Observations of aerosol-cloud interactions with varying vertical separation between biomass-burning aerosols and stratocumulus clouds over the South-East Atlantic

Cloud and aerosol data collected above, within and below clouds from six research flights during the ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) field campaign in September 2016 were used to determine how the microphysical properties of marine stratocumulus clouds over the South-East Atlantic varied depending on whether overlying biomass-burning aerosols were present immediately above, or separated from cloud tops. Forty vertical cloud profiles were classified into two regimes, Mixing and Separated, according to whether the plume of densest aerosols was mixing into or separated from cloud tops. A statistical analysis showed that more numerous and smaller cloud droplets were sampled in the mixing regime with the median Nc 100 to 150 cm-3 higher and the median Re 1.5 to 2 µm lower than the separated regime. In addition, the median liquid water content (LWC) near cloud base was 0.06 g m-3 lower while similar LWC was sampled within cloud and near cloud top the LWC was 0.08 g m-3 higher. Inhomogeneous mixing near cloud top led to a decrease in the total number concentration and LWC by up to 28% and 20% respectively and led to an increase in the median volume diameter up to 29 µm for the separated regime. These patterns were observed regardless of the large-scale meteorological conditions. Precipitation suppression was observed during the mixing regime as a lower probability of high drizzle (D > 50 µm) concentration (>0.1 cm-3) was observed during the mixing regime (0.2) compared to the separated regime (0.24)