The Influence of Elevated Smoke Layers on Stratocumulus Clouds Over the SE Atlantic in the NASA Goddard Earth Observing System (GEOS) Model

Previous evaluations of simulated aerosol transport over the south-east Atlantic by global aerosol models, including the Goddard Earth Observing System (GEOS) atmospheric general circulation model, showed that the bulk of the modeled smoke aerosol layer resided similar to 1-2 km lower than Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar observations. Using this finding as the motivation, this study examines the changes in model-simulated cloud properties in response to redistributing the vertical placement of the aerosol over the ocean. Ten years (2006-2015) of CALIOP-retrieved smoke aerosol extinction profiles were used to redistribute the model-simulated aerosol mass on a monthly mean basis, keeping the column aerosol mass conserved. The results from the model sensitivity experiments show that elevating the aerosol layer to higher levels in agreement with CALIOP observations causes an increase in cloud fractions by similar to 33% for shallow marine boundary layers (MBL) and a decrease by similar to 30% for deeper MBL. For a shallow MBL, aerosol-induced warming within the cloud layers for the lower altitude aerosol case decreases relative humidity at these levels and leads to a reduction of overall cloud amount compared to the elevated aerosol case. For a deeper MBL, however, aerosol heating within the upper cloud levels in the lower altitude aerosol case increases the underlying MBL stability, which suppresses the cloud vertical extent, enhances cloud cover, and delays the stratocumulus to cumulus transition. Finally, aerosol redistribution impacts on radiative forcing are investigated, which appear to be mainly driven by the changes in cloud area fractions rather than in-cloud liquid water path changes between the model experiments.