A011-0010
Cold air outbreaks during ACTIVATE: Observationally constrained large-eddy simulations of microphysically-mediated cloud regime transitions

Monday, 7 December 2020
Poster
Florian Tornow1,2, Andrew S Ackerman2, Ann M Fridlind2, Brian Cairns2, Ewan Crosbie3,4, Stefan Kaufmann5, Simon Kirschler5, Xiangyu Li6, Richard Moore4, Claire E Robinson3,4, Taylor J Shingler3,4, Michael Shook4, Armin Sorooshian7, Kenneth Lee Thornhill II4, Bastiaan van Diedenhoven1,2, Christiane Voigt5, Hailong Wang6, Edward Winstead3,4 and Luke D Ziemba4, (1)Columbia University, New York, United States, (2)NASA Goddard Institute for Space Studies, New York, NY, United States, (3)Science Systems and Applications, Inc., Hampton, VA, United States, (4)NASA Langley Research Center, Hampton, VA, United States, (5)German Aerospace Center DLR Oberpfaffenhofen, Institute of Atmospheric Physics, Oberpfaffenhofen, Germany, (6)Pacific Northwest National Laboratory, Richland, WA, United States, (7)University of Arizona, Department of Chemical and Environmental Engineering, Tucson, AZ, United States
Abstract:
Cold air outbreaks typically produce boundary layer clouds composed of both liquid and ice condensate. Driven by large surface heat fluxes, these clouds undergo rapid transitions from a stratiform to a convective regime and produce varying amounts of surface precipitation, the formation of which can deplete boundary-layer aerosol which in turn accelerates the transition between regimes.

The ACTIVATE campaign set out to better understand cloud-aerosol interactions by using in-situ and remote sensing instrumentation - sampling cloud and aerosol properties in and around the cloud fields – in concert with large-eddy simulations to verify process understanding. This study targets a cold air outbreak on March 1st 2020 over the Gulf Stream in the northwest Atlantic. We show how simulated micro- and macrophysical cloud and precipitation fields improve in comparison to observed ones, when ensuring that dynamic and thermodynamic variables as well as aerosol properties in the boundary layer and free troposphere match their respective observations. We further demonstrate how measurements -- mostly obtained from the stratiform regime -- have a significant impact on the simulated transition to the convective regime. This study highlights key parameters necessary to realistically simulate cold air outbreaks and steers future observations in upcoming campaign flights. Future sampling should target expected in-cloud portions of heightened riming prior to precipitation as well as boundary layers immediately following precipitation events and keep measuring particle size distributions and hygroscopicity, in order to verify transition drivers that we identified in our simulations. To avoid early icing of instrument inlets, these targets need to be carefully arranged in sequence. Continued dropsonde measurements at strategic locations will provide profiles of wind, temperature, and humidity and ensure that large-scale conditions in reanalysis fields are realistic.

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