The Role of the Size Distribution Shape in Determining Differences between Condensation Rates in Bin and Bulk Microphysics Schemes

Abstract

The condensation rates predicted by bin and bulk microphysics schemes in the same model framework are compared in a novel way using simulations of non-precipitating shallow cumulus clouds. The bulk scheme generally predicts lower condensation rates than does the bin scheme, even when the saturation ratio and the integrated diameter of the droplet size distribution are identical. Despite other fundamental disparities between the bin and bulk condensation parameterizations, the differences in condensation rates are predominantly explained by accounting for the shape of the cloud droplet size distributions simulated by the bin scheme. This shape is not well constrained by observations and thus it is difficult to know how to appropriately specify it in double-moment bulk microphysics schemes. However, this study shows that enhancing our observations may be important since the choice of distribution shape can have a large impact on condensation rates, changing them by 50 % or more in some cases.