The influence of droplet nucleation parameterization scheme on cloud and precipitation development of convective clouds

A cloud model is a potentially invaluable tool in studying cloud evolution. In particular, a spectral bin model is appropriate for the study of detailed cloud microphysical evolution and its interaction with the cloud dynamics. However, spectral bin models as well as bulk models employ parameterizations of some parts of cloud microphysical processes partially because of computational expense and partly because of the difficulty of treating cloud microphysical processes in the model. The parameterization of droplet nucleation process is a representative example. Modelers use several different approaches to calculate nucleation process in a cloud model. The purpose of this study is to examine the influence of parameterization scheme of droplet nucleation process on cloud and precipitation development of convective clouds. We used a 2D convective cloud model with bin microphysics and simulated the effect of CCN on cloud and precipitation development. Twomey's formula was used to calculate the number concentration of activated CCN and a gamma form was assumed for the initial size distribution of newly activated droplets by the nucleation process. Several tests were performed in order to determine an adequate initial droplet size distribution shape for different CCN concentrations. Basically we prescribed that the initial droplet size distribution was shifted to smaller sizes when CCN concentration was higher. Depending on the distribution shape, the cloud evolution appeared different. The crucial factor was found to be the difference of latent heat release, the main energy source for cloud convection. Although higher in activated droplet concentration, the polluted (higher CCN concentration) cloud had less amount of activated liquid water and therefore less latent heat release, owing to the smaller initial droplet size distribution, when compared to the maritime cloud. Furthermore, small droplet sizes of the polluted cloud led to faster evaporation because of the higher surface-to-volume ratio of droplets. The stronger evaporation for the polluted cloud led to weaker convection because the buoyancy was compensated more significantly for this cloud by the stronger evaporative cooling. More detailed analyses will be presented at the conference.