Dependence of Cloud and Precipitation Properties over the South-East Atlantic on Aerosol Concentrations Above and Below Clouds

Instances of contact and separation between biomass-burning aerosols and marine stratocumulus have been observed over the South-East Atlantic Ocean. In-situ measurements of aerosol and cloud properties were made onboard the NASA P-3 aircraft during the ObseRvations of Aerosols above Clouds and their intEractionS ORACLES field campaign in Sep. 2016, Aug. 2017 and Oct. 2018. Variations in vertical profiles of droplet concentration N_c, effective radius R_e, and precipitation susceptibility S_o (change in rain rate R with N_c) as a function of cloud thickness H, were determined for varying above- and below-cloud aerosol concentration N_a. The data were sorted into 4 regimes according to whether a prominent aerosol layer (N_a > 500 cm^-3) was in contact or separated from cloud tops, and whether the clouds occurred within a clean or dirty boundary layer (BL).

The N_c and R_e were calculated using the number distribution function n(D) for 3 < D < 1280 μm from the Cloud and Aerosol Spectrometer and the 2D‐Stereo probe. N_a was calculated using n(D) for 0.1 < D < 3 μm from the Passive Cavity Aerosol Spectrometer Probe, and rain rate R was derived using droplet mass m(D), fall speed u(D) and n(D) for 50 < D < 1280 μm. Across the 3 campaigns, a total of 359 vertical profiles were flown through clouds and for 181 contact, or C-cases (N_a > 500 cm^-3 within 100 m above cloud), the mean N_c was 77 cm^-3 higher and mean R_e 1.78 μm lower, compared to 178 separated, or S-cases (N_a < 500 cm^-3 within 100 m above cloud). Within clean BLs (N_a < 400 cm^-3 within 100 m below cloud), mean N_c for C-cases was 42% higher than S-cases. Within dirty BLs (N_a > 400 cm^-3 within 100 m below cloud), mean N_c for C-cases was 53% higher, and the N_c change increased from 20% near cloud base to 75% near cloud top. Although cloud-top entrainment increased N_c throughout the cloud layer, its effect was weaker (stronger) near cloud base (top) within dirty BLs.

For all data combined, the average S_o was positive (0.94) implying R decreased with an increase in N_c. For C- and S-cases, the average S_o was 0.60 and 1.01, respectively, with the difference arising from low S_o (-0.06) for thin (H < 131 m) C-case clouds. When the thin, C-case clouds were further classified into clean & dirty BL cases, S_o was 0.67 and -0.57, respectively. Condensational growth is limited by low H in thin clouds and increasing N_c in clean conditions decreased R_e, hindering collision-coalescence. However, in dirty conditions with low R_e, increasing N_c only increased the number and collision efficiency of small droplets. The mean N_c (average S­_o) in the cleanest and dirtiest conditions (S-case & clean BL and C-case & dirty BL) was 84 cm^-3 and 206 cm^-3 (1.17 and 0.43), respectively. When condensational growth is no longer limited by H, R_e decreases with N_c and clean clouds with the highest R_e are the most susceptible to precipitation suppression.