Contour Dynamics of Tornado-like Vortices

Abstract

Contour dynamics (CD) is applied to study the mechanism responsible for the breakup of an isolated tornado-like vortex into multiple vortices, the nonlinear interaction between a tornado and its parent storm, and the impact of tornadoes, which are subgrid features in any existing prediction model, on the more resolvable storm-scale motion.The genesis of the multiple vortex via the linear and eventually nonlinear barotropic/inertial instability of an observed tornado's mean tangential wind profile is studied in unprecedented detail using a high-resolution CD model. The most unstable eigenmodes obtained from the linear stability analyses were used as the initial perturbations to initialize the CD model. Despite that multiple vortices at the fully nonlinear stage bear little resemblance to the linear eigenmode structure, it is found that normal-mode barotropic/inertial instability can, indeed, trigger the formation of these vortices. In addition, the number of the secondary vortices identified in the fully nonlinear phase is found to be equal to the most unstable azimuthal wavenumber.The interaction between a tornado and its environment (the parent storm) is studied using highly idealized initial conditions. The formation of the hook-echo-like flow pattern by the outside air spiraling in is revealed in the high-resolution CD simulations. It is found that the rotation of the parent storm may be strongly influenced by the feedback from the tornado.