Toward Improving Aerosol Radiative Forcing in Global Climate Models

Effective radiative forcing of anthropogenic aerosols (ERF_aer) is an important metric to quantify the simulated aerosol effects on climate, which remains the largest uncertainty in climate projection using global climate models. Many models simulate overly strong ERF_aer and its direct and indirect forcing components (ERF_ari and ERF_aci), including the Department of Energy’s Energy Exascale Earth System Model (E3SM). For E3SM, two most important factors contributing to the large aerosol forcing are the overestimated burden and lifetime of anthropogenic aerosols that is attributed to the insufficient aerosol wet removal and high frequency of extremely low cloud droplet number concentrations (Nc < 10 # cm^-3). To address the former, we improve aerosol wet removal treatments for deep convective clouds (e.g., cloud-borne aerosol detrainment, aerosol secondary activation, and cloud-borne aerosol removal). These changes effectively decrease the aerosol burden and lifetime, and reduce the positive biases in aerosol optical depth and aerosol mass concentration in E3SM. The ERF_ari and ERF_aci, and the bias in the TOA radiation budget are also significantly decreased. For the latter, the measurement shows that the frequency of extremely low Nc cases is around 15 %, whereas E3SM shows higher than 50 % over the majority of the region over the globe. We identify the reasons leading to the problem and develop a physical solution instead of posing a lower bound to address the issue. The physical solution significantly reduces the extremely low Nc case frequency to about 25 %, similarly to the result of posing a lower bound. It improves LWC but drastically changes the global radiative balance. The effort in retuning the model for radiative balance will be presented.