University of Oulu

Bougiatioti, A., Nenes, A., Lin, J. J., Brock, C. A., de Gouw, J. A., Liao, J., Middlebrook, A. M., and Welti, A.: Drivers of cloud droplet number variability in the summertime in the southeastern United States, Atmos. Chem. Phys., 20, 12163–12176,, 2020.

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

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Author: Bougiatioti, Aikaterini1,2; Nenes, Athanasios2,3,4; Lin, Jack J.2;
Organizations: 1Institute for Environmental Research and Sustainable Development, National Observatory of Athens, P. Penteli, 15236, Greece
2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
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
5Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
6Cooperative Institute for Research in Environmental Sciences, Univ. of Colorado, Boulder, CO 80309, USA
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 0.5 MB)
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Language: English
Published: Copernicus Publications, 2020
Publish Date: 2021-02-11


Here we analyze regional-scale data collected on board the NOAA WP-3D aircraft during the 2013 Southeast Nexus (SENEX) campaign to study the aerosol–cloud droplet link and quantify the sensitivity of droplet number to aerosol number, chemical composition, and vertical velocity. For this, the observed aerosol size distributions, chemical composition, and vertical-velocity distribution are introduced into a state-of-the-art cloud droplet parameterization to show that cloud maximum supersaturations in the region range 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 with a dry diameter of 90 nm and above. An important finding is that the standard deviation of the vertical velocity (σw) exhibits considerable diurnal variability (ranging from 0.16 m s⁻¹ during nighttime to over 1.2 m s⁻¹ during day), and it tends to covary with total aerosol number (Na). This σw–Na covariance amplifies the predicted response in cloud droplet number (Nd) to Na increases by 3 to 5 times compared to expectations based on Na changes alone. This amplified response is important given that droplet formation is often velocity-limited and therefore should normally be insensitive to aerosol changes. We also find that Nd cannot exceed a characteristic concentration that depends solely on σw. Correct consideration of σw and its covariance with time and Na is important for fully understanding aerosol–cloud interactions and the magnitude of the aerosol indirect effect. Given that model assessments of aerosol–cloud–climate interactions do not routinely evaluate for overall turbulence or its covariance with other parameters, datasets and analyses such as the one presented here are of the highest priority to address unresolved sources of hydrometeor variability, bias, and the response of droplet number to aerosol perturbations.

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Series: Atmospheric chemistry and physics
ISSN: 1680-7316
ISSN-E: 1680-7324
ISSN-L: 1680-7316
Volume: 20
Issue: 20
Pages: 12163 - 12176
DOI: 10.5194/acp-20-12163-2020
Type of Publication: A1 Journal article – refereed
Field of Science: 114 Physical sciences
1172 Environmental sciences
Funding: This research has been supported by the US Environmental Protection Agency (STAR grant no. R835410), the General Secretariat for Research and Technology (“Supporting of Postdoctoral Researchers” action of the Operational Program “Education and Lifelong Learning”), and the H2020 European Research Council (PyroTRACH; grant no. 726165).
Copyright information: © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.