Abstract

Warm rain collision coalescence has been persistently difficult to parameterize in bulk microphysics schemes. Here we use a flexible bulk microphysics scheme with bin scheme process parameterizations, called AMP, to investigate reasons for the difficulty. AMP is configured in a variety of ways to mimic bulk schemes and is compared to simulations with the bin scheme upon which AMP is built. We find that the biggest limitation in traditional bulk schemes is the use of separate cloud and rain categories. When the drop size distribution is instead represented by a continuous distribution with or without an explicit functional form, the simulation of cloud-to-rain conversion is substantially improved. We find that the use of an assumed double-mode gamma distribution and the choice of predicted distribution moments do somewhat influence the ability of AMP to simulate rain production, but much less than using a single liquid category compared to separate cloud and rain categories. Traditional two category configurations of AMP are always too slow in producing rain due to their struggle to capture the emergence of the rain mode. Single category configurations may produce rain either too slowly or too quickly, with too slow production more likely for initially narrow droplet size distributions. However, the average error magnitude is much smaller using a single category than two categories. Optimal moment combinations for the single category approach appear to be linked more to the information content they provide for constraining the size distributions than to their correlation with collision-coalescence rates.