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https://doi.org/10.5194/acp-2020-1039
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-1039
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  27 Oct 2020

27 Oct 2020

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This preprint is currently under review for the journal ACP.

Impact of the Variability in Vertical Separation between Biomass-Burning Aerosols and Marine Stratocumulus on Cloud Microphysical Properties over the Southeast Atlantic

Siddhant Gupta1,2, Greg M. McFarquhar1,2, Joseph R. O'Brien3, David J. Delene3, Michael R. Poellot3, Amie Dobracki4, James R. Podolske5, Jens Redemann2, Samuel E. LeBlanc5,6, Michal Segal-Rozenhaimer5,6,7, and Kristina Pistone5,6 Siddhant Gupta et al.
  • 1Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, OK, USA
  • 2School of Meteorology, University of Oklahoma, Norman, OK, USA
  • 3Department of Atmospheric Sciences, University of North Dakota, Grand Forks, ND, USA
  • 4Department of Atmospheric Sciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
  • 5NASA Ames Research Center, Moffett Field, CA, USA
  • 6Bay Area Environmental Research Institute, Moffett Field, CA, USA
  • 7Department of Geophysics and Planetary Sciences, Porter School of Environmental and Earth Sciences, Tel Aviv University, Tel Aviv, Israel

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 the 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 due to 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 was driven by the 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 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, Re, and LWC 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.

Siddhant Gupta et al.

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Data sets

ORACLES Science Team: Suite of Aerosol, Cloud, and Related Data Acquired Aboard P3 During ORACLES 2016, Version 2 NASA Ames Earth Science Project Office https://doi.org/10.5067/Suborbital/ORACLES/P3/2016_V2

Siddhant Gupta et al.

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Short summary
Observations from the 2016 NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign examine how biomass-burning aerosols from southern Africa affect marine stratocumulus cloud decks over the southeast Atlantic. Instances of contact and separation between aerosols and clouds are examined to quantify the impact of aerosol mixing into cloud top on cloud drop numbers and sizes. This information is needed for improving Earth system models and satellite retrievals.
Observations from the 2016 NASA ObseRvations of Aerosols above CLouds and their intEractionS...
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