The role of Tibetan summer low clouds in the simulation of the East Asian summer monsoon rain belt

Abstract

AbstractIt has been challenging to simulate the East Asian summer monsoon (EASM) using general circulation models. By evaluating the cloud layers unified by binormals (CLUBB) model and its revised version in the version‐5 Community Atmosphere Model, we find that EASM simulations benefit from improving the reproduction of low clouds over the Tibetan Plateau. When a cloud‐top radiative cooling scheme (RAD) is coupled with CLUBB, it significantly improves the resulting EASM rain belt and western Pacific subtropical high (WPSH) simulations compared to the default simulations without RAD; in these default simulations, the low‐level southwesterlies, WPSH ridge, and EASM rain belt are displaced northward. The moisture budget analyses indicate that the improvements in EASM simulations are mainly contributed to by the improved presentation of low‐level stationary eddy meridional flow convergence over East Asia; this convergence shifts northward during the default model runs. Because the RAD scheme enables the model to better represent the subgrid radiation–turbulence interaction, the model produces stronger turbulent fluxes and lower clouds but reduces incoming solar radiation over the Tibetan Plateau. It thus shifts the Tibetan High southward, ultimately resulting in an improved simulation of the low‐level southwesterlies. These improvements in CLUBB_RAD highlight the importance of improving the representation of low clouds when simulating EASM rainfall.