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

Abstract. 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 (N_a) > 500 cm⁻³ was within 100 m of cloud tops, and 41 separated profiles where the aerosol layer with N_a > 500 cm⁻³ was located more than 100 m above cloud tops. For contact profiles, the average cloud droplet concentration (N_c) in the cloud layer was up to 68 cm⁻³ higher, the effective radius (R_e) 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 N_a (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 N_c 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 N_c (30 %) and LWC (16 %) near cloud tops.

Below-cloud N_a was sampled during 61 profiles, and most contact profiles (20 out of 28) were within high-N_a (> 350 cm⁻³) boundary layers while most separated profiles (22 out of 33) were within low-N_a (< 350 cm⁻³) boundary layers. Although, the differences in below-cloud N_a were statistically insignificant, contact profiles within low-N_a boundary layers had up to 34.9 cm⁻³ higher N_c compared to separated profiles. This was driven by the weaker impact of droplet evaporation in the presence of biomass-burning aerosols within 100 m above cloud tops. For contact profiles within high-N_a boundary layers, the presence of biomass-burning aerosols led to higher below-cloud N_a (up to 70.5 cm⁻³) and additional droplet nucleation above cloud base along with weaker droplet evaporation. Consequently, the contact profiles in high-N_a boundary layers had up to 88.4 cm⁻³ higher N_c compared to separated profiles. These results motivate investigations of aerosol-cloud-precipitation interactions over the southeast Atlantic since the changes in N_c, R_e, and LWC induced by the presence of above-cloud biomass burning aerosols are likely to impact precipitation rates, liquid water path and cloud fraction, and modulate closed to open cell transitions.