The Influence of Biomass Burning Aerosols on Stratocumulus Clouds over the Southeast Atlantic

Biomass burning (BB) aerosol plumes originating from the seasonal (July-October) burning of savannah grasslands and agricultural residue traverse thousands of kilometers westward over the South-East Atlantic, where one of the planet’s three semi-permanent marine stratocumulus cloud decks reside. This gives rise to a unique and persistent feature of absorbing aerosols above clouds for the entire austral spring season that merits special scrutiny. Previous evaluations of global model simulations of aerosol transport over the region showed that largest differences between the modeled and observed aerosol vertical distribution occurred when aerosol plumes transitioned over the ocean from the burning sources on land. Specific to the Goddard Earth Observing System (GEOS) Atmospheric GCM, the bulk of the modeled BB aerosol layer resided ~1-2 km lower than that of CALIOP lidar observations over the ocean. Using this finding as the motivation, the objective of the present study is to examine the changes in model simulated cloud and marine boundary layer properties in response to the changes in aerosol vertical distribution over the ocean. Ten years (2006-15) of CALIOP smoke aerosol extinction profiles were used to redistribute the model simulated aerosol mass on a monthly mean basis, while keeping the column aerosol mass conserved. The redistributed model aerosol mass profiles were then used to prescribe the aerosol distribution in the GEOS free-running AGCM simulations. Initial results show an increase of cloud fractions by ~40% at areas of high aerosol loading over the ocean, when an elevated aerosol layer over the clouds was placed as opposed to having no absorbing aerosols over the ocean. Also, the presence of an elevated aerosol layer compared to an aerosol layer near the cloud-tops seems to hasten the stratocumulus to cumulus-like transition (SCT) of clouds in the model away from the coast. The possible explanations for the cloud increase and early onset of SCT transition will be discussed with respect to the changes in boundary layer properties owing to the redistribution of aerosol mass in the vertical within the model. Finally, aerosol impacts on top of the atmosphere and surface radiative forcing will be discussed over this key region of BB.