Robert Wood

University of Washington
The concentration Nd of cloud droplets in marine boundary layer (MBL) clouds is a primary determinant of their ability to reflect sunlight and modulates their ability to precipitate. Studies have focused upon aerosol source variability as the key driver of variability in Nd. In this talk, I will explore the impact of both precipitation and source variability on Nd. I show that a simple aerosol budget model for the MBL can skillfully predict the geographical variability of Nd observed using MODIS in regions of extensive marine low clouds. It is shown that light precipitation is a key control on the Nd climatology. Light precipitation, measured using CloudSat, results in reduction in Nd by factors of 2–3 over the remote oceans. Within 500 km of coastlines the reduction in Nd due to precipitation is weak but in these regions the model is not able to accurately predict Nd because of strong pollution sources. In general, both free-tropospheric and surface aerosol sources are likely needed to maintain Nd against precipitation losses. The results demonstrate that light precipitation rates typical of marine stratocumulus impact the radiative properties of marine low clouds not only by impacting cloud dynamics but by modulating MBL aerosol.