Idealized 2D Cloud-resolving Simulations for Tidally Locked Habitable Planets

Abstract

Cloud is critical for planetary climate and habitability, but it is also one of the most challenging aspects of studying planets in and beyond the solar system. Here we use a cloud-resolving model (CRM) with high resolution (2 km) in a 2D configuration to simulate the clouds and circulation on tidally locked aquaplanets. We find that the substellar area is covered by deep convective clouds, the nightside is dominated by low-level clouds, and the two are linked by a global-scale Walker circulation. We further find that uniform surface warming causes the substellar cloud width to decrease, but a reduction in the day–night surface temperature contrast or an increase in the longwave radiative cooling rate causes the substellar cloud width to increase. These relationships can be roughly interpreted in accordance with simple thermodynamic theories. Comparing the results between the CRM and the global 3D general circulation model (GCM), we find that they are qualitatively consistent, including the Walker circulation, the substellar clouds, and the responses of the substellar ascending area and strength to changes in the surface temperature or in its zonal contrast. But large quantitative differences exist, such as the magnitude of the cloud water path, the cloud width, and their responses to external forcings. These results increase our confidence in using GCMs to model exoplanetary climates, although large quantitative uncertainties are always likely to exist. Future work is required to use 3D CRMs with realistic radiative transfer and the Coriolis force to examine the clouds and climates of tidally locked planets.