ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-4039-2009 How important is the vertical structure for the representation of aerosol impacts on the diurnal cycle of marine stratocumulus? Sandu I. 1 2 Brenguier J.-L. 1 Thouron O. 1 Stevens B. 2 3 CNRM-GAME, Meteo-France/CNRS, Toulouse, France Max Planck Institute for Meteorology, Hamburg, Germany University of California at Los Angeles, Los Angeles, CA, USA 19 06 2009 9 12 4039 4052 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/4039/2009/acp-9-4039-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/4039/2009/acp-9-4039-2009.pdf

Large-Eddy Simulations (LES) are performed to examine the impact of hygroscopic aerosols on the diurnal cycle of marine stratocumulus clouds, under varying meteorological forcing conditions. When the cloud condensation nuclei concentration increase is sufficient to inhibit drizzle formation in the cloud layer, the precipitating and the non-precipitating cloud layers exhibit contrasting evolutions, with noticeable differences in liquid water path. Aerosol-induced modifications of the droplet sedimentation and drizzle precipitation result in noticeable changes of the entrainment velocity at cloud top, but also in significant changes of the vertical stratification in the boundary layer. This set of simulations is then used to evaluate whether a model which does not explicitly represent the effects of the interactions occurring within the boundary layer on its vertical stratification (i.e. such as a mixed-layer model) is capable of reproducing at least the sign, if not the amplitude, of these aerosol impacts on the liquid water path. It is shown that the evolution of the vertical structure is key to the responses we simulate, and must be considered in bulk models that wish to predict the impact of aerosol perturbations on the radiative properties of stratocumulus-topped boundary layers.

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