11 Mar 2022
11 Mar 2022
Status: this preprint is currently under review for the journal ACP.

Seasonal updraft speeds change cloud droplet number concentrations in low level clouds over the Western North Atlantic

Simon Kirschler1,2, Christiane Voigt1,2, Bruce Anderson3, Ramon Campos Braga4, Gao Chen3, Andrea F. Corral5, Ewan Crosbie3, Hossein Dadashazar5, Richard F. Ferrare3, Valerian Hahn1,2, Johannes Hendricks1, Stefan Kaufmann1,2, Richard Moore3, Mira L. Pöhlker6,7, Claire Robinson3, Amy J. Scarino3, Dominik Schollmayer1,2, Michael A. Shook3, K. Lee Thornhill3, Edward Winstead3, Luke D. Ziemba3, and Armin Sorooshian5,8 Simon Kirschler et al.
  • 1Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany
  • 2Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Mainz, Germany
  • 3NASA Langley Research Center, Hampton, VA, USA
  • 4Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 5Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona, USA
  • 6Experimental Aerosol and Cloud Microphysics Department, Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 7Faculty of Physics and Earth Sciences, Leipzig Institute for Meteorology, University of Leipzig, Leipzig, Germany
  • 8Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, Arizona, USA

Abstract. Low level clouds over the Western North Atlantic show a seasonal cycle in cloud properties which anticorrelates to aerosol concentrations. To determinate the impact of dynamic and aerosol processes within marine low clouds we examine the seasonal impact of updraft speed w and cloud condensation nuclei concentration at 0.43 % supersaturation (NCCN0.43 %) on the cloud droplet number concentration (NC) of low level clouds over the Western North Atlantic Ocean. Aerosol and cloud properties were measured with instruments on board the NASA LaRC Falcon HU-25 during the ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment) mission in summer (August) and winter (February-March) 2020. The data are grouped in different NCCN0.43 % loadings and the density functions of NC and w near the cloud bases are compared. For low updrafts (w < 1.3 m s-1), NC in winter are mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts (w  > 3 m s-1), NC are impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol limited and for high pollution it is influenced by aerosols and updraft. The aerosol size distribution in winter shows a bimodal distribution in clean marine environments, which transforms to a unimodal distribution in high pollution levels due to altering processes, whereas unimodal distributions prevail in summer with a significant difference in their aerosol concentration and composition. The increase in pollution level is accompanied with an increase of organic aerosol and sulfate compounds in both seasons. We demonstrate that NC can be explained by cloud condensation nuclei activation through upwards processed air masses with varying fractions of activated aerosols. The activation highly depends on w and thus supersaturation between the different seasons, while the aerosol size distribution additionally affects NC within a season. Our results quantify the seasonal influence of w and NCCN0.43 % on NC and can be used to improve the representation of low marine clouds in models.

Simon Kirschler et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-171', Anonymous Referee #1, 11 Apr 2022
  • CC1: 'Publisher's comment on acp-2022-171', Copernicus Publications, 29 Apr 2022
  • RC2: 'Comment on acp-2022-171', Anonymous Referee #2, 10 May 2022

Simon Kirschler et al.

Data sets

Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment Data Armin Sorooshian

Simon Kirschler et al.


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Short summary
In this study we show that the vertical velocity dominantly impacts the cloud droplet number concentration (Nc) of low-level clouds over the Western North Atlantic in the winter and summer season, while the cloud condensation nuclei concentration, aerosol size distribution and chemical composition impacts Nc within a season. The observational data presented in this study can evaluate and improve the representation of aerosol-cloud interactions for a wide range of conditions.