The Effect of Planetary Rotation Period on Clouds in a Global Climate Model with a Bin Microphysics Scheme

TL;DR

This study compares a size-resolved bin microphysics model with a traditional parameterized scheme in a climate model to assess cloud differences on exoplanets with varying rotation rates, impacting climate and observational interpretations.

Contribution

It demonstrates the differences between bin microphysics and parameterized schemes in exoplanet climate simulations, highlighting the importance of resolved microphysics for accurate cloud representation.

Findings

CARMA produces fewer liquid clouds and more ice clouds than MG

Ice cloud size distribution differs significantly, affecting spectral retrievals

Net cloud radiative effect decreases by 4-10 W/m^2

Abstract

Clouds are the largest source of uncertainty in climate simulations. For exoplanets, cloud simulation is particularly challenging because of the lack of observational data to tune parameterized cloud models. Here we apply Community Aerosol and Radiation Model for Atmospheres (CARMA), a size-resolved bin cloud microphysics model, to the atmospheric global climate model Community Atmosphere Model (CAM6) and simulate exoplanets with a range of planetary rotation rates. CARMA produces fewer liquid clouds than the native CAM6 parameterized cloud microphysics scheme (Morrison-Gettelman two-moment microphysics, MG), more ice clouds, and a significantly different ice cloud size distribution. Overall, this leads to a decrease in the magnitude of the net CRE by 4-10 $W/m^2$, which is unlikely to change the determination of habitability from a climate perspective in most cases. The difference in ice cloud size distribution is likely to strongly affect transmission spectral retrievals. Our work confirms that the MG parameterized cloud microphysics scheme can produce reasonable climate simulation when extrapolated to some exoplanet contexts and highlights the value of resolved cloud microphysics for evaluating parameterized schemes and for interpreting observations.