Does the threshold representation associated with the autoconversion process matter? H. Guo1, Y. Liu1, and J. E. Penner2 1Atmospheric Science Division, Brookhaven National Laboratory, Upton, NY, USA 2Department of Atmospheric, Oceanic and Space Sciences, the University of Michigan, Ann Arbor, MI, USA
Abstract. 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.
Citation: Guo, H., Liu, Y., and Penner, J. E.: Does the threshold representation associated with the autoconversion process matter?, Atmos. Chem. Phys., 8, 1225-1230, doi:10.5194/acp-8-1225-2008, 2008.