The Importance of Shallow Cloud Microphysics for Aerosol-Cloud Interactions

Aerosol Cloud Interactions (ACI) are the consequence of perturbed aerosols affecting cloud drop number, with corresponding microphysical and radiative effects. ACI are sensitive to both cloud microphysical processes (the “C” in ACI) and aerosol emissions and processes (the “A” in ACI). This work highlights the importance of cloud microphysical processes in shallow clouds for ACI, using idealized and global tests of a cloud microphysics scheme used for global climate prediction. Uncertainties in cloud microphysical processes cause uncertainties of up to -35 to +50% in ACI, stronger than uncertainties due to natural aerosol emissions -20 to +30%). The different dimensions and sensitivities of ACI to microphysical processes are analyzed in detail, showing that warm rain precipitation processes are critical for understanding ACI and that uncertain cloud lifetime effects are 1/3 of simulated ACI. Buffering of different processes is important, as is the mixed phase and coupling of the microphysics to the condensation and turbulence schemes in the model.