Rapid development of tornado like vortices by simulated supercells

Abstract

The Regional Atmospheric Modeling System (RAMS) is used to examine the evolution of low-level vorticity beneath a modeled supercell thunderstorm. The simulations are performed using seven-species bulk microphysics in a horizontally-homogeneous domain, and are initialized through the use of a warm bubble. A mesocyclonic circulation becomes apparent a kilometer above the ground after 45 minutes of simulation time, and vertical vorticity of similar magnitude develops along the gust front near the surface by 55 minutes. Approximately five minutes later a transition occurs; the maximum vorticity beneath cloud base becomes vertically co-located, and translates with nearly constant grid-relative velocity. The vorticity increases by a factor of five during the five minutes following the co-location of the vorticity. An intense pressure deficit develops within the region of closed streamlines. It is shown that the concentration of vorticity is not dynamically forced by vertical pressure gradients or buoyancy, but occurs in a quasi-horizontal framework. The process is similar to models of non-supercell tornado-genesis in the literature that invoke the non-linear pooling of vorticity by barotropic processes, but with modification due to the presence of large-scale plane convergence. Specifically, the plane convergence allows the vorticity to concentrate at much faster time scales and without need of the coalescence of several discrete vorticity centers. The relevance to observed tornadic vortices and the implications for future modeling work are discussed.