Vortex Generation in a Numerical Thunderstorm Model

Abstract

Detailed analysis of mesocyclone formation was performed using a three-dimensional numerical thunderstorm model. The vorticity equation was used to determine rates of generation and advection of vorticity in the modeled mesocyclones. Experiments were run with varied base state wind profiles to examine the role of vertical wind shear in the formation of vortices within thunderstorms. Analysis of mechanisms involved in vortex growth showed that during early and mature storm regimes important contributions were made by three processes: tilting of horizontal vorticity into the vertical stretching of these vortices by wind convergence, and vertical advection of vorticity. The combination of these processes served to create strong vortex couplets through extensive vertical columns in the modeled storms. Numerical experiments also showed that mature storm mesovortex strength was a nonlinear function of ambient vertical wind shear magnitude. Maximum vortex strength reached or exceeded 10?2 s?1 vorticity. It also was found that turning of the ambient wind with height affected severe storm structure through vortex couplet orientation. Centers of tilting of horizontal vorticity were largely responsible for the relative positions of the cyclonic and anticyclonic mesocyclones.