Abstract
Abstract Terrestrial-type exoplanets orbiting nearby red dwarf stars (M dwarfs) are the first potentially habitable exoplanets suitable for atmospheric characterization in the near future. Understanding the stability of water in cold-trap regions on such planets is critical because it directly impacts transmission spectroscopy observations, the global energy budget, and long-term surface water evolution. Here we diagnose the humidity distribution in idealized general circulation model simulations of terrestrial-type exoplanets. We use the “tracer of last saturation” technique to study the saturation statistics of air parcels. We find that on synchronously rotating planets the water vapor abundance in the nightside upper troposphere depends weakly on planetary rotation, while more water vapor builds up in the nightside lower troposphere on fast-rotating planets. We then discuss how last-saturation statistics can elucidate the multiple moist climate equilibrium states on synchronously and asynchronously rotating arid planets. We show that the multiple moist climate states arise from the cold-trapping competition between the substellar upper atmosphere and cold surface regions. We find that fast synchronously rotating planets tend to trap surface water on the nightside as a result of their weak atmospheric and strong surface cold traps compared to the slow-rotating case. These results elucidate the nature of the water cycle on arid rocky exoplanets and will aid interpretation of atmospheric observations in the future.
Highlights
As hundreds of Earth-sized planets are discovered beyond our solar system1, the new frontier of exoplanet science will soon be characterization of atmospheres on these terrestrial planets
We find that on synchronously rotating planets, the water vapor abundance in the nightside upper troposphere depends weakly on planetary rotation, while more water vapor builds up in the nightside lower troposphere on fast rotating planets
We show that the multiple moist climate states arise from the cold-trapping competition
Summary
As hundreds of Earth-sized planets are discovered beyond our solar system, the new frontier of exoplanet science will soon be characterization of atmospheres on these terrestrial planets. An idealized model of non-local large-scale control of tropospheric humidity, called the last-saturation model or advection–condensation model, has been proposed for understanding the humidity distribution in Earth’s atmosphere (Pierrehumbert 1998; Pierrehumbert et al 2007; Pierrehumbert & Ding 2016) This model assumes the mixing ratio of water vapor in an air parcel is conserved along any segment of a trajectory unless it becomes saturated and loses water by precipitation. Galewsky et al (2005) first developed this method to study the dryness of the Earth’s subtropical troposphere in both idealized GCM simulations and a reanalysis dataset They found that the subsaturation is primarily controlled by the isentropic transport by midlatitude eddies and that diabatic descending transport from the tropical upper troposphere plays a secondary role. To study a wider range of climate states in an efficient way, we choose instead to simplify the radiation and convection calculations and make the moist GCM more idealized
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