Aerosol-Ice-Cloud Interactions in a Perturbed Parameter Ensemble

The effective radiative forcing (ERF) from aerosol-cloud interactions (ERFaci) remains poorly constrained and increases uncertainty in projections of future warming. While aerosol effects on liquid clouds have been studied extensively in both models and observations, less attention has been paid to the ERF from aerosol-ice-cloud interactions (ERFaci _ice ). A new method is introduced for isolating the shortwave (SW) ERFaci _ice in global climate models (GCMs) using histograms of monthly averaged ice cloud fraction partitioned by ice crystal effective radius (r _e ) and ice water path (IWP). Combining the cloud histograms with radiative kernels enables estimation of the total SW ERFaci _ice , which can be further decomposed into contributions from r _e , IWP, and cloud amount changes. The approach is illustrated with a new perturbed parameter ensemble (PPE) using the Community Atmosphere Model, version 6 (CAM6). The overall cooling effect from SW ERFaci _ice is estimated to be -0.43 Wm ^-2 in the ensemble mean, a non-negligible contribution to the total ERFaci. A pathway is proposed that relates aerosol-induced subtropical low cloud changes to tropical ice cloud changes via changes in boundary layer moisture. The PPE results demonstrate that dynamical changes can drive the strength of the SW aerosol forcing on ice clouds in certain regions, adding another dimension of complexity to analyzing aerosol-cloud interactions.