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Discussion papers
https://doi.org/10.5194/acp-2020-225
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-225
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Mar 2020

Submitted as: research article | 17 Mar 2020

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This preprint is currently under review for the journal ACP.

Drivers of cloud droplet number variability in the summertime Southeast United States

Aikaterini Bougiatioti1,2, Athanasios Nenes2,3,4, Jack J. Lin2,a, Charles A. Brock5, Joost de Gouw5,6,b, Jin Liao5,6,c,d, Ann M. Middlebrook5, and Andre Welti5,6,e Aikaterini Bougiatioti et al.
  • 1Institute for Environmental Research & Sustainable Development, National Observatory of Athens, P. Penteli, 15236, Greece
  • 2School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
  • 3Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, 1015, Lausanne, Switzerland
  • 4Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, 26504, Greece
  • 5Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, 80305, USA
  • 6Cooperative Institute for Research in Environmental Sciences, Univ. of Colorado, Boulder, CO, 80309, USA
  • anow at: Nano and Molecular Systems Research Unit, Box 3000, 90014 University of Oulu, Oulu, Finland
  • bnow at: Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO, USA
  • cnow at: Atmospheric Chemistry and Dynamic Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • dnow at: Universities Space Research Association, GESTAR, Columbia, MD, USA
  • enow at: Atmospheric Composition Research Unit, Finnish Meteorological Institute, Helsinki, Finland

Abstract. The Southeast United States has experienced a different climate warming trend compared to other places worldwide. Several hypotheses have been proposed to explain this trend, one being the interaction of anthropogenic and biogenic aerosol precursors that synergistically promote aerosol formation, elevate cloud droplet concentration and induce regional cooling. We examine these aerosol-cloud droplet links by analyzing regional scale data collected onboard the NOAA WP-3D aircraft during the 2013 Southeast Nexus (SENEX) campaign to quantify the sensitivity of droplet number to aerosol number, chemical composition and vertical velocity on a regional scale. The observed aerosol size distributions, chemical composition and vertical velocity distribution (Gaussian with standard deviation σw) are introduced into a state-of-the-art cloud droplet parameterization to show that cloud maximum supersaturations in the region are low, ranging from 0.02 to 0.52 % with an average of 0.14 ± 0.05 %. Based on these low values of supersaturation, the majority of activated droplets correspond to particles of diameter 90 nm and above. Droplet number shows little sensitivity to total aerosol owing to their strong competition for water vapor. Given, however, that σw exhibits considerable diurnal variability (ranging from 0.16 m/s during nighttime to over 1.2 m/s during day), its covariance with total aerosol number (Na) during the same period amplifies predicted response in cloud droplet number (Nd) by 3 to 5 times. Therefore, correct consideration of vertical velocity and its covariance with time and aerosol amount is important for fully understanding aerosol-cloud interactions and the magnitude of the aerosol indirect effect. Datasets and analysis such as the one presented here can provide the required constraints for addressing this important problem.

Aikaterini Bougiatioti et al.

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Aikaterini Bougiatioti et al.

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Short summary
The number concentration of droplets in clouds in the summertime southeastern United State is influenced by aerosol variations but limited by the strong competition for supersaturated water vapor. Concurrent variations in vertical velocity magnifies the response of cloud droplet number to aerosol increases by up to a factor of 5. Omitting the covariance omit the covariance of vertical velocity with aerosol number may therefore bias estimates of the cloud albedo effect from aerosols.
The number concentration of droplets in clouds in the summertime southeastern United State is...
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