Dependence of Cloud and Precipitation Properties over the South-East Atlantic on Aerosol Concentrations Above and Below Clouds
2:00 – 3:00 pm MST
Observations of marine stratocumulus clouds and biomass-burning aerosols were made over the southeast Atlantic Ocean during the NASA ObseRvations of Aerosols above Clouds and their intEractionS ORACLES field campaign. These observations were used to quantify microphysical changes in the marine stratocumulus due to a layer of biomass-burning aerosols overlaying the clouds. The southeast Atlantic acts as a natural laboratory for investigating aerosol-cloud interactions because of variability in the vertical displacement between the above-cloud aerosols and cloud tops. Instances of contact and separation between the layers were observed during 359 cloud profiles from 25 ORACLES research flights conducted in September 2016, August 2017, and October 2018.
In-situ cloud and aerosol probes were installed on the NASA P-3 aircraft. The droplet number distribution function n(D) was measured for droplets with diameter D between 3 to 1280 μm using the Cloud and Aerosol Spectrometer CAS and the 2-Dimensional Stereo Probe 2-DS. N(D) for accumulation-mode aerosols (0.1 < D < 3 μm) was measured using the Passive Cavity Aerosol Spectrometer Probe PCASP. Droplet concentration Nc and effective radius Re were calculated using the droplet n(D) and aerosol concentration Na was calculated using the aerosol n(D). Rain rate R was derived using the droplet mass and fall speeds for drizzle (D > 50 μm). 181 cloud profiles, when a prominent aerosol layer (Na > 500 cm-3) was sampled within 100 m above cloud tops, were termed “contact” or C-cases. 178 cloud profiles, when the level of Na > 500 cm-3 was sampled at least 100 m above cloud tops, were termed “separated” or S-cases. On average, C-cases had higher Nc (by up to 80 cm-3) and lower Re (by up to 2.7 μm) compared to S-cases. These cases were further classified into clean boundary layers (Na < 400 cm-3) or pollutedboundary layers (Na > 400 cm-3). The differences between Nc/Re were lower within clean boundary layers (41 cm-3/1.5 μm) compared to polluted boundary layers (79 cm-3/2.7 μm).
Precipitation susceptibility So (change in rain rate R with Nc) was quantified as a function of cloud thickness H. Positive So implies a decrease in R with an increase in Nc and S-cases were more susceptible to precipitation suppression with higher So (1.12) compared to C-cases (0.92). For C-case thin (H < 131 m) clouds, So was positive (1.6) in clean but negative (-0.5) in polluted boundary layers. For clean cases, this is consistent with higher Nc leading to lower Re by hindering collision-coalescence and drizzle formation and leading to lower R. For polluted conditions, when Re was already low, negative So is consistent with higher Nc increasing the number and collision efficiency of small droplets and leading to higher R. On average, S-case clouds within clean boundary layers had the lowest Nc, the highest R, and the highest susceptibility (So) to aerosol-induced precipitation suppression. The implications of these findings for the understanding of aerosol-cloud interactions occurring in marine stratocumulus and for the understanding of their climate impact are discussed.