Name: Siddhant Gupta
Title: Dependence of Cloud and Precipitation Properties over the South-East Atlantic on Aerosol Concentrations Above and Below Clouds
Location: NWC 1350
Time: 3:00 PM
Series: Weather and Climate Systems
Abstract: 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 Nc, effective radius Re, and precipitation susceptibility So (change in rain rate R with Nc) as a function of cloud thickness H, were determined for varying above- and below-cloud aerosol concentration Na. The data were sorted into 4 regimes according to whether a prominent aerosol layer (Na > 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 Nc and Re were calculated using the number distribution function n(D) for 3 < D < 1280 Î¼m from the Cloud and Aerosol Spectrometer and the 2Dâ€Stereo probe. Na was calculated using n(D) for 0.1 < D < 3 Î¼m 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 Î¼m. Across the 3 campaigns, a total of 359 vertical profiles were flown through clouds and for 181 contact, or C-cases (Na > 500 cm-3 within 100 m above cloud), the mean Nc was 77 cm-3 higher and mean Re 1.78 Î¼m lower, compared to 178 separated, or S-cases (Na < 500 cm-3 within 100 m above cloud). Within clean BLs (Na < 400 cm-3 within 100 m below cloud), mean Nc for C-cases was 42% higher than S-cases. Within dirty BLs (Na > 400 cm-3 within 100 m below cloud), mean Nc for C-cases was 53% higher, and the Nc change increased from 20% near cloud base to 75% near cloud top. Although cloud-top entrainment increased Nc throughout the cloud layer, its effect was weaker (stronger) near cloud base (top) within dirty BLs.
For all data combined, the average So was positive (0.94) implying R decreased with an increase in Nc. For C- and S-cases, the average So was 0.60 and 1.01, respectively, with the difference arising from low So (-0.06) for thin (H < 131 m) C-case clouds. When the thin, C-case clouds were further classified into clean & dirty BL cases, So was 0.67 and -0.57, respectively. Condensational growth is limited by low H in thin clouds and increasing Nc in clean conditions decreased Re, hindering collision-coalescence. However, in dirty conditions with low Re, increasing Nc only increased the number and collision efficiency of small droplets. The mean Nc (average SÂ¬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, Re decreases with Nc and clean clouds with the highest Re are the most susceptible to precipitation suppression.