroturbulent atmospheric circulation of Earth-like planets mediates their equator-to-pole heat transport. For fast-rotating terrestrial planets, baroclinic instabilities in the mid-latitudes lead to turbulent eddies that act to transport heat poleward. In this work, we derive a scaling theory for the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanet atmospheres. This theory is built on the work of Jansen & Ferrari (2013), and determines how unstable the atmosphere is to baroclinic instability (the baroclinic “criticality”) through a balance between the baroclinic eddy heat flux and radiative heating/cooling. We compare our scaling theory to General Circulation Model (GCM) simulations and find that the theoretical predictions for equator-to-pole temperature contrast and bulk lapse rate broadly agree with GCM experiments with varying rotation rate and surface pressure throughout the baroclincally unstable regime. Our theoretical results show that baroclinic instabilities are a strong control of heat transport in the atmospheres of Earth-like exoplanets, and our scalings can be used to estimate the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanets. These scalings can be tested by spectroscopic retrievals and full-phase light curves of terrestrial exoplanets with future space telescopes.

Read this paper on arXiv…

T. Komacek, M. Jansen, E. Wolf, et. al.
Thu, 8 Aug 19
41/78

Comments: Accepted at ApJ, 8 pages, 4 figures

Scaling Relations for Terrestrial Exoplanet Atmospheres from Baroclinic Criticality [EPA]

Posted on by

http://arxiv.org/abs/1908.02661


The macroturbulent atmospheric circulation of Earth-like planets mediates their equator-to-pole heat transport. For fast-rotating terrestrial planets, baroclinic instabilities in the mid-latitudes lead to turbulent eddies that act to transport heat poleward. In this work, we derive a scaling theory for the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanet atmospheres. This theory is built on the work of Jansen & Ferrari (2013), and determines how unstable the atmosphere is to baroclinic instability (the baroclinic “criticality”) through a balance between the baroclinic eddy heat flux and radiative heating/cooling. We compare our scaling theory to General Circulation Model (GCM) simulations and find that the theoretical predictions for equator-to-pole temperature contrast and bulk lapse rate broadly agree with GCM experiments with varying rotation rate and surface pressure throughout the baroclincally unstable regime. Our theoretical results show that baroclinic instabilities are a strong control of heat transport in the atmospheres of Earth-like exoplanets, and our scalings can be used to estimate the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanets. These scalings can be tested by spectroscopic retrievals and full-phase light curves of terrestrial exoplanets with future space telescopes.

Read this paper on arXiv…

T. Komacek, M. Jansen, E. Wolf, et. al.
Thu, 8 Aug 19
16/78

Comments: Accepted at ApJ, 8 pages, 4 figures

Simulation of the 2018 Global Dust Storm on Mars Using the NASA Ames Mars GCM: A Multi-Tracer Approach [EPA]

Posted on by

http://arxiv.org/abs/1908.02453


Global dust storms are the most thermodynamically significant dust events on Mars. They are produced from the combination of multiple local and regional lifting events and maintained by positive radiative-dynamic feedbacks. The most recent of these events, which began in June 2018, was monitored by several spacecraft in orbit and on the surface, but many questions remain regarding its onset, expansion and decay. We model the 2018 global dust storm with the NASA Ames Mars Global Climate Model to better understand the evolution of the storm and how the general circulation and finite surface dust reservoirs impact it. The global dust storm is characterized by the rapid eastward transport of dust in the equatorial regions and subsequent lifting. We highlight the rapid transfer of dust between western and eastern hemispheres reservoirs, which may play an important role in the storm development through the replenishment of surface dust. Both the Hadley cell circulation and the diurnal cycle of atmospheric heating increase in intensity with increasing dustiness. Large dust plumes are predicted during the mature stage of the storm, injecting dust up to 80 km. The water ice cloud condensation level migrates to higher altitudes, leading to the enrichment of water vapor in the upper atmosphere. In our simulations, the intensity of the Hadley cell is significantly stronger than that of non-dusty conditions. This feedback is strongly sensitive to the radiative properties of dust, which depends on the effective size of the lifted dust distribution.

Read this paper on arXiv…

T. Bertrand, R. Wilson, M. Kahre, et. al.
Thu, 8 Aug 19
71/78

Comments: 40 pages, 16 figures, submitted to JGR planets

Ethane in Titan's Stratosphere from Cassini CIRS Far- and Mid-Infrared Spectra [EPA]

Posted on by

http://arxiv.org/abs/1908.01926


The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission in the far- and mid-infrared (from 10 cm$^{-1}$ to 1500 cm$^{-1}$), enabling spatiotemporal studies of ethane on Titan across the span of the Cassini mission from 2004 through 2017. Many previous measurements of ethane on Titan have relied on modeling the molecule’s mid-infrared $\nu_{12}$ band, centered on 822 cm$^{-1}$. Other bands of ethane at shorter and longer wavelengths were seen, but have not been modeled to measure ethane abundance. Spectral line lists of the far-infrared $\nu_{4}$ torsional band at 289 cm$^{-1}$ and the mid-infrared $\nu_{8}$ band centered ay 1468 cm$^{-1}$ have recently been studied in the laboratory. We model CIRS observations of each of these bands (along with the $\nu_{12}$ band) separately and compare retrieved mixing ratios from each spectral region. Nadir observations of of the $\nu_{4}$ band probe the low stratosphere below 100 km. Our equatorial measurements at 289 cm$^{-1}$ show an abundance of (1.0$\pm$0.4) $\times$10$^{-5}$ at 88 km, from 2007 to 2017. This mixing ratio is consistent with measurements at higher altitudes, in contrast to the depletion that many photochemical models predict. Measurements from the $\nu_{12}$ and $\nu_{8}$ bands are comparable to each other, with the $\nu_{12}$ band probing an altitude range that extends deeper in the atmosphere. We suggest future studies of planetary atmospheres may observe the $\nu_{8}$ band, enabling shorter wavelength studies of ethane. There may also be an advantage to observing both the ethane $\nu_{8}$ band and nearby methane $\nu_{4}$ band in the same spectral window.

Read this paper on arXiv…

N. Lombardo, C. Nixon, M. Sylvestre, et. al.
Wed, 7 Aug 19
3/61

Comments: 19 pages, 10 figures

Carbon dioxide retrieval of Argus 1000 space data by using GENSPECT line-by-line radiative transfer model [CL]

Posted on by

http://arxiv.org/abs/1908.00628


The micro-spectrometer Argus 1000 being in space continuously monitors the sources and sinks of the trace gases. It is commonly believed that among other gases $\text{CO}\text{2}$ is the major contributor causing the greenhouse effect. Argus 1000 along its orbit gathers the valuable spectral data that can be analyzed and retrieved. In this paper we present the retrieval of $\text{CO}\text{2}$ gas in the near infrared window $1580$ to $1620$ nm by using line-by-line code GENSPECT. The retrieved Argus 1000 space data taken over British Columbia on May 31, 2010 indicates an enhancement of $\text{CO}_\text{2}$ by about $25\text{%}$ – $30\text{%}$.

Read this paper on arXiv…

R. Jagpal, R. Siddiqui, S. Abrarov, et. al.
Mon, 5 Aug 19
50/53

Comments: 16 pages, 7 figures

nsCouette — A high-performance code for direct numerical simulations of turbulent Taylor-Couette flow [CL]

Posted on by

http://arxiv.org/abs/1908.00587


We present nsCouette, a highly scalable software tool to solve the Navier-Stokes equations for incompressible fluid flow between differentially heated and independently rotating, concentric cylinders. It is based on a pseudospectral spatial discretization and dynamic time-stepping. It is implemented in modern Fortran with a hybrid MPI-OpenMP parallelization scheme and thus designed to compute turbulent flows at high Reynolds and Rayleigh numbers. An additional GPU implementation (C-CUDA) for intermediate problem sizes and a basic version for turbulent pipe flow (nsPipe) are also provided.

Read this paper on arXiv…

J. Lopez, D. Feldmann, M. Rampp, et. al.
Mon, 5 Aug 19
53/53

Comments: ~3000 words, 3 figures (in colour)

Near-resonance tidal evolution of the Earth-Moon system influenced by orbital-scale climate change [EPA]

Posted on by

http://arxiv.org/abs/1907.09121


We build a coupled conceptual model of the climate and the tidal evolution of the Earth-Moon system to find the influence of the former on the latter. Energy balance model is applied to calculate steady temperature field from the mean annual insolation as a function of varying astronomical parameters. Harmonic oscillator model is applied to integrate the lunar orbit and the Earth’s rotation with the tidal torque dependent on the dominant natural frequency of ocean. An ocean geometry acts as a bridge between temperature and oceanic frequency. On assumptions of a fixed hemispherical continent and an equatorial circular lunar orbit, considering only the 41 kyr periodicity of the Earth’s obliquity $\varepsilon$ and the $M_2$ tide, simulations are performed near tidal resonance for $10^6$ yr. It is verified that the climate can influence the tidal evolution via ocean. Compared with the tidal evolution with constant $\varepsilon$, that with varying $\varepsilon$ is slowed down; the Earth-Moon distance oscillates in phase with $\varepsilon$ before the resonance maximum but exactly out of phase after that; the displacement of the oscillation is in positive correlation with the difference between oceanic frequency and tidal frequency.

Read this paper on arXiv…

N. Wang and Z. He
Tue, 23 Jul 19
12/72

Comments: 14 pages, 5 figures, accepted for publication in RAA

Martian Year 34 Column Dust Climatology from Mars Climate Sounder Observations: Reconstructed Maps and Model Simulations [EPA]

Posted on by

http://arxiv.org/abs/1907.08187


We have reconstructed longitude-latitude maps of column dust optical depth (CDOD) for Martian year (MY) 34 (May 5, 2017 — March 23, 2019) using observations by the Mars Climate Sounder (MCS) aboard NASA’s Mars Reconnaissance Orbiter spacecraft. Our methodology works by gridding standard and newly available estimates of CDOD from MCS limb observations, using the “iterative weighted binning” methodology. In this work, we reconstruct four gridded CDOD maps per sol, at different Mars Universal Times. Together with the seasonal and day-to-day variability, the use of several maps per sol allows to explore also the daily variability of CDOD in the MCS dataset, which is shown to be particularly strong during the MY 34 equinoctial Global Dust Event (GDE). Regular maps of CDOD are then produced by daily averaging and spatially interpolating the irregularly gridded maps using a standard “kriging” interpolator, and can be used as “dust scenario” for numerical model simulations. In order to understand whether the daily variability of CDOD has a physical explanation, we have carried out numerical simulations with the “Laboratoire de M\’et\’eorologie Dynamique” Mars Global Climate Model. Using a “free dust” run initiated at $L_s \sim 210^\circ$ with the corresponding kriged map, but subsequently free of further CDOD forcing, we show that the model is able to account for some of the observed daily variability in CDOD. The model serves also to confirm that the use of the MY 34 daily-averaged dust scenario in a GCM produces results consistent with those obtained for the MY 25 GDE.

Read this paper on arXiv…

L. Montabone, A. Spiga, D. Kass, et. al.
Fri, 19 Jul 19
16/78

Comments: 30 pages, 20 figures, submitted to JGR Planets

Atmospheric Electricity at the Ice Giants [CL]

Posted on by

http://arxiv.org/abs/1907.07151


Lightning was detected by Voyager 2 at Uranus and Neptune, and weaker electrical processes also occur throughout planetary atmospheres from galactic cosmic ray (GCR) ionisation. Lightning is an indicator of convection, whereas electrical processes away from storms modulate cloud formation and chemistry, particularly if there is little insolation to drive other mechanisms. The ice giants appear to be unique in the Solar System in that they are distant enough from the Sun for GCR-related mechanisms to be significant for clouds and climate, yet also convective enough for lightning to occur. This paper reviews observations (both from Voyager 2 and Earth), data analysis and modelling, and considers options for future missions. Radio, energetic particle and magnetic instruments are recommended for future orbiters, and Huygens-like atmospheric electricity sensors for in situ observations. Uranian lightning is also expected to be detectable from terrestrial radio telescopes.

Read this paper on arXiv…

K. Aplin, G. Fischer, T. Nordheim, et. al.
Wed, 17 Jul 19
22/75

Comments: Submitted to Space Science Reviews

Buoyancy-Driven Entrainment in Dry Thermals [CL]

Posted on by

http://arxiv.org/abs/1906.07224


Turner (1957) proposed that dry thermals entrain because of buoyancy (via a constraint which requires an increase in the radius $a$). This however, runs counter to the scaling arguments commonly used to derive the entrainment rate, which rely on either the self-similarity of Scorer (1957) or the turbulent entrainment hypothesis of Morton et al (1956). The assumption of turbulence-driven entrainment was investigated by Lecoanet and Jeevanjee (2018), who found that the entrainment efficiency $e$ varies by less than $20\%$ between laminar (Re = 630) and turbulent (Re = 6300) thermals. This motivated us to utilize Turner’s argument of buoyancy-controlled entrainment in addition to the thermal’s vertical momentum equation to build a model for thermal dynamics which does not invoke turbulence or self-similarity. We derive simple expressions for the thermals’ kinematic properties and their fractional entrainment rate $\epsilon$ and find close quantitative agreement with the values in direct numerical simulations. We then directly validate the role of buoyancy-driven entrainment by running simulations where gravity is turned off midway through a thermal’s rise. The entrainment efficiency $e$ is observed to drop to less than 1/3 of its original value in both the laminar and turbulent cases when $g=0$, affirming the central role of buoyancy in entrainment in dry thermals.

Read this paper on arXiv…

B. McKim, N. Jeevanjee and D. Lecoanet
Wed, 19 Jun 19
24/60

Comments: 10 pages, 7 figures

Investigating the Semiannual Oscillation on Mars using data assimilation [EPA]

Posted on by

http://arxiv.org/abs/1906.06943


A Martian semiannual oscillation (SAO), similar to that in the Earth’s tropical stratosphere, is evident in the Mars Analysis Correction Data Assimilation reanalysis dataset (MACDA) version 1.0, not only in the tropics, but also extending to higher latitudes. Unlike on Earth, the Martian SAO is found not always to reverse its zonal wind direction, but only manifests itself as a deceleration of the dominant wind at certain pressure levels and latitudes. Singular System Analysis (SSA) is further applied on the zonal-mean zonal wind in different latitude bands to reveal the characteristics of SAO phenomena at different latitudes. The second pair of principal components (PCs) is usually dominated by a SAO signal, though the SAO signal can be strong enough to manifest itself also in the first pair of PCs. An analysis of terms in the Transformed Eulerian Mean equation (TEM) is applied in the tropics to further elucidate the forcing processes driving the tendency of the zonal-mean zonal wind. The zonal-mean meridional advection is found to correlate strongly with the observed oscillations of zonal-mean zonal wind, and supplies the majority of the westward (retrograde) forcing in the SAO cycle. The forcing due to various non-zonal waves supplies forcing to the zonal-mean zonal wind that is nearly the opposite of the forcing due to meridional advection above ~3 Pa altitude, but it also partly supports the SAO between 40 Pa and 3 Pa. Some distinctive features occurring during the period of the Mars year (MY) 25 global-scale dust storm (GDS) are also notable in our diagnostic results with substantially stronger values of eastward and westward momentum in the second half of MY 25 and stronger forcing due to vertical advection, transient waves and thermal tides.

Read this paper on arXiv…

T. Ruan, N. Lewis, S. Lewis, et. al.
Tue, 18 Jun 19
7/73

Comments: Accepted for publication in Icarus. 20 pages, 5 figures

Entropy Rain: Dilution and Compression of Thermals in Stratified Domains [SSA]

Posted on by

http://arxiv.org/abs/1906.02342


Large-scale convective flows called giant cells were once thought to transport the Sun’s luminosity in the solar convection zone, but recent observations have called their existence into question. In place of large-scale flows, some authors have suggested the solar luminosity may instead be transported by small droplets of rapidly falling, low entropy fluid. This “entropy rain” could propagate as dense vortex rings, analogous to rising buoyant thermals in the Earth’s atmosphere. In this work, we develop an analytical theory describing the evolution of dense, negatively buoyant thermals. We verify the theory with 2D and 3D simulations of laminar, axisymmetric thermals in highly stratified atmospheres. Our results show that dense thermals fall in two categories: a stalling regime in which the droplets slow down and expand, and a falling regime in which the droplets accelerate and shrink as they propagate downwards. We estimate that solar downflows are in the falling regime and maintain their entropy perturbation against diffusion until they reach the base of the convection zone. This suggests that entropy rain may be an effective nonlocal mechanism for transporting the solar luminosity.

Read this paper on arXiv…

E. Anders, D. Lecoanet and B. Brown
Fri, 7 Jun 19
11/49

Comments: Submitted to ApJ. 11 pages, including 6 figures, 2 tables, and 3 appendices

Introduction of water-vapor broadening coefficients and their temperature dependence exponents into the HITRAN database, Part I: CO2, N2O, CO, CH4, O2, NH3, and H2S [CL]

Posted on by

http://arxiv.org/abs/1906.01475


The amount of water vapor in the terrestrial atmosphere is highly variable both spatially and temporally. In the tropics it sometimes constitutes 4-5% of the atmosphere. At the same time collisional broadening of spectral lines by water vapor is much larger than that by nitrogen and oxygen. Therefore, in order to accurately characterize and model spectra of the atmospheres with significant amounts of water vapor, the line-shape parameters for spectral lines broadened by water vapor are required. In this work, the line-broadening coefficients (and their temperature dependence exponents) due to the pressure of water vapor for lines of CO2, N2O, CO, CH4, O2, NH3, and H2S from both experimental and theoretical studies were collected and carefully reviewed. A set of semi-empirical models based on these collected data was created and then used to estimate water broadening and its temperature dependence for all transitions of selected molecules in the HITRAN2016 database.

Read this paper on arXiv…

Y. Tan, R. Kochanov, L. Rothman, et. al.
Wed, 5 Jun 19
35/74

Comments: Submitted to the Journal of Geophysical Research: Atmospheres on May 23, 2019. 3 Tables, 11 Figures

A challenge for Martian lightning: Limits of collisional charging at low pressure [EPA]

Posted on by

http://arxiv.org/abs/1905.11138


Collisional charging is one potential initial step in generating lightning. In this work, we study the charging of colliding monodisperse, spherical basalt grains depending on ambient pressure. We used grains of 1.0 to 1.2 mm in one set and 2.0 to 2.4 mm in another set. We varied the ambient pressure between 0.03 mbar and 80 mbar. This especially includes Martian pressure being 6 mbar on average. At a few mbar the net charge gathering on colliding grains has a minimum. A smooth incline in charging occurs for larger pressures. Toward lower pressure the charge increases steeply. The pressure dependence is in agreement to a model where the maximum charge is limited by a gas discharge occurring between two charged colliding grains shortly after or before a collision. The capability of building up charge is at a minimum exactly in the range of Martian pressures. The charges on grains are at least a factor 5 smaller than at the highest pressure tested and still smaller compared to ambient pressure on Earth. This implies that on Mars collisional charging and the potential of subsequent generation of lightning or other large scale discharges are strongly reduced compared to Earth. This might result in less frequent and less energetic lightning on Mars.

Read this paper on arXiv…

G. Wurm, L. Schmidt, T. Steinpilz, et. al.
Tue, 28 May 19
2/82

Comments: N/A

Hazes and clouds in a singular triple vortex in Saturn's atmosphere from HST/WFC3 multispectral imaging [EPA]

Posted on by

http://arxiv.org/abs/1905.11301


In this paper we present a study of the vertical haze and cloud structure over a triple vortex in Saturn’s atmosphere in the planetographic latitude range 55N-69N (del Rio- Gaztelurrutia et al. , 2018) using HST/WFC3 multispectral imaging. The observations were taken during 29-30 June and 1 July 2015 at ten different filters covering spectral range from the 225 nm to 937 nm, including the deep methane band at 889 nm. Absolute reflectivity measurements of this region at all wavelengths and under a number of illumination and observation geometries are fitted with the values produced by a radiative transfer model. Most of the reflectivity variations in this wavelength range can be attributed to changes in the tropospheric haze. The anticyclones are optically thicker ($\tau \sim$ 25 vs $\sim$ 10), more vertically extended ($\sim$ 3 gas scale heights vs $\sim$ 2) and their bases are located deeper in the atmosphere (550 mbar vs 500 mbar) than the cyclone.

Read this paper on arXiv…

J. Sanz-Requena, S. Perez-Hoyos, A. Sanchez-Lavega, et. al.
Tue, 28 May 19
55/82

Comments: 36 pages, 16 figures

Enhanced Habitability on High Obliquity Bodies near the Outer Edge of the Habitable Zone of Sun-like Stars [EPA]

Posted on by

http://arxiv.org/abs/1905.09398


High obliquity planets represent potentially extreme limits of terrestrial climate, as they exhibit large seasonality, a reversed annual-mean pole-to-equator gradient of stellar heating, and novel cryospheres. A suite of 3-D global climate model simulations with a dynamic ocean is performed with Earthlike atmospheres for low and high obliquity planets with various stellar fluxes, CO2 concentrations, and initial conditions to explore the propensity for high obliquity climates approaching the outer edge of the Habitable Zone to undergo global glaciation. We also simulate planets with thick CO2 or H2 atmospheres, such as those expected to develop near or beyond the outer edge of the Habitable Zone.
We show that high obliquity planets are hotter than their low obliquity counterparts due to ice-albedo feedbacks for cold climates, and water vapor in warm climates. We suggest that the water vapor greenhouse trapping is greater on high obliquity bodies due to the different dynamical regimes that occur between the two states.
While equatorial ice-belts are stable at high obliquity in some climate regimes, it is harder to achieve global glaciation than for a low obliquity planet. Temperate polar conditions can be present at high obliquity at forcings for which low obliquity planets would be in a hard snowball state. We suggest the conditions on high obliquity planets are likely to be more favorable for a robust biosphere to develop approaching the outer edge of the HZ. However, the influence of obliquity diminishes for dense atmospheres, in agreement with calculations from 1-D Energy Balance Models.

Read this paper on arXiv…

C. Colose, A. Genio and M. Way
Fri, 24 May 19
5/60

Comments: 27 pages, 12 Figures

New cloud morphologies discovered on the Venus's night during Akatsuki [EPA]

Posted on by

http://arxiv.org/abs/1905.08913


During the years 2016 to 2018, the instruments Akatsuki/IR2 (JAXA) and IRTF/SpeX (NASA) acquired a large set of images at 1.74, 2.26 and 2.32 {\mu}m to study the nightside mid-to-lower clouds (48-60 km) of Venus. Here we summarize the rich variety of cloud morphologies apparent in these images: from frequent wave packets and billows caused by shear instabilities, to features reported decades ago like the circum-equatorial belts, bright blotches and equatorial troughs, and previously unseen features like dark spots, sharp dark streaks at mid latitudes and fully-developed vortices.

Read this paper on arXiv…

J. Peralta, A. Sánchez-Lavega, T. Horinouchi, et. al.
Thu, 23 May 19
45/67

Comments: 17 pages, 2 figures, 1 table

Dust particle size and optical depth on Mars retrieved by the MSL Navigation Cameras [EPA]

Posted on by

http://arxiv.org/abs/1905.01073


In this paper we show that Sun-viewing images obtained by the Mars Science Laboratory (MSL) Navigation Cameras (Navcam) can be used for retrieving the dust optical depth and constrain the aerosol physical properties at Gale Crater by evaluating the sky brightness as a function of the scattering angle. We have used 65 Sun-pointing images covering a period of almost three Martian years, from MSL mission sol 21 to sol 1646 (MY 31 to 33). Radiometric calibration and geometric reduction were performed on MSL Navcam raw image data records to provide the observed sky radiance as a function of the scattering angle for the near-Sun region (scattering angle from 4{\deg} to 30{\deg}). These curves were fitted with a multiple scattering radiative transfer model for a plane-parallel Martian atmosphere model using the discrete ordinates method. Modelled sky brightness curves were generated as a function of two parameters: the aerosol particle size distribution effective radius and the dust column optical depth at the surface. A retrieval scheme was implemented for deriving the parameters that generated the best fitting curve under a least-square error criterion. The obtained results present a good agreement with previous work, showing the seasonal dependence of both dust column optical depth and the effectiveparticle radius.

Read this paper on arXiv…

H. Chen-Chen, S. Perez-Hoyos and A. Sanchez-Lavega
Mon, 6 May 19
36/58

Comments: N/A

Why do some probabilistic forecasts lack reliability? [CL]

Posted on by

http://arxiv.org/abs/1904.08791


In this work, we investigate the reliability of the probabilistic binary forecast. We mathematically prove that a necessary, but not sufficient, condition for achieving a reliable probabilistic forecast is maximizing the Peirce skill score (PSS) at the threshold probability of the climatological base rate. The condition is confirmed by using artificially synthesized forecast-outcome pair data and previously published probabilistic solar flare forecast models. The condition gives a partial answer as to why some probabilistic forecast system lack reliability, because the system, which does not satisfy the proved condition, can never be reliable. Therefore, the proved condition is very important for the developers of a probabilistic forecast system. The result implies that those who want to develop a reliable probabilistic forecast system must adjust or train the system so as to maximize PSS near the threshold probability of the climatological base rate.

Read this paper on arXiv…

Y. Kubo
Mon, 22 Apr 19
2/36

Comments: 12 pages, 6 figures, 1 table, accepted for publication in the Journal of Space Weather and Space Climate (JSWSC)

Martian cloud climatology and life cycle extracted from Mars Express OMEGA spectral images [EPA]

Posted on by

http://arxiv.org/abs/1904.06422


A Martian water-ice cloud climatology has been extracted from OMEGA data covering 7 Martian years (MY 26-32). We derived two products, the Reversed Ice Cloud Index (ICIR) and the Percentage of Cloudy Pixels (PCP), indicating the mean cloud thickness and nebulosity over a regular grid (1{\deg} longitude x 1{\deg} latitude x 1{\deg} Ls x 1 h Local Time) adapted for comparisons with data from high-resolution Martian Global Climate Models. The ICIR has been shown to be a proxy of the water-ice column. The PCP confirms the existence and location of the main cloud structures mapped with the ICIR, but also gives a more accurate image of the cloud cover. We observed a denser cloud coverage over Hellas Planitia, the Lunae Planum region and over large volcanoes (Tharsis volcanoes, Olympus and Elysium Montes) in the aphelion belt. For the first time, thanks to the fact that Mars Express is not in Sun-synchronous orbit, we can explore the clouds diurnal cycle at a given season by combining seven years of observations. However, because of the excentric orbit, the temporal coverage remains limited. To study the diurnal cloud life cycle, we averaged the data over larger regions: from specific topographic features (covering a few degrees in longitude and latitude) up to large climatic bands (covering all longitudes). We found that in the tropics (25{\deg}S-25{\deg}N) around northern summer solstice, the diurnal thermal tide modulates the abundance of clouds, which is reduced around noon (LT). At northern midlatitudes (35{\deg}N-55{\deg}N), clouds corresponding to the edge of the north polar hood are observed mainly in the morning and around noon during northern winter (Ls=260-30{\deg}). Over Chryse Planitia, low lying morning fogs dissipate earlier and earlier in the afternoon during northern winter. Over Argyre, clouds are present over all daytime during two periods, around Ls = 30 and 160{\deg}.

Read this paper on arXiv…

A. Szantai, J. Audouard, F. Forget, et. al.
Tue, 16 Apr 19
57/88

Comments: 39 pages, 18 figures, 2 tables. Article submitted to Icarus

The paradoxes of the Late Hesperian Mars ocean [EPA]

Posted on by

http://arxiv.org/abs/1904.03919


The long-standing debate on the existence of ancient oceans on Mars has been recently revived by evidence for tsunami resurfacing events that date from the Late Hesperian geological era. It has been argued that these tsunami events originated from the impact of large meteorites on a deglaciated or nearly deglaciated ocean present in the northern hemisphere of Mars. Here we show that the presence of such a late ocean faces a paradox. If cold, the ocean should have been entirely frozen shortly after its formation, thus preventing the formation of tsunami events. If warm, the ice-free ocean should have produced fluvial erosion of Hesperian Mars terrains much more extensively than previously reported. To solve this apparent paradox, we suggest a list of possible tests and scenarios that could help to reconcile constraints from climate models with tsunami hypothesis. These scenarios could be tested in future dedicated studies.

Read this paper on arXiv…

M. Turbet and F. Forget
Tue, 9 Apr 19
74/105

Comments: Nature Scientific Reports, in press

Air-Sea Interactions on Titan: Lake Evaporation, Atmospheric Circulation, and Cloud Formation [EPA]

Posted on by

http://arxiv.org/abs/1904.00120


Titan’s abundant lakes and seas exchange methane vapor and energy with the atmosphere via a process generally known as air-sea interaction. This turbulent exchange process is investigated with an atmospheric mesoscale model coupled to a slab model representation of an underlying lake. The impact of lake size, effective lake mixed layer depth, background wind speed, air-lake temperature differential, atmospheric humidity, and diabatic heating of the atmosphere on air-sea interaction processes is studied through 67 two-dimensional simulations. The general, quasi-steady solution is a non-linear superposition of a plume circulation driven by the buoyancy of evaporated methane and an opposing thermally direct (sea breeze) circulation driven by the thermal contrast between the cold marine layer over the lake and the warmer inland air. The specific solution depends on the value of selected atmosphere and lake property parameters and ranges from a persistent and strong methane-rich plume circulation over the lake with little to no sea breeze, or a rapidly developing sea breeze with a highly suppressed plume circulation. The solutions that appear most consistent with limited observational constraints are those where a sea breeze circulation is able to offset the opposing plume circulation. This scenario results in a cool, moist, and statically stable shallow marine layer with nearly calm winds and small turbulent flux exchanges with an underlying lake that is at least 2 K colder than the atmosphere. Other configurations produced extreme scenarios with strong surface winds that could trigger waves, supersaturated layers at the top of the plume circulation that would be conducive to cloud formation, and lakes cold enough to freeze. These extreme scenarios are unlikely to be realistic based on limited observational constraints.

Read this paper on arXiv…

S. Rafkin and A. Soto
Tue, 2 Apr 19
5/90

Comments: In review at Icarus

An axisymmetric limit for the width of the Hadley cell on planets with large obliquity and long seasonality [EPA]

Posted on by

http://arxiv.org/abs/1903.11656


Hadley cells dominate the meridional circulation of terrestrial atmospheres. The Solar System terrestrial atmospheres, Venus, Earth, Mars and Titan, exhibit a large variety in the strength, width and seasonality of their Hadley circulation. Despite the Hadley cell being thermally driven, in all planets, the ascending branch does not coincide with the warmest latitude, even in cases with very long seasonality (e.g., Titan) or very small thermal inertia (e.g., Mars). In order to understand the characteristics of the Hadley circulation in case of extreme planetary characteristics, we show both theoretically, using axisymmetric theory, and numerically, using a set of idealized GCM simulations, that the thermal Rossby number dictates the character of the circulation. Given the possible variation of thermal Rossby number parameters, the rotation rate is found to be the most critical factor controlling the circulation characteristics. The results also explain the location of the ascending branch on Mars and Titan.

Read this paper on arXiv…

I. Guendelman and Y. Kaspi
Fri, 29 Mar 19
63/78

Comments: N/A

The effects of crustal magnetic fields and solar EUV flux on ionopause formation at Mars [CL]

Posted on by

http://arxiv.org/abs/1903.10473


We study the ionopause of Mars using a database of 6,893 ionopause detections obtained over 11 years by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounder. The ionopause, in this work, is defined as a steep density gradient that appears in MARSIS remote sounding ionograms as a horizontal line at frequencies below 0.4 MHz. We find that the ionopause is located on average at an altitude of $363 \pm 65$ km. We also find that the ionopause altitude has a weak dependence on solar zenith angle and varies with the solar extreme ultraviolet flux on annual and solar cycle time scales. Furthermore, our results show that less than $10\%$ of the ionopauses are observed when the crustal field strength at 400 km is greater than 20 nT. The strong crustal fields act as mini-magnetospheres that alter the solar wind interaction and prevent the ionopause from forming.

Read this paper on arXiv…

F. Chu, Z. Girazian, D. Gurnett, et. al.
Tue, 26 Mar 19
63/72

Comments: N/A

The CO2-broadened H2O continuum in the 100-1500 cm-1 region. Measurements, predictions and empirical model [EPA]

Posted on by

http://arxiv.org/abs/1903.08972


Transmission spectra of H$_2$O+CO$_2$ mixtures have been recorded, at 296, 325 and 366 K, for various pressures and mixture compositions using two experimental setups. Their analysis enables to retrieve values of the ‘continuum’ absorption by the CO$_2$-broadened H$_2$O line wings between 100 and 1500 cm$^{-1}$. The results are in good agreement with those, around 1300 cm$^{-1}$, of the single previous experimental study available. Comparisons are also made with direct predictions based on line-shape correction factors $\chi$ calculated, almost thirty years ago, using a quasistatic approach and an input H$_2$O-CO$_2$ intermolecular potential. They show that this model quite nicely predicts, with slightly overestimated values, the continuum over a spectral range where it varies by more than three orders of magnitude. An empirical correction is proposed, based on the experimental data, which should be useful for radiative transfer and climate studies in CO$_2$ rich planetary atmospheres.

Read this paper on arXiv…

H. Tran, M. Turbet, S. Hanoufa, et. al.
Fri, 22 Mar 19
3/70

Comments: Accepted for publication in the Journal of Quantitative Spectroscopy & Radiative Transfer (JQSRT); 11 pages and 3 figures

Global Stability Properties of the Climate: Melancholia States, Invariant Measures, and Phase Transitions [CL]

Posted on by

http://arxiv.org/abs/1903.08348


For a wide range of values of the incoming solar radiation, the Earth features at least two attracting states, which correspond to competing climates. The warm climate is analogous to the present one; the snowball climate features global glaciation and conditions that can hardly support life forms. Paleoclimatic evidences suggest that in past our planet flipped between these two states. The main physical mechanism responsible for such instability is the ice-albedo feedback. In a previous work, we defined the Melancholia states that sit between the two climates. Such states are embedded in the boundaries between the two basins of attraction and feature extensive glaciation down to relatively low latitudes. Here, we explore the global stability properties of the system by introducing random perturbations as modulations to the intensity of the incoming solar radiation. We observe noise-induced transitions between the competing basins of attractions. In the weak noise limit, large deviation laws define the invariant measure and the statistics of escape times. By empirically constructing the instantons, we show that the Melancholia states are the gateways for the noise-induced transitions. In the region of multistability, in the zero-noise limit, the measure is supported only on one of the competing attractors. For low (high) values of the solar irradiance, the limit measure is the snowball (warm) climate. The changeover between the two regimes corresponds to a first order phase transition in the system. The framework we propose seems of general relevance for the study of complex multistable systems. At this regard, we relate our results to the debate around the prominence of contigency vs. convergence in biological evolution. Finally, we propose a new method for constructing Melancholia states from direct numerical simulations, thus bypassing the need to use the edge-tracking algorithm.

Read this paper on arXiv…

V. Lucarini and T. Bodai
Thu, 21 Mar 19
36/66

Comments: 37 Pages, 10 Figures

Lightning and charge processes in brown dwarf and exoplanet atmospheres [EPA]

Posted on by

http://arxiv.org/abs/1903.04565


The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and evolutionary states. We use results from 3D simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionisation processes which will affect their atmosphere chemistry: The dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H$_2$O. Such planetary atmospheres exhibit a substantial degree of thermal ionisation and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionised, H$_3^+$-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather-system driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H$_3^+$ in the upper atmosphere of a brown dwarf (e.g. LSR-J1835) which may react with it to form hydronium, H$_3$O$^+$, as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H$_3$O$^+$ as an H$_3^+$ product.

Read this paper on arXiv…

C. Helling and P. Rimmer
Wed, 13 Mar 19
76/125

Comments: 16 pages, accepted for publication in the Philosophical Transactions A of the Royal Society

Morphology and dynamics of Venus's middle clouds with Akatsuki/IR1 [EPA]

Posted on by

http://arxiv.org/abs/1903.02883


The Venusian atmosphere is covered by clouds with super-rotating winds whose accelerating mechanism is still not well understood. The fastest winds, occurring at the cloud tops ($\sim$70 km height), have been studied for decades thanks to their visual contrast in dayside ultraviolet images. The middle clouds ($\sim$50-55 km) can be observed at near-infrared wavelengths (800-950 nm), although with very low contrast. Here we present the first extensive analysis of their morphology and motions at lower latitudes along 2016 with 900-nm images from the IR1 camera onboard Akatsuki. The middle clouds exhibit hemispherical asymmetries every 4-5 days, sharp discontinuities in elongated “hook-like” stripes, and large contrasts (3-21%) probably associated with large changes in the optical thickness. Zonal winds obtained with IR1 images and with ground-based observations reveal mean zonal winds peaking at the equator, while their combination with Venus Express unveils long-term variations of 20 m s$^{-1}$ along 10 years.

Read this paper on arXiv…

J. Peralta, N. Iwagami, S. Sánchez-Lavega, et. al.
Fri, 8 Mar 19
1/56

Comments: 18 pages, 4 figures

From cosmic explosions to terrestrial fires? [EPA]

Posted on by

http://arxiv.org/abs/1903.01501


Multiple lines of evidence point to one or more moderately nearby supernovae, with the strongest signal ~2.6 Ma. We build on previous work to argue for the likelihood of cosmic ray ionization of the atmosphere and electron cascades leading to more frequent lightning, and therefore an increase in nitrate deposition and in wildfires. The potential exists for a large increase in the pre-human nitrate flux onto the surface, which has previously been argued to lead to CO2 drawdown and cooling of the climate. Evidence for increased wildfires exists in an increase in soot and carbon deposits over the relevant period. The wildfires would have contributed to the transition from forest to savanna in northeast Africa, long argued to have been a factor in the evolution of hominin bipedalism.

Read this paper on arXiv…

A. Melott and B. Thomas
Wed, 6 Mar 19
64/75

Comments: 20 pages, 3 figures. To be published in the Journal of Geology

Analytical Estimation of the Width of Hadley Cells in the Solar System [EPA]

Posted on by

http://arxiv.org/abs/1903.00393


The angular width of Earth’s Hadley cell has been related to the square root of the product of the tropospheric thickness and the buoyancy frequency, and to the inverse of the square root of the angular velocity and the planetary radius. Here, this formulation is examined for other planetary bodies in the solar system. Generally good consistency is found between predictions and observations for terrestrial planets provided the pressure scale height rather than the tropopause height is assumed to determine the thickness of the tropospheric circulation. For gas giants, the relevant thickness is deeper than the scale height, possibly due to the internal heat produced by Kelvin-Helmholtz contraction. On Earth, latent heat release within deep convection may play a similar role in deepening and widening the Hadley Cell.

Read this paper on arXiv…

K. Rees and T. Garrett
Mon, 4 Mar 19
41/48

Comments: N/A

Optical turbulence forecast in the Adaptive Optics realm [IMA]

Posted on by

http://arxiv.org/abs/1902.07989


(35-words maximum) In this talk I present the scientific drivers related to the optical turbulence forecast applied to the ground-based astronomy supported by Adaptive Optics, the state of the art of the achieved results and the most relevant challenges for future progresses.

Read this paper on arXiv…

E. Masciadri, A. Turchi and L. Fini
Fri, 22 Feb 19
33/52

Comments: 1 figure, Orlando, Florida United States, 25 – 28 June 2018, ISBN: 978-1-943580-44-6,Turbulence & Propagation, JW5I.1 Adaptive Optics: Analysis, Methods and Systems

The environmental effects of very large bolide impacts on early Mars explored with a hierarchy of numerical models [EPA]

Posted on by

http://arxiv.org/abs/1902.07666


We use a hierarchy of numerical models (a 3-D Global Climate Model, a 1-D radiative-convective model and a 2-D Mantle Dynamics model) to explore the environmental effects of very large impacts on the atmosphere, surface and interior of early Mars.
Using a combination of 1-D and 3-D climate simulations, we show that the environmental effects of the largest impact events recorded on Mars are characterized by: (i) a short impact-induced warm period; (ii) a low amount of hydrological cycling of water; (iii) deluge-style precipitation; and (iv) precipitation patterns that are uncorrelated with the observed regions of valley networks. We show that the impact-induced stable runaway greenhouse state predicted by Segura et al. 2012 is physically inconsistent. We confirm the results of Segura et al. 2008 and Urata & Toon 2013 that water ice clouds can significantly extend the duration of the post-impact warm period, and even for cloud coverage lower than predicted in Ramirez & Kasting 2017. However, the range of cloud microphysical properties for which this scenario works is very narrow.
Using 2-D Mantle Dynamics simulations we find that large impacts can raise the near-surface internal heat flux up to several hundreds of mW/m$^2$ (i.e. up to $\sim$ 10 times the ambient flux) for several millions years at the edges of the impact crater. However, such internal heat flux is insufficient to keep the martian surface above the melting point of water.
Our numerical results support the prediction of Palumbo & Head 2018 that very large impact-induced rainfall could have caused degradation of craters and formed smooth plains, potentially erasing much of the previously visible morphological surface history. Such hot rainfalls may have also led to the formation of aqueous alteration products on Noachian-aged terrains.

Read this paper on arXiv…

M. Turbet, C. Gillmann, F. Forget, et. al.
Thu, 21 Feb 19
23/54

Comments: Submitted to Icarus. Abstract significantly abridged to meet ArXiv size limit

Tropical and Extratropical General Circulation with a Meridional Reversed Temperature Gradient as Expected in a High Obliquity Planet [EPA]

Posted on by

http://arxiv.org/abs/1902.06832


Planets with high obliquity receive more radiation in the polar regions than at low latitudes, and thus, assuming an ocean-covered surface with sufficiently high heat capacity, their meridional temperature gradient was shown to be reversed for the entire year. The objective of this work is to investigate the drastically different general circulation of such planets, with an emphasis on the tropical Hadley circulation and the mid-latitude baroclinic eddy structure. We use a 3D dry dynamic core model, accompanied by an eddy-free configuration and a generalized 2D Eady model. When the meridional temperature gradient $T_y$ is reversed, the Hadley cell remains in the same direction, because the surface wind pattern and hence the associated meridional Ekman transport are not changed, as required by the baroclinic eddy momentum transport. The Hadley cell under reversed $T_y$ also becomes much shallower and weaker, even when the magnitude of the gradient is the same as in the normal case. The shallowness is due to the bottom-heavy structure of the baroclinic eddies in the reverse case, and the weakness is due to the weak wave activity. We propose a new mechanism to explain the mid-latitude eddy structure for both cases, and verify it using the generalized Eady model. With seasonal variations included, the annual mean circulation resembles that under perpetual annual mean setup. Approaching the solstices, a strong cross-equator Hadley cell forms in both cases, and about 2/3 of the Hadley circulation is driven by eddies, as shown by eddy-free simulations and using a decomposition of the Hadley cell.

Read this paper on arXiv…

W. Kang, M. Cai and E. Tziperman
Wed, 20 Feb 19
38/42

Comments: N/A

A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients [EPA]

Posted on by

http://arxiv.org/abs/1902.05574


HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50 to 400 K. We present 15 new reaction rate coefficients with no previously known value; 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to be inefficient and suggest that the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.

Read this paper on arXiv…

B. Pearce, P. Ayers and R. Pudritz
Mon, 18 Feb 19
20/37

Comments: 34 pages, 8 figures, 14 tables, accepted for publication in J Phys Chem A

A theoretical model for realistic local climates [CL]

Posted on by

http://arxiv.org/abs/1902.05598


We address the question of writing a model that —considering the doubly-periodic forcing induced by solar radiation and the laws of irradiance and conduction— reproduces realistic annual and daily modulations of temperatures on the surface of a small region on Earth. The interest in such exercise on theoretical and numeric integration is twofold: the application of this model can give realistic projections of local climate evolution on Earth or on a chosen exoplanet; the creation of such model can shed some light on the effective exchanges of energy among the many actors of this system (atmosphere, water, soil,…). The presence of at least two different thermal reservoirs has as evident consequence the well known phenomenon of lag of seasons, and the less discussed lag of noons (delay in daily temperatures evolution with respect to daily solar radiation).
In this article, after describing the motivating phenomena, we develop a physical model, we apply it to many types of climatic zones on Earth, and we compare the results with real temperature data. We finally make some theoretical application to orbits with non zero eccentricity, more in line with actual extra-solar planetary systems.

Read this paper on arXiv…

G. Bona and A. Giacobbe
Mon, 18 Feb 19
27/37

Comments: 19 pages, 12 figures

Magnetic eddy viscosity of mean sheared flows in two-dimensional magnetohydrodynamics [CL]

Posted on by

http://arxiv.org/abs/1902.01105


Induction of magnetohydrodynamics (MHD) fluids at magnetic Reynolds number (Rm) less than~1 has long been known to cause magnetic drag. Here, we show that when $\Rm \gg 1$, and additionally in a hydrodynamic-dominated regime in which the magnetic energy is much smaller than the kinetic energy, induction due to a mean shared flow leads to a magnetic eddy viscosity. The magnetic viscosity is derived from simple physical arguments, where a coherent response due to shear flow builds up in the magnetic field until decorrelated by turbulent motion. The dynamic viscosity coefficient is approximately $B_p^2/(2\m_0) \tc$, the poloidal magnetic energy density multiplied by the correlation time. We confirm the magnetic eddy viscosity through numerical simulations of two-dimensional incompressible MHD. We also consider the three-dimensional case, and in cylindrical or spherical geometry we find a nonzero viscosity whenever there is differential rotation. These results thus serve as a dynamical generalization of Ferraro’s law of isorotation. The magnetic eddy viscosity leads to transport of angular momentum and may be of importance to zonal flows in the interior of gas giants.

Read this paper on arXiv…

J. Parker and N. Constantinou
Tue, 5 Feb 19
13/86

Comments: 14 pages, 8 figures

The Effects of Gravity on the Climate and Circulation of a Terrestrial Planet [EPA]

Posted on by

http://arxiv.org/abs/1901.11426


The climate and circulation of a terrestrial planet are governed by, among other things, the distance to its host star, its size, rotation rate, obliquity, atmospheric composition and gravity. Here we explore the effects of the last of these, the Newtonian gravitational acceleration, on its atmosphere and climate. We first demonstrate that if the atmosphere obeys the hydrostatic primitive equations, which are a very good approximation for most terrestrial atmospheres, and if the radiative forcing is unaltered, changes in gravity have no effect at all on the circulation except for a vertical rescaling. That is to say, the effects of gravity may be completely scaled away and the circulation is unaltered. However, if the atmosphere contains a dilute condensible that is radiatively active, such as water or methane, then an increase in gravity will generally lead to a cooling of the planet because the total path length of the condensible will be reduced as gravity increases, leading to a reduction in the greenhouse effect. Furthermore, the specific humidity will decrease, leading to changes in the moist adiabatic lapse rate, in the equator-to-pole heat transport, and in the surface energy balance because of changes in the sensible and latent fluxes. These effects are all demonstrated both by theoretical arguments and by numerical simulations with moist and dry general circulation models.

Read this paper on arXiv…

S. Thomson and G. Vallis
Fri, 1 Feb 19
7/61

Comments: 17 pages, 9 figures. Submitted to QJRMS on 23/01/19

Simulating Non-hydrostatic atmospheres on Planets (SNAP): formulation, validation and application to the Jovian atmosphere [EPA]

Posted on by

http://arxiv.org/abs/1901.02955


A new non-hydrostatic and cloud-resolving atmospheric model is developed for studying moist convection and cloud formation in planetary atmospheres. It is built on top of the Athena++ framework, utilizing its static/adaptive mesh-refinement, parallelization, curvilinear geometry, and dynamic task scheduling. We extend the original hydrodynamic solver to vapors, clouds, and precipitation. Microphysics is formulated generically so that it can be applied to both Earth and Jovian planets. We implemented the Low Mach number Approximate Riemann Solver (LMARS) for simulating low speed atmospheric flows in addition to the usual Roe and HLLC Riemann solvers. Coupled with a fifth-order Weighted Essentially Nonoscillatory (WENO) subgrid-reconstruction method, the sharpness of critical fields such as clouds is well-preserved, and no extra hyperviscosity or spatial filter is needed to stabilize the model. Unlike many atmospheric models, total energy is used as the prognostic variable of the thermodynamic equation. One significant advantage of using total energy as a prognostic variable is that the entropy production due to irreversible mixing process can be properly captured. The model is designed to provide a unified framework for exploring planetary atmospheres across various conditions, both terrestrial and Jovian. First, a series of standard numerical tests for Earth’s atmosphere is carried out to demonstrate the performance and robustness of the new model. Second, simulation of an idealized Jovian atmosphere in radiative-convective equilibrium shows that 1) the temperature gradient is superadiabatic near the water condensation level because of the changing of the mean molecular weight, and 2) the mean profile of ammonia gas shows a depletion in the subcloud layer down to nearly 10 bars. Relevance to the recent Juno observations is discussed.

Read this paper on arXiv…

C. Li and X. Chen
Fri, 11 Jan 19
41/45

Comments: 35 pagers, 11 figures, accepted to ApJS

Errors induced by the neglect of polarization in radiance calculations for three-dimensional cloudy atmospheres [EPA]

Posted on by

http://arxiv.org/abs/1901.01836


Remote sensing instruments observe radiation being scattered and absorbed by molecules, aerosol particles, cloud droplets and ice crystals. In order to interpret and accurately model such observations, the vector radiative transfer equation needs to be solved, because scattering polarizes the initially unpolarized incoming solar radiation. A widely used approximation in radiative transfer theory is the neglect of polarization which allows to greatly simplify the radiative transfer equation. It is well known that the error caused by multiple Rayleigh scattering can be larger than 10\%, depending on wavelength and sun-observer geometry (Mishchenko et al., 1994). For homogeneous plane-parallel layers of liquid cloud droplets the error is comparatively small (below 1\%) (Hansen 1971). However, in reality clouds are not plane-parallel layers of water droplets but complex three-dimensional (3D) structures and observations of clouds usually include pixels consisting of clear and cloudy parts. In this study we revisit the question of the magnitude of error due to the neglect of polarization in radiative transfer theory for a realistic 3D cloudy atmosphere. We apply the Monte Carlo radiative transfer model MYSTIC with and without neglecting polarization and compare the results. At a phase angle of 90{\deg} and 400nm wavelength we find the maximum overestimation error of about 8% for complete clear-sky conditions. The error is reduced to about 6% in clear-sky regions surrounded by clouds due to scattering from clouds into the clear regions. Within the clouds the error is up to 4% with the highest values in cloud shadows. In backscattering direction the radiance is underestimated by about 5% in clear regions between clouds. For other sun-observer geometries, the error ranges between the two extremes. The error decreases with wavelength and in the absorption bands.

Read this paper on arXiv…

C. Emde and B. Mayer
Tue, 8 Jan 19
28/99

Comments: N/A

IPRT polarized radiative transfer model intercomparison project – Three-dimensional test cases (phase B) [EPA]

Posted on by

http://arxiv.org/abs/1901.01828


Initially unpolarized solar radiation becomes polarized by scattering in the Earth’s atmosphere. In particular molecular scattering polarizes electromagnetic radiation, but also scattering of radiation at aerosols, cloud droplets and ice crystals polarizes. Each atmospheric constituent produces a characteristic polarization signal, thus spectro-polarimetric measurements are frequently employed for remote sensing of aerosol and cloud properties. Retrieval algorithms require efficient radiative transfer models. Usually, these apply the plane-parallel approximation, assuming that the atmosphere consists of horizontally homogeneous layers. For remote sensing applications, the radiance is considered constant over the instantaneous field-of-view of the instrument and each sensor element is treated independently in plane-parallel approximation, neglecting horizontal radiation transport between adjacent pixels. In order to estimate the errors due to the IPA approximation, three-dimensional (3D) vector radiative transfer models are required. So far, only a few such models exist. Therefore, the International Polarized Radiative Transfer (IPRT) working group of the International Radiation Commission (IRC) has initiated a model intercomparison project in order to provide benchmark results for polarized radiative transfer. The group has already performed an intercomparison for one-dimensional (1D) multi-layer test cases (Emde et al., 2015). This paper presents the continuation of the intercomparison project for 2D and 3D test cases: a step cloud, a cubic cloud, and a more realistic scenario including a 3D cloud field generated by a Large Eddy Simulation model and typical background aerosols. The commonly established benchmark results for 3D polarized radiative transfer are available at the IPRT website (this http URL).

Read this paper on arXiv…

C. Emde, V. Barlakas, C. Cornet, et. al.
Tue, 8 Jan 19
41/99

Comments: N/A

IPRT polarized radiative transfer model intercomparison project – Phase A [EPA]

Posted on by

http://arxiv.org/abs/1901.01813


The polarization state of electromagnetic radiation scattered by atmospheric particles such as aerosols, cloud droplets, or ice crystals contains much more information about the optical and microphysical properties than the total intensity alone. For this reason an increasing number of polarimetric observations are performed from space, from the ground and from aircraft. Polarized radiative transfer models are required to interpret and analyze these measurements and to develop retrieval algorithms exploiting polarimetric observations. In the last years a large number of new codes have been developed, mostly for specific applications. Benchmark results are available for specific cases, but not for more sophisticated scenarios including polarized surface reflection and multi-layer atmospheres. The International Polarized Radiative Transfer (IPRT) working group of the International Radiation Commission (IRC) has initiated a model intercomparison project in order to fill this gap. This paper presents the results of the first phase A of the IPRT project which includes ten test cases, from simple setups with only one layer and Rayleigh scattering to rather sophisticated setups with a cloud embedded in a standard atmosphere above an ocean surface. All scenarios in the first phase A of the intercomparison project are for a one-dimensional plane-parallel model geometry. The commonly established benchmark results are available at the IPRT website (this http URL).

Read this paper on arXiv…

C. Emde, V. Barlakas, C. Cornet, et. al.
Tue, 8 Jan 19
81/99

Comments: N/A

Internal gravity waves in the energy and flux budget turbulence-closure theory for shear-free stably stratified flows [CL]

Posted on by

http://arxiv.org/abs/1812.10059


We have advanced the energy and flux budget (EFB) turbulence closure theory that takes into account a two-way coupling between internal gravity waves (IGW) and the shear-free stably stratified turbulence. This theory is based on the budget equation for the total (kinetic plus potential) energy of IGW, the budget equations for the kinetic and potential energies of fluid turbulence, and turbulent heat fluxes for waves and fluid flow. The waves emitted at a certain level, propagate upward, and the losses of wave energy cause the production of turbulence energy. We demonstrate that due to the nonlinear effects more intensive waves produce more strong turbulence, and this, in turns, results in strong damping of IGW. As a result, the penetration length of more intensive waves is shorter than that of less intensive IGW. The anisotropy of the turbulence produced by less intensive IGW is stronger than that caused by more intensive waves. The low amplitude IGW produce turbulence consisting up to 90 % of potential energy. This resembles the properties of the observed high altitude tropospheric strongly anisotropic (nearly two-dimensional) turbulence.

Read this paper on arXiv…

N. Kleeorin, I. Rogachevskii, I. Soustova, et. al.
Thu, 27 Dec 18
43/80

Comments: 10 pages, 11 figures, REVTEX4-1

Internal gravity waves in the energy and flux budget turbulence-closure theory for shear-free stably stratified flows [CL]

Posted on by

http://arxiv.org/abs/1812.10059


We have advanced the energy and flux budget (EFB) turbulence closure theory that takes into account a two-way coupling between internal gravity waves (IGW) and the shear-free stably stratified turbulence. This theory is based on the budget equation for the total (kinetic plus potential) energy of IGW, the budget equations for the kinetic and potential energies of fluid turbulence, and turbulent heat fluxes for waves and fluid flow. The waves emitted at a certain level, propagate upward, and the losses of wave energy cause the production of turbulence energy. We demonstrate that due to the nonlinear effects more intensive waves produce more strong turbulence, and this, in turns, results in strong damping of IGW. As a result, the penetration length of more intensive waves is shorter than that of less intensive IGW. The anisotropy of the turbulence produced by less intensive IGW is stronger than that caused by more intensive waves. The low amplitude IGW produce turbulence consisting up to 90 % of potential energy. This resembles the properties of the observed high altitude tropospheric strongly anisotropic (nearly two-dimensional) turbulence.

Read this paper on arXiv…

N. Kleeorin, I. Rogachevskii, I. Soustova, et. al.
Thu, 27 Dec 18
57/80

Comments: 10 pages, 11 figures, REVTEX4-1

The Relationship between Precipitation and Aerosol: Evidence from Satellite Observation [CL]

Posted on by

http://arxiv.org/abs/1812.02036


The interaction of aerosol-cloud-precipitation has an important impact on the global climate. The understanding of this issue is related to the uncertainty of climate change prediction. The traditional indirect effect of aerosols suggests that when the number of aerosols increases, it will act to suppress precipitation. However, recent studies on satellite observations have found that aerosols are positively correlated with precipitation, which is contrary to conventional views. This study attempts to use the A-Train satellite product to verify the correlation between aerosol and precipitation, and further reveals the possible physical mechanism of the positive relationship between aerosols and precipitation in satellite observations through the three-dimensional structure of clouds. The study found that the precipitation intensity is positively correlated with the aerosol optical depth, while the relationship between the cloud droplet concentration and the precipitation intensity is related to the liquid water path; and when the number of aerosols increases, the radar reflectance spectrum is widened, and the precipitation increases, while the cloud droplet concentration shows the opposite phenomenon. It can be concluded that the possible cause of positive correlation between aerosol and precipitation is the negative correlation between CDNC and AOD.

Read this paper on arXiv…

C. Fan, Y. Fan, P. Wu, et. al.
Thu, 6 Dec 18
16/52

Comments: 13 pages

Influence of gravity waves on the climatology of high-altitude Martian carbon dioxide ice clouds [EPA]

Posted on by

http://arxiv.org/abs/1812.00346


Carbon dioxide (CO$_2$) ice clouds have been routinely observed in the middle atmosphere of Mars. However, there are still uncertainties concerning physical mechanisms that control their altitude, geographical, and seasonal distributions. Using the Max Planck Institute Martian General Circulation Model (MPI-MGCM), incorporating a state-of-the-art whole atmosphere subgrid-scale gravity wave parameterization [Yi\u{g}it et al., 2008], we demonstrate that internal gravity waves generated by lower atmospheric weather processes have wide reaching impact on the Martian climate. Globally, GWs cool the upper atmosphere of Mars by $\sim$10 % and facilitate high-altitude CO$_2$ ice cloud formation. CO$_2$ ice cloud seasonal variations in the mesosphere and the mesopause region appreciably coincide with the spatio-temporal variations of GW effects, providing insight into the observed distribution of clouds. Our results suggest that GW propagation and dissipation constitute a necessary physical mechanism for CO$_2$ ice cloud formation in the Martian upper atmosphere during all seasons.

Read this paper on arXiv…

E. Yiğit, A. Medvedev and P. Hartogh
Tue, 4 Dec 18
63/78

Comments: Accepted for publication in Annales Geophysicae

Site testing study based on weather balloons measurements [IMA]

Posted on by

http://arxiv.org/abs/1812.00028


We present wind and temperature profiles at Dome C measured by balloon born sonds during the polar summer. Data from 197 flights have been processed for 4 campaigns between 2000 and 2004. We show the exceptionnal wind conditions at Dome C, Average ground wind speed is 3.6 m/s. We noticed in mid-november the presence of high altitude strong winds (40 m/s) probably due to the polar vortex which disappear in summer. These winds seem to have no effect on seeing measurements made with a DIMM at the same period. Temperature profiles exhibit a minimum at height 5500 m (over the snow surface) that defines the tropopause. Surface layer temperature profile has negative gradient in the first 50 m above ground in the afternoon and a strong inversion layer (5{\deg}C over 50 m) around midnight. Wind profiles are compared with other astronomical sites, and with a meteorological model from Meteo France.

Read this paper on arXiv…

E. Aristidi, A. Agabi, M. Azouit, et. al.
Tue, 4 Dec 18
74/78

Comments: N/A

Global climate modeling of Saturn's atmosphere. Part II: multi-annual high-resolution dynamical simulations [EPA]

Posted on by

http://arxiv.org/abs/1811.01250


The Cassini mission unveiled the intense and diverse activity in Saturn’s atmosphere: banded jets, waves, vortices, equatorial oscillations. To set the path towards a better understanding of those phenomena, we performed high-resolution multi-annual numerical simulations of Saturn’s atmospheric dynamics. We built a new Global Climate Model [GCM] for Saturn, named the Saturn DYNAMICO GCM, by combining a radiative-seasonal model tailored for Saturn to a hydrodynamical solver based on a icosahedral grid suitable for massively-parallel architectures. The impact of numerical dissipation, and the conservation of angular momentum, are examined in the model before a reference simulation employing the Saturn DYNAMICO GCM with a $1/2^{\circ}$ latitude-longitude resolution is considered for analysis. Mid-latitude banded jets showing similarity with observations are reproduced by our model. Those jets are accelerated and maintained by eddy momentum transfers to the mean flow, with the magnitude of momentum fluxes compliant with the observed values. The eddy activity is not regularly distributed with time, but appears as bursts; both barotropic and baroclinic instabilities could play a role in the eddy activity. The steady-state latitude of occurrence of jets is controlled by poleward migration during the spin-up of our model. At the equator, a weakly-superrotating tropospheric jet and vertically-stacked alternating stratospheric jets are obtained in our GCM simulations. The model produces Yanai (Rossby-gravity), Rossby and Kelvin waves at the equator, as well as extratropical Rossby waves, and large-scale vortices in polar regions. Challenges remain to reproduce Saturn’s powerful superrotating jet and hexagon-shaped circumpolar jet in the troposphere, and downward-propagating equatorial oscillation in the stratosphere.

Read this paper on arXiv…

A. Spiga, S. Guerlet, E. Millour, et. al.
Tue, 6 Nov 18
3/77

Comments: 85 pages (double spacing), 21 figures, version submitted to Icarus on October 26th 2018

Effects of the second virial coefficient on the tropospheric adiabatic lapse rate [CL]

Posted on by

http://arxiv.org/abs/1810.10411


We report the effect of considering as equation of state a virial expansion up to second order in the calculation of the adiabatic lapse rate. In the process, we have derived many thermodynamic functions necessary to our aim, like the internal energy, the heat capacity, and the adiabatic curves. We apply these results to Earth, Venus, and Titan. In each case, we consider three physically relevant virial coefficients: the hard-sphere, van der Waals, and the square-well potential. For Venus and Titan, we obtain corrections toward the experimental value of the corresponding tropospheric lapse rate.

Read this paper on arXiv…

E. Navarro, B. Díaz, M. García-Ariza, et. al.
Thu, 25 Oct 18
11/65

Comments: 7 pages, 6 figures. Comments are welcome,

Nightside Winds at the Lower Clouds of Venus with Akatsuki/IR2: Longitudinal, local time and decadal variations from comparison with previous measurements [EPA]

Posted on by

http://arxiv.org/abs/1810.05418


We present measurements of the wind speeds at the nightside lower clouds of Venus from observations by JAXA’s mission Akatsuki during 2016, complemented with new wind measurements from ground-based observations acquired with TNG/NICS in 2012 and IRTF/SpeX in 2015 and 2017. Zonal and meridional components of the winds were measured from cloud tracking on a total of 466 Akatsuki images of Venus acquired by the camera IR2 using the 2.26-$\mathrm{\mu m}$ filter, with spatial resolutions ranging 10–80 km per pixel and covering from 2016 March 22 to October 31. More than 149,000 wind vectors were obtained with an automatic technique of template matching, and 2,947 wind vectors were inferred with the manual procedure. The meridional profiles for both components of the winds are found to be consistent with results from the Venus Express mission during 2006–2008, although stronger wind variability is found for the zonal component at equatorial latitudes where Akatsuki observations have better viewing geometry than Venus Express. The zonal winds at low latitudes also suggest a zonal variability that could be associated with solar tides or vertically propagating orographic waves. Finally, the combination of our wind measurements from TNG/NICS, IRTF/SpeX and Akatsuki images with previously published and based in data from 1978 to 2017 suggests variations of up to 30 m s$^{-1}$ in the winds at the lower clouds of the Venus nightside.

Read this paper on arXiv…

J. Peralta, K. Muto, R. Hueso, et. al.
Mon, 15 Oct 18
11/56

Comments: 25 pages, 13 figures, Supplemental Material available at ApJS

Vientos, Turbulencia y Ondas en las Nubes de Venus [EPA]

Posted on by

http://arxiv.org/abs/1810.05477


This PhD thesis consists on a study of the atmospheric dynamics of the planet Venus with data from two space missions separated in time: the Galileo mission and Venus Express. Concretely, images obtained with different wavelengths have been used to study the motions of the clouds at different vertical levels of the atmosphere, enabling to track the global atmospheric dynamics of the planet. In this thesis simultaneous measurements of the 3-dimensional structure of the winds are presented for the first time, finding and analyzing different sources of temporal variability such as the thermal waves produced by the solar insolation, global oscillations of the wind speeds and variability for periods covering from months to years. Furthermore, a study of the spatial scales of the Venusian clouds and the atmospheric turbulence have been undertaken. Frequent periodic patterns can be also seen on the lower clouds of Venus in high spatial resolution images from the instrument VIRTIS onboard Venus Express. These correspond to a type of atmospheric waves called gravity waves, and their characteristics and role in te atmospheric dynamics of Venus are analyzed at the end of the thesis.

Read this paper on arXiv…

J. Peralta
Mon, 15 Oct 18
49/56

Comments: 152 pages, 44 figures, PhD thesis in Spanish