Introduction of a new cloud droplet nucleation scheme in a convective cloud model with bin microphysics

There were many previous studies that examined the effects of aerosols on cloud development and precipitation. While some observational and numerical studies report a decrease in precipitation in polluted clouds, in some other studies aerosol induced precipitation enhancement was reported. These mixed results are due to the fact that cloud development is affected not only by aerosol conditions but also by environmental conditions that determine cloud dynamics. Numerical cloud models are a valuable tool that can be used for studying the complex process of cloud development. Among the most important processes that should be represented well in the model in the study of aerosol effects on cloud and precipitation development is the cloud droplet nucleation process that calculates newly activated cloud droplet distribution, which is crucially related to the aerosol distribution. This study aims at improving the nucleation scheme in the cloud model originally developed by Takahashi (Takahashi, 1976, J. Atmos. Sci., 33, 269-286). Although bin microphysics is employed, this model assumes a prescribed initial droplet spectrum. One problem is that the prescribed initial droplet spectrum is too broad and fixed although supersaturation condition may change. In this study, we propose much more realistic and theoretically reasonable nucleation scheme than the original one as well as other currently available nucleation schemes. New approach assumes that newly activated droplets have the smallest size bin and the concentration of these droplets is determined by critical supersaturation as a function of effective hygroscopicity parameter, κ and of the dry particle volume equivalent diameter of aerosols (Petters and Kreidenweis, 2007, Atmos. Chem. Phys., 7,1961-1971). More detail will be discussed at the conference.