Resolving Clouds on Tidally Locked Rocky Planets Part I: 2D Idealized Experiments

Abstract:

Cloud is critical for planetary climate and habitability and is one of the most challenging parts in studying exoplanets as well as planets in the solar system. Previous numerical simulations using global atmospheric general circulation models (AGCMs) found that for 1:1 tidally locked rocky planets with oceans strong convergence and convection produce optically thick clouds over the substellar region. These clouds are effective in cooling the surface and expanding the habitable zone. One weakness of these studies is that the clouds are parameterized based on our knowledge on Earth and whether it is applicable to tidally locked planets or not is unknown. One method to improve our understanding is using cloud resolving models (CRMs) with very fine resolution. In this study, we try to simulate the clouds on tidally locked planets using a CRM -- the Simulating Nonhydrostatic Atmospheres on Planets (SNAP) model. As the first step, the model is set to be 2D along the equator (z-x), and the domain size is 40,000 km × 35 km with a horizontal resolution of 2 km and a vertical resolution of 250 m. Heating is provided by a fixed sea surface temperature (SST) obtained from previous AGCM experiments, and longwave cooling is parameterized by a constant cooling rate. We find that the substellar region is covered by deep convective clouds, the night side is dominated by low-level clouds trapped in the planetary boundary layer, and these two are linked by a global-scale Walker circulation. This spatial pattern is similar to that found in AGCMs. In the model, water amount, horizontal extend, and vertical altitude of the clouds are determined by SST magnitude, SST gradient, and radiative cooling magnitude. Theories of the weak temperature gradient approximation and moist static energy are applicable to this system.