ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-8917-2009 Sensitivity studies of different aerosol indirect effects in mixed-phase clouds Lohmann U. 1 Hoose C. 2 Institute of Atmospheric and Climate Science, ETH Zurich, Switzerland University of Oslo, Department of Geosciences, Oslo, Norway 26 11 2009 9 22 8917 8934 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/9/8917/2009/acp-9-8917-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/8917/2009/acp-9-8917-2009.pdf

Aerosols affect the climate system by changing cloud characteristics. Using the global climate model ECHAM5-HAM, we investigate different aerosol effects on mixed-phase clouds: The glaciation effect, which refers to a more frequent glaciation due to anthropogenic aerosols, versus the de-activation effect, which suggests that ice nuclei become less effective because of an anthropogenic sulfate coating. The glaciation effect can partly offset the indirect aerosol effect on warm clouds and thus causes the total anthropogenic aerosol effect to be smaller. It is investigated by varying the parameterization for the Bergeron-Findeisen process and the threshold coating thickness of sulfate (SO<sub>4</sub>-crit), which is required to convert an externally mixed aerosol particle into an internally mixed particle. Differences in the net radiation at the top-of-the-atmosphere due to anthropogenic aerosols between the different sensitivity studies amount up to 0.5 W m<sup>&minus;2</sup>. This suggests that the investigated mixed-phase processes have a major effect on the total anthropogenic aerosol effect.

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