Impact of the Variability in Vertical Separation between BiomassBurning Aerosols and Marine Stratocumulus on Cloud Microphysical Properties over the Southeast Atlantic

Marine stratocumulus cloud properties over the southeast Atlantic Ocean are impacted by contact between above-cloud biomass-burning aerosols and cloud tops. Different vertical separations (0 to 2000 m) between the aerosol layer and cloud tops were observed on six research flights in September 2016 during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. There were 30 contact profiles where the aerosol layer with aerosol concentration (Na) > 500 cm-3 was within 100 m of cloud tops, and 41 separated profiles where the aerosol layer with Na > 500 cm-3 was located more than 100 m above cloud tops. For contact profiles, the average cloud droplet concentration (Nc) in the cloud layer was up to 68 cm-3 higher, the effective radius (Re) up to 1.3 µm lower and the liquid water content (LWC) within 0.01 g m-3 compared to separated profiles. Free tropospheric humidity was higher in the presence of biomass-burning aerosols and contact profiles had a smaller decrease in humidity (and positive buoyancy) across cloud tops due to higher median above-cloud Na (895 cm-3) compared to separated profiles (30 cm-3). Due to droplet evaporation from entrainment mixing of warm, dry free tropospheric air into the clouds, the median Nc and LWC for contact profiles decreased with height by 21% and 9% in the top 20% of the cloud layer. The impact of droplet evaporation was stronger during separated profiles as a greater decrease in humidity (and negative buoyancy) across cloud tops led to greater decreases in median Nc (30%) and LWC (16%) near cloud tops.