Comparison of hydrometeor fall speed distributions in bin and bulk microphysical schemes

Using the Weather Research and Forecast (WRF) model, idealized two dimensional simulations of squall lines were performed to compare fall speed distributions from a spectral (bin) microphysical scheme to three widely-used bulk schemes. The bin model exhibited wider spectra of fall speed distributions compared to any of the bulk schemes. More specifically, the bin model had more frequent low fall speeds for all hydrometeors. The bin model also exhibited the slowest falling rain due to a relatively crude method for treating the melting of ice. A microphysical budget analysis indicated that the melting of graupel contributed to the smaller fall speeds in the bin model, whereas both snow melting and graupel melting were mostly responsible for the larger fall speeds in the bulk schemes. The method for dealing with the melting of ice as well as a different solution for condensation in the bin scheme might have contributed to a weaker system in the bin scheme compared to the bulk schemes. The weaker flow in the bin scheme might have also acted to limit the amount of advection experienced by the hydrometeors resulting in stratiform and anvil regions that are similar to or narrower than those simulated in the bulk schemes. Comparison of hydrometeor fall speeds also reveals potential flaws in specific physical aspects found in one or more of the bulk schemes.