Siddhant Gupta - April 3

Importance of Above and Below Cloud Aerosols in Determining Cloud Properties in Stratocumulus over the South East Atlantic Ocean

Siddhant Gupta

Wednesday, April 3^rd

NWC 5600/3:00 pm

Biomass-Burning Aerosols (BBA) from the African continent are transported over a large stratocumulus cloud deck off the west coast of Africa by the southern branch of the African Easterly Jet. The BBA typically overlap the cloud deck and can descend into the clouds with advection off the coast. Instances of both separation and contact between the cloud and aerosol layers have been observed using spaceborne lidars. In-situ measurements and remote sensing retrievals of aerosol and cloud properties above, within, and below stratocumulus clouds over the South-East Atlantic Ocean measured during the 2016 and 2017 ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) field experiments are presented.

Cloud droplet size distributions for 3 < D < 1280 µm were sampled by Cloud and Aerosol Spectrometer (CAS) and Two-Dimensional Stereo (2DS) probes on board a NASA P-3 aircraft to determine droplet concentration (N_c), effective radius (r_e) and liquid water content (LWC). The Passive Cavity Aerosol Spectrometer Probe (PCASP) was used to measure aerosol size distribution for accumulation-mode aerosols (0.1 < D < 3 µm) and calculate aerosol concentration (N_a). The following trends were observed across six P-3 Research Flights (PRFs) based in Walvis Bay, Namibia in September 2016 and eight PRFs based in Sao Tome in August 2017. Cloud properties were examined for clean (below-cloud N_a < 200 cm^-3) and dirty clouds when there was separation (above-cloud N_a < 500 cm^-3) or contact with BBA within 100 m above cloud tops. N_c increases by 105 to 132 cm^-3 (95% confidence intervals) and r_e decreases by 1.1 to 1.3 µm with contact between the two layers, relative to cases with separation, LWCs were within 0.04 g m^-3 for the different cases. The increase in N_c due to contact with above-cloud BBA is greater in dirty clouds, compared to clean clouds. For cases with separation between the two layers, cloud-top entrainment was hypothesized with negative buoyancy near cloud tops due to evaporative cooling in entrained air parcels.

An increase in N_c associated with contact between the BBA and cloud tops can also impact precipitation development. Precipitation susceptibility (S_o), the negative change in rain rate (R) with increase in N_c, is quantified. Positive S_o up to 1.2 for cases with above-cloud N_a < 500 cm^-3 indicates a decrease in rain rate with increasing N_c. In contrast, negative S_o up to -0.8 was observed for cases with above-cloud N_a > 500 cm^-3 which suggests an increase in R with increase in N_c associated with contact between the BBA and cloud tops.