Biomass burning aerosol transport and vertical distribution over the South African-Atlantic region

Optically thick smoke aerosol plumes originating from biomass burning (BB) in the southwestern African Savanna during the austral spring are transported westward by the free tropospheric winds to primarily overlie vast stretches of stratocumulus cloud decks in the southeast Atlantic. We evaluated the simulations of long-range transport of BB aerosol by the Goddard Earth Observing System (GEOS-5) and four other global aerosol models over the complete South African-Atlantic region using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations to find any distinguishing or common model biases. Models, in general, captured the vertical distribution of aerosol over land but exhibited some common features after long-range transport of smoke plumes that were distinct from that of CALIOP. Most importantly, the model-simulated BB aerosol plumes quickly descend to lower levels just off the western coast of the continent, while CALIOP data suggest that smoke plumes continue their horizontal transport at elevated levels above the marine boundary layer. This is crucial because the sign of simulated aerosol semidirect effect can change depending on whether the bulk of the absorbing aerosols is present within or above the cloud levels in a model. The levels to which the aerosol plumes get subsided and the steepness of their descent vary amongst the models and amongst the different subregions of the domain. Investigations into possible causes of differences between GEOS-5 and CALIOP aerosol transport over the ocean revealed a minimal role of aerosol removal process representation in the model as opposed to model dynamics.