EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Dilution of boundary layer cloud condensation nucleus concentrations by free tropospheric entrainment during marine cold air outbreaks

Florian Tornow1,2, Andrew Ackerman2, Ann Fridlind2, Brian Cairns2, Ewan Crosbie3,4, Simon Kirschler5,6, Richard Moore3, Claire Robinson3,4, Chellappan Seethala7, Michael Shook3, Christiane Voigt5,6, Edward Winstead3,4, Luke Ziemba3, Paquita Zuidema7, and Armin Sorooshian8,9
Florian Tornow et al.
  • 1Earth Institute, Columbia University, NY, 10025, NY
  • 2NASA Goddard Institute for Space Studies, NY, 10025, NY
  • 3NASA Langley Research Center, Hampton, VA 23681, USA
  • 4Science, Systems, and Applications, Inc., Hampton, VA 23681, USA
  • 5Deutsches Zentrum fuer Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany
  • 6Johannes Gutenberg-Universitaet, Mainz, Germany
  • 7Rosenstiel School of Marine and Atmosphere Science, University of Miami, FL, 33149, USA
  • 8Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona, 85721, USA
  • 9Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, Arizona, 85721, USA

Mid-latitude marine cold air outbreaks (CAOs) occur in the post-frontal sector of extratropical cyclones. Once advected over the ocean, the marine boundary layer (MBL) quickly deepens and hosts near-overcast clouds that transition into an open-cellular cloud field downwind, mediated by a reduction in aerosol concentrations. Typically, the MBL experiences strong large-scale subsidence that is often associated with free-tropospheric (FT) dry intrusions. Apart from being relatively warm and dry, FT air may have substantially different aerosol properties and, thus, different cloud condensation nuclei (CCN) concentrations compared to the MBL.

In this study, we examine the difference between MBL and FT air by using in-situ and remote sensing observations collected during NASA's ACTIVATE (Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment) field campaign in the northwest Atlantic. Analysis of the 8 CAO flights in 2020 reveals predominantly far lesser CCN concentrations in the FT than in the MBL. We investigate one representative flight more deeply, through a fetch-dependent MBL CCN budget that has contributions from sea-surface fluxes, hydrometeor collision-coalescence, and entrainment of FT air. We find a dominant role of FT entrainment in reducing MBL CCN concentrations upwind of strong precipitation that results in cloud regime transition, consistent with satellite-retrieved gradients in droplet number concentration upwind of precipitation.

The FT circulation and its relative lack of CCN can accelerate overcast-to-broken cloud transitions, especially where MBL air is CCN-rich (e.g., near continents), and thereby dramatically reduce regional albedo.


How to cite: Tornow, F., Ackerman, A., Fridlind, A., Cairns, B., Crosbie, E., Kirschler, S., Moore, R., Robinson, C., Seethala, C., Shook, M., Voigt, C., Winstead, E., Ziemba, L., Zuidema, P., and Sorooshian, A.: Dilution of boundary layer cloud condensation nucleus concentrations by free tropospheric entrainment during marine cold air outbreaks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10285,, 2022.

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