The Sensitivity of Numerically Simulated Cyclic Mesocyclogenesis to Variations in Model Physical and Computational Parameters

Abstract

In a previous paper, a three-dimensional numerical model was used to study the evolution of successive mesocyclones produced by a single supercell storm, that is, cyclic mesocyclogenesis. Not all supercells, simulated or observed, exhibit this behavior, and few previous papers in the literature mention it. As a first step toward identifying and understanding the conditions needed to produce cyclic redevelopments within supercell updrafts, this paper examines the effect on cyclic mesocyclogenesis of variations in model physical and computational parameters. Specified changes in grid spacing, numerical diffusion, microphysics options, and the coefficient of surface friction are found to alter, in some cases dramatically, the number and duration of simulated mesocyclone cycles. For example, a decrease from 2.0 to 0.5 km in horizontal grid spacing transforms a nearly perfectly steady, noncycling supercell into one that exhibits three distinct mesocyclone cycles during the same time period. Decreasing the minimum vertical grid spacing at the ground tends to speed up the cycling process, while increasing it has the opposite effect. Ice microphysics is shown to cut short the initial cycling, while both simple surface friction and increased numerical diffusion tend to slow it down. Combining competing effects (i.e., ice microphysics with friction) tends to bring the simulation back to the evolution found in the control case. Explanations for these results are offered in the context of nonlinear feedbacks associated with the cycling process. In addition, the implications of these findings in our understanding of storm behavior as well as in the context of storm-scale numerical weather prediction are discussed.