16A.1 A diagnostic study of a numerically generated supercell tornado vortex

Thursday, 14 October 2010: 4:30 PM
Grand Mesa Ballroom F (Hyatt Regency Tech Center)
Gregory J. Tripoli, University of Wisconsin, Madison, WI; and M. L. Büker

A numerical simulation of an idealized supercell tornado is studied through dynamical decomposition. The study reveals that a process of vortex mergers occur in the presence of a vorticity skirt, or a region of vorticity gradient between the primary vortex and the environment, similar to those reported in ongoing studies of the tropical cyclone core vortex. This vorticity skirt enhances the vortex's ability to absorb surrounding vorticity through vortex merger processes. Three primary sources of vorticity appear to be important. First, downdraft pulses produced by variability in the precipitation field generate vortons (elementary vortex loops) that are merged into the main vortex body. These mergers are hypthosized to occur through a two-stage process: gyroscopically driven alignment of the vorton occurs in the presence of the main vortex body, followed by upgradient forcing (see Büker and Tripoli, 2010, Severe storms Conf.) These weakly negatively buoyant entities form a subvortex within the vortex wall of the mesocyclone that is driven to the surface, causing tornadogenesis. The second source of vorticity includes the collection of randomly oriented vortons behind the rear flanking gust front where near-neutral stability and the presence of vortons resulting from previous downdrafts and turbulence shed from the main vortex populate the environment. The third source region occurs near the surface, where vortons are initially horizontally oriented and created by surface friction, particularly near the tornado core. These merger processes steepen the vorticity gradient on the wall of the primary vortex and reduce the vorticity skirt influence, that eventually help isolate the primary vortex from this environment. Also, as the vorticity gradient on the outside of the tornadic vortex wall steepens, the production of turbulence by deformation is reduced in scale, weakening the ability of this deformation to mix across the walls and thus maintain the vorticity skirt. This effectively decouples the vortex from destructive mixing with the surrounding environment. Numerical simulations of this processes and diagnostic forcing will be presented.
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