Impact of the variability in vertical separation between biomass burning aerosols and marine stratocumulus on cloud microphysical properties over the Southeast Atlantic

Abstract

Abstract. Marine stratocumulus cloud properties over the Southeast Atlantic Ocean are impacted by contact between above-cloud biomass burning aerosols and cloud tops. Different vertical separations (0 to 2000 m) between the aerosol layer and cloud tops were observed on six research flights in September 2016 during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. There were 30 contact profiles, where an aerosol layer with aerosol concentration (Na) > 500 cm−3 was within 100 m of cloud tops, and 41 separated profiles, where the aerosol layer with Na > 500 cm−3 was located more than 100 m above cloud tops. For contact profiles, the average cloud droplet concentration (Nc) in the cloud layer was up to 68 cm−3 higher, the effective radius (Re) up to 1.3 µm lower, and the liquid water content (LWC) within 0.01 g m−3 compared to separated profiles. Free-tropospheric humidity was higher in the presence of biomass burning aerosols, and contact profiles had a smaller decrease in humidity (and positive buoyancy) across cloud tops with higher median above-cloud Na (895 cm−3) compared to separated profiles (30 cm−3). Due to droplet evaporation from entrainment mixing of warm, dry free-tropospheric air into the clouds, the median Nc and LWC for contact profiles decreased with height by 21 and 9 % in the top 20 % of the cloud layer. The impact of droplet evaporation was stronger during separated profiles as a greater decrease in humidity (and negative buoyancy) across cloud tops led to greater decreases in median Nc (30 %) and LWC (16 %) near cloud tops. Below-cloud Na was sampled during 61 profiles, and most contact profiles (20 out of 28) were within high-Na (> 350 cm−3) boundary layers, while most separated profiles (22 out of 33) were within low-Na (< 350 cm−3) boundary layers. Although the differences in below-cloud Na were statistically insignificant, contact profiles within low-Na boundary layers had up to 34.9 cm−3 higher Nc compared to separated profiles. This is consistent with a weaker impact of droplet evaporation in the presence of biomass burning aerosols within 100 m above cloud tops. For contact profiles within high-Na boundary layers, the presence of biomass burning aerosols led to higher below-cloud Na (up to 70.5 cm−3) and additional droplet nucleation above the cloud base along with weaker droplet evaporation. Consequently, the contact profiles in high-Na boundary layers had up to 88.4 cm−3 higher Nc compared to separated profiles. These results motivate investigations of aerosol–cloud–precipitation interactions over the Southeast Atlantic since the changes in Nc and Re induced by the presence of above-cloud biomass burning aerosols are likely to impact precipitation rates, liquid water path, and cloud fraction, and modulate closed-to-open-cell transitions.

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

“…Three important factors affecting So were discussed (Sorooshian et al, 2019): above-cloud Na, below-cloud Na, and 650 meteorological conditions. This study analyzed ORACLES data from all three IOPs and the first two conclusions were consistent with the analysis of ORACLES 2016 (Gupta et al, 2021). Future work will compare in situ data with Rp retrievals from the Airborne Precipitation Radar (Dzambo et al, 2021) to evaluate the sensitivity of So to the use of satellite retrievals of Rp (Bai et al, 2018).…”

“…The above-cloud aerosol plume was associated with elevated water vapor content which influenced cloud-top humidity and dynamics following the mechanisms discussed by Ackerman et al (2004). 115 During the 2016 IOP, variable vertical displacement (0 to 2000 m) was observed between above-cloud aerosols and the MSC (Gupta et al, 2021;hereafter G21). Instances of contact and separation between the aerosol and cloud layers were associated with differences in the aboveand below-cloud Na, water vapor mixing ratio (wv), and cloud-top entrainment processes.…”

Joint cloud water path and rainwater path retrievals from airborne ORACLES observations

“…Stratocumulus clouds (Sc hereafter) are the dominant cloud type over Earth's subtropical oceans. Significant advances in our understanding of subtropical Sc have been documented in many previous studies including their typical cloud fractions (Stephens et al, 2012;King et al, 2013;, radiative impacts (Hang et al, 2019), and associated precipitation processes using data from past field experiments (Stevens et al, 2003), long-term ground-based data collection (Mann et al, 2014;Yang et al, 2018), and spaceborne remote sensors (Fox and Illingworth, 1997;L'Ecuyer and Stephens, 2002;Lebsock et al, 2011;Chen et al, 2011;Lebsock et al, 2011;Werner and Deneke, 2020). Despite these advances, until recently observations of cloud and precipitation processes over the Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

“…Results so far have shown that smoke from southern African biomass burning is often found within the remote southeast Atlantic boundary layer (Zuidema et al, 2018) and can have important radiative (Zhang and Zuidema, 2019) and microphysical (Gupta et al, 2021) effects. Important caveats have also been identified, including that methods that relate cloud properties to above-cloud rather than below-cloud aerosol concentrations likely misrepresent aerosol microphysical effects on clouds (Diamond et al, 2018); that at high smoke concentrations, clouds move from an aerosol-limited to an updraft-limited regime in which cloud sensitivity to further aerosol increases is limited (Kacarab et al, 2020); and that some of the lowest aerosol concentrations observed at Ascension Island occur during the biomass burning season, likely due to in-cloud scavenging (Pennypacker et al, 2020).…”