Analysis of the Semidirect Effect on Stratocumulus Clouds in the SE Atlantic Using In Situ and Remote Sensing Measurements from the ORACLES Campaign

Marine stratocumulus clouds (MSc) play an important role in climate, producing a net cooling effect in the global radiation budget. Biomass burning aerosols, when found in the same locations as the semi-permanent decks of MSc, alter the impact of these clouds. Solar absorption by biomass burning aerosol located above the cloud deck heats the atmosphere, which can further stabilize the lower troposphere and thereby strengthen the cloud deck, known as the semi-direct effect. This effect is thought to be active over the southeast Atlantic, but to date has only been demonstrated in modeling and satellite analyses. Questions remain on the importance of a meteorological contribution and accompanying changes in the boundary layer thermodynamic and microphysical structure. The NASA ORACLES aircraft campaign in the southeast Atlantic provides multiple sources of in-situ and remotely-sensed measurements of cloud properties, such as liquid water path, cloud droplet number concentration, the above-cloud aerosol optical depth, and in-situ derived thermodynamic profiles. These measurements are coordinated with remotely-sensed sea surface temperatures as a first step towards investigating the presence of the semi-direct effect over the southeast Atlantic during the ORACLES campaign. The focus is on September of 2016, when large aerosol loadings were present above a much cleaner boundary layer, so that the semi-direct effect should in theory have been active during this time.