Improving Aerosol Radiative Forcing and Climate in E3SM: Impacts of New Cloud Microphysics and Improved Wet Removal Treatments.

Numerous Earth system models exhibit excessive aerosol effective forcing at the top of the atmosphere (TOA), including the Department of Energy's Energy Exascale Earth System Model (E3SM). Here, in the context of the E3SM version 3 effort, the predicted particle property (P3) stratiform cloud microphysics scheme and an enhanced deep convection parameterization suite (ZM_plus) are implemented into E3SM. The ZM_plus includes a convective cloud microphysics scheme, a multi‐scale coherent structure parameterization for mesoscale convective systems, and a revised cloud base mass flux formulation considering impacts of the large‐scale environment. The P3 scheme improved cloud and radiation particularly over the Northern Hemisphere and the frequency of heavy precipitation over the tropics, and the ZM_plus improved clouds in the tropics. P3 decreases aerosol effective forcing by 0.15 W m−2, while the ZM_plus increases it by 0.27 W m−2, resulting from excessive direct (0.31 W m−2) and indirect forcing (−1.79 W m−2). The excessive aerosol forcings are due to aerosol overestimation associated with insufficient aerosol wet removal. By improving the physical treatments in the aerosol wet removal, we effectively mitigate anthropogenic aerosol overestimation and thus attenuate direct (0.09 W m−2) and indirect aerosol forcing (−1.52 W m−2). Adjustment to primary organic matter hygroscopicity reduces direct and indirect forcing to more reasonable values: −0.13 W m−2 and −1.31 W m−2, respectively. On climatology, improved aerosol treatments mitigate overestimation of aerosol optical depth. Plain Language Summary: The Energy Exascale Earth System Model (E3SM) exhibits strong direct and indirect aerosol forcings, after advanced stratiform and convective cloud treatments are implemented. This study identifies the primary cause of these excessive aerosol forcings as the significant overestimation of anthropogenic aerosols due to insufficient removal of aerosols by precipitation. To address this issue, we made aerosol wet removal representation more physical, which effectively reduced the overestimation of aerosols, bringing both direct and indirect forcings to the expected ranges. Furthermore, cloud and aerosol climatology are notably improved as the result of these developments in cloud and aerosol treatments. Key Points: The new cloud microphysics scheme P3 improves simulations of cloud properties in the NH and aerosol forcingInsufficient wet removal leads to an overly strong aerosol forcing after incorporating the enhanced deep convection parameterization suiteThe aerosol wet removal improvements lead to more reasonable aerosol forcing and improved aerosol climatology in Energy Exascale Earth System Model [ABSTRACT FROM AUTHOR]