The Relative Impact of Ice Fall Speeds and Microphysics Parameterization Complexity on Supercell Evolution

Many properties of hydrometeors are not well understood, notably size distributions and diameter-fall speed relationships. Thus, many different parameterizations of these properties exist for the modeling community to use. In the past, many studies have observed significant changes in storms when different size distributions are used; but fall speed parameterizations have not been studied as thoroughly. In this study, four supercell simulations were run with the Regional Atmospheric Modeling System (RAMS), two using the standard 2-moment bulk microphysics scheme and two using the Hebrew University Spectral Bin Model. Two different sets of diameter-fall speed relationships were incorporated in each scheme, one with higher fall speeds for hail, graupel, and aggregates at all diameters than the other. Despite fundamental differences between the parameterization schemes, both high fall speed storms split with strong left-movers while neither low fall speed storm splits. Higher fall speeds lead to higher density of hydrometeors in air at low levels and thus more melting. Additionally, higher fall speeds lead to more condensate loading. Thus, stronger downdrafts and cold pools exist in the high fall speed storms, and these stronger cold pools lead to storm splitting and the intensification of a left-mover.