Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 6 2009 10.5194/acpd-9-25853-2009 http://www.atmos-chem-phys-discuss.net/9/25853/2009/ http://www.atmos-chem-phys-discuss.net/9/25853/2009/acpd-9-25853-2009.html http://www.atmos-chem-phys-discuss.net/9/25853/2009/acpd-9-25853-2009.pdf 25853 25883 2009-12-02 The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree? J. Uchida C. S. Bretherton breth@washington.edu P. N. Blossey Department of Applied Mathematics, University of Washington, Seattle, Washington, USA Department of Atmospheric Science, University of Washington, Seattle, Washington, USA The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM), to see if the two model types agree on the strength of the second aerosol indirect effect. Good agreement can be obtained if the MLM entrainment closure explicitly reduces entrainment efficiency proportional to the rate of cloud droplet sedimentation at cloud top for cases in which the LES-simulated boundary layer remains well mixed, with a single peak in the vertical profile of vertical velocity variance. <br><br> To achieve this agreement, the MLM entrainment closure and the drizzle parameterization must be modified from their observationally-based defaults. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. 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