Improvement of Low-cloud Simulations with a Revised Cloud Microphysics Scheme in an Atmospheric General Circulation Model

Clouds play an important role in global atmospheric energy and water vapor budgets, and the simulation of low cloud suffers from large biases in many atmospheric general circulation models. In this study, cloud microphysical processes, such as raindrop evaporation and cloud water accretion in a double-moment 6-class cloud microphysics scheme, are revised to enhance the simulation of low clouds with the Global-to-Regional Integrated Forecast System (GRIST) model. Validation of the revised scheme in a single-column version of the GRIST demonstrates a reasonable reduction of liquid water biases. The revised parameterization simulates medium- and low-level cloud fractions in better agreement with the observations than the original scheme. Long-term global simulations indicate mitigation of the originally overestimated low-level cloud fraction and cloud water mixing ratio in mid-to-high latitude regions, primarily due to the enhanced accretion process and weakened raindrop evaporation. The reduced low cloud with the revised scheme shows better consistency with the satellite observations, especially in mid and high latitudes. Consequent improvements are observed in the simulated cloud shortwave radiative forcing and the vertical distribution of total cloud. Annual precipitation in midlatitude regions is also ameliorated, especially over oceans, with significantly increased large-scale and decreased convective precipitation.