ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-1225-2008 Does the threshold representation associated with the autoconversion process matter? Guo H. 1 Liu Y. 1 Penner J. E. 2 Atmospheric Science Division, Brookhaven National Laboratory, Upton, NY, USA Department of Atmospheric, Oceanic and Space Sciences, the University of Michigan, Ann Arbor, MI, USA 04 03 2008 8 5 1225 1230 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/8/1225/2008/acp-8-1225-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/1225/2008/acp-8-1225-2008.pdf

Different ad hoc threshold functions associated with the autoconversion process have been arbitrarily used in atmospheric models. However, it is unclear how these ad hoc functions impact model results. Here systematic investigations of the sensitivities of climatically-important properties: CF (cloud fraction), LWP (liquid water path), and AIE (aerosol indirect effect) to threshold functions have been performed using a 3-D cloud-resolving model. It is found that the effect of threshold representations is larger on instantaneous values than on daily averages; and the effect depends on the percentage of clouds in their transitional stages of converting cloud water to rain water. For both the instantaneous values and daily averages, the sensitivity to the specification of critical radius is more significant than the sensitivity to the "smoothness" of the threshold representation (as embodied in the relative dispersion of droplet size distribution) for drizzling clouds. Moreover, the impact of threshold representations on the AIE is stronger than that on CF and LWP.

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