Large Eddy Simulation of Mixed‐Phase Clouds With Aerosol Perturbation: Importance of Effective Radius

Abstract Aerosol‐cloud interactions are a major source of uncertainty in estimating anthropogenic effective radiative forcing. Although warm polluted clouds have been studied to investigate the influence of aerosols on clouds, studies on mixed‐phase polluted clouds are limited. Here we performed Large Eddy Simulations of mixed‐phase clouds with various cloud phase partitioning and aerosol emissions under multiple environmental conditions to investigate the processes governing the responses of their radiative properties to aerosol emissions and the dependence of these responses on environmental conditions. The cloud radiative responses for the Mixed‐Phase Arctic Cloud Experiment were mainly caused by responses in the effective radius of the cloud droplets, which were significantly larger than those in the liquid water content (LWC). Budget analysis of the cloud LWC showed that this was because aerosol emissions barely suppressed precipitation as the contributions from collision‐coalescence processes were weaker than those of the other microphysical processes, even in simulations without aerosol emissions in the model used here. Sensitivity experiments to vary the three uncertain parameters in the cloud microphysics scheme were conducted to investigate the influence of model uncertainty on the responses. The results showed that the mean cloud droplet mass after evaporation and the environmental conditions as well as the phase partitioning affected the effective radius, which changed the signs of the radiative responses. These results emphasize the significance of the effective radius and its responses to aerosols dependent on environmental conditions with varying cloud phases in determining the radiative forcing due to the aerosol‐cloud interactions of mixed‐phase clouds.