NIR-Driven Moist Upper Atmospheres of Synchronously Rotating Temperate Terrestrial Exoplanets [EPA]

H$_2$O is a key molecule in characterizing atmospheres of temperate terrestrial planets, and observations of transmission spectra are expected to play a primary role in detecting its signatures in the near future. Detectability of H$_2$O absorption features in transmission spectra depends on the abundance of water vapor in the upper part of the atmosphere. While the stratospheric water vapor mixing ratio of the Earth is less than $10^{-5}$ due to the cold trap, the efficiency of the cold trap depends on atmospheric properties. Here we study the 3D distribution of atmospheric H$_2$O for synchronously rotating Earth-sized aquaplanets using the GCM ROCKE-3D, and examine the effects of total incident flux and stellar spectral type. We observe a more gentle increase of the water vapor mixing ratio in response to increased incident flux than 1D models suggest, in qualitative agreement with the climate-stabilizing effect of clouds around the substellar point previously observed in GCMs applied to synchronously rotating planets. However, the water vapor mixing ratio in the upper atmosphere starts to increase while the surface temperature is still moderate. This is explained by the circulation in the upper atmosphere driven by the radiative heating due to absorption by water vapor and cloud particles, causing efficient vertical transport of water vapor. Consistently, the water vapor mixing ratio is found to be well correlated with the near-infrared portion of the incident flux. Our results imply that various levels of water vapor mixing ratio in the upper atmosphere may be expected for synchronously rotating temperate terrestrial planets, and that for the more highly irradiated ones the H$_2$O absorption features in the transmission spectra are strengthened by a factor of a few, loosening the observational demands for a direct H$_2$O detection. (abridged)

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Y. Fujii, A. Genio and D. Amundsen
Fri, 21 Apr 17

Comments: under review