Relationships between mixed–phase microphysical collection and modeled precipitation in various regimes

It is well known that the explicit prediction of precipitation by a numerical model is often highly sensitive to its microphysics scheme. In particular, care must be taken with the representation of microphysics in mixed-phase regions, because of the existence of numerous competing processes, and because the collection rates between different precipitation-sized species can be quite large. It has been found that mixed-phase fractional collection rates can exceed unity for timesteps of plausible sizes, due to failure to account for the discrete nature of collection properly. These problems become most acute when explicit bulk microphysics scheme are used to model midlatitude systems on relatively large spatial scales.

Here the numerical prediction of hydrometeor and stratiform precipitation magnitudes is examined for different microphysics specifications applied across varying meteorological regimes and length scales. The cases include a tropical cyclone remnant that transforms to a midlatitude-type system over time. The numerical model used is the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®). The microphysics specifications are the default model scheme and various modifications: among these are the replacement of the graupel category by a denser, faster-falling hail category; and the use of bulk collection equations that properly bound collection across all hydrometeor sizes. Among other results, it was found that the bounded scheme produced little effect during the tropical stage; however, during the midlatitude stage, the maximum precipitation intensity decreased, while the area experiencing precipitation increased and became more uniform. Further comparisons and implications will be discussed.