Precipitation driving of droplet concentration variability in marine low clouds

The concentration N-d of cloud droplets in marine low clouds is a primary determinant of their ability to reflect sunlight and modulates their ability to precipitate. Previous studies have focused upon aerosol source variability as the key driver of variability in N-d. Here, we use a highly simplified aerosol budget model to examine the impact of precipitation on N-d. This model considers: precipitation (coalescence) scavenging, constrained using new satellite measurements of light precipitation; entrainment of aerosol from above cloud combined with constant aerosol concentration based on recent field observations of aerosol particles in the free troposphere; and sea-surface aerosol production estimated using a wind speed dependent source function. Despite the highly simplified nature of this model, it skillfully predicts the geographical variability of N-d in regions of extensive marine low clouds. Inclusion of precipitation results in reduction in N-d by factors of 2-3 over the remote oceans. Within 500 km of coastlines the reduction in N-d due to precipitation is weak but in these regions the model is not able to accurately predict N-d because of strong pollution sources. In general, neither free-tropospheric nor surface CCN sources alone are sufficient to maintain N-d against precipitation losses. The results demonstrate that even the light precipitation rates typical of marine stratocumulus profoundly impact the radiative properties of marine low clouds.