Precipitation Susceptibility of Marine Stratocumulus with Variable Above and Below-Cloud Aerosol Concentrations over the Southeast Atlantic: Results from ORACLES

Marine stratocumulus with variable above- and below-cloud aerosol concentration Na were sampled over the southeast Atlantic Ocean during the NASA ObseRvations of Aerosols above Clouds and their intEractionS ORACLES field campaign in Sep. 2016, Aug. 2017, and Oct. 2018. In-situ measurements of aerosol and cloud properties from 329 cloud profiles across 24 research flights in clean and polluted boundary layers were examined. The droplet concentration Nc and effective radius Re were compared for different aerosol conditions and precipitation susceptibility So (negative rate of change in rain rate Rp with change in Nc) was determined as a function of cloud thickness H (where positive So implies that Rp decreased with increase in Nc). Nc and Re were calculated using the droplet number distribution function n(D) for diameter D > 3 m measured using in situ cloud probes on the NASA P-3B aircraft. The out-of-cloud Na was calculated using n(D) for aerosols with 0.1 < D < 3 m. Rp was derived using the droplet mass and fall speeds for drizzle drops (D > 50 m). During 173 contact or C-cases, Na > 500 cm-3 was sampled within 100 m above cloud tops, and during 156 separated or S-cases, Na < 500 cm-3 was sampled up to 100 m above cloud tops. On average, C-cases had 90 cm-3 higher Nc and 1.6 m lower Re compared to S-cases. The aerosol-induced changes in Nc and Re led to precipitation suppression with 50 % lower Rp for C-cases compared to S-cases. The Nc and Re differences were lower within clean boundary layers with below-cloud Na < 350 cm-3 (31 cm-3 and 0.5 m) compared to polluted boundary layers (108 cm-3 and 1.8 m). For C-cases and S-cases combined, So had little variation for thin clouds with 29 < H < 256 m (0.54 to 0.68) before increasing to 1.13 for H > 256 m. C-cases had lower So (0.87) relative to S-cases (1.08) with different trends in So versus H. The difference in So was highest for thin clouds (H < 129 m) as C-cases had low So (-0.06) compared to S-cases (1.47) due to poor correlation between Rp and Nc. So increased with H for C-cases and decreased with H for S-cases with 29 < H < 256 m before increasing for S-cases with H > 256 m. These trends in So versus H were dependent on the differences in Nc and Re for C-cases and S-cases and the impact of Nc and Re on droplet growth processes like collision-coalescence with height in cloud. Along with insignificant differences in meteorological conditions, these results confirm the hypothesis that disagreements between So for stratocumulus calculated in previous studies are due to differences in aerosol conditions.