Early Performance of a Combined Cloud-Rain Liquid Category for Simulating Warm Rain Production

Climate models continue to struggle with the simulation of warm rain from low-level stratocumulus and cumulus clouds. We are investigating whether some of this struggle could be mitigated by a fundamental change to the structure of the bulk microphysics schemes used by climate models and high resolution models alike. Bulk schemes have separate categories for cloud droplets and rain. For each category, the mass mixing ratio is predicted, and increasingly likely also the number mixing ratio. Such a structure necessitates the transfer of droplets from one category to another due to collision-coalescence. Such a transfer is ultimately an artificial process that is necessitated by the structure of bulk schemes. Simulations have repeatedly been shown to be sensitive to the choices regarding when and how to transfer the droplets. We have developed a bulk scheme called AMP which is flexible in its structure – it can either have a traditional structure with separate cloud and rain categories or a single liquid category that accounts for cloud and rain simultaneously. Both configurations use the same number of prognostic variables. Box model simulations of collision-coalescence have shown that the single category approach can substantially improve the evolution the drop size distribution. The improvement holds in idealized single column tests as well as preliminary large eddy simulations. We will discuss the advantages of the single category approach and reasons why it is performing better than traditional two separate category approach.