EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Cloud droplet variability in the summertime in the southeast United States: day vs. night

Aikaterini Bougiatioti1,2, Athanasios Nenes2,3,4, Jack Lin2,5, Charles Brock6, Joost de Gouw6,7,8, Jin Liao6,7,9, Ann Middlebrook6, and Andre Welti6,7,10
Aikaterini Bougiatioti et al.
  • 1National Observatory of Athens, IERSD, Athens, Greece (
  • 2Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, 26504 Patras, Greece
  • 3Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
  • 4Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, 26504 Patras, Greece
  • 5Nano and Molecular Systems Research Unit, University of Oulu, 90014 Oulu, Finland
  • 6Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 7Cooperative Institute for Research in Environmental Sciences, Univ. of Colorado, Boulder, CO 80309, USA
  • 8Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO 80309, USA
  • 9Atmospheric Chemistry and Dynamic Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
  • 10Atmospheric Composition Research Unit, Finnish Meteorological Institute, 00560 Helsinki, Finland

During the 2013 Southeast Nexus (SENEX) campaign, in-situ observational data were collected on board the NOAA WP-3D aircraft to study the aerosol-cloud droplet link and examine the sensitivity of the cloud droplet number to aerosol physicochemical parameters. In order to do so, observed aerosol number size distributions, chemical composition and vertical-velocity distributions were introduced into a state-of-the-art cloud droplet parameterization from which cloud droplet number and cloud maximum supersaturations were derived. We find that the standard deviation of the vertical velocity (σw) exhibits significant diurnal variability ranging from 0.16 m s-1 during nighttime to over 1.2 m s-1 during day. Total aerosol number (Na) covaries with σw , with lower values observed during nighttime. The covariance between σw and Na enhances the apparent response of Nd to changes in Na levels by a factor of 5. For the same “cleaner” environments where Na values are limited and not impacted by local sources, the relative response of Nd to σw is almost twice as great during night, compared to the day (24% during day vs. 42% during night). On the other hand, in environment with enhanced concentrations, especially of accumulation-mode particles, the majority of droplet number variability is attributed to changes in total aerosol number rather than changes in σw. Chemical composition is found to on-average have a limited effect on Nd variability (4%). Finally, we identify an upper limit to the number of droplets that can form in clouds which depends only on σw independently from total aerosol number. Doubling σw from 0.2 to 0.3 m s-1increases this limiting droplet number by 52%.When Nd values approach this upper limit the observed droplet variability is driven by σw and, subsequently, by vertical-velocity changes only. Therefore only by using this -σw relationship in regions where velocity-limited conditions are expected, σw can be estimated from retrievals of droplet number and vice versa.

How to cite: Bougiatioti, A., Nenes, A., Lin, J., Brock, C., de Gouw, J., Liao, J., Middlebrook, A., and Welti, A.: Cloud droplet variability in the summertime in the southeast United States: day vs. night, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16245,, 2021.

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