Nonaxisymmetric, unsteady tornadic corner flows

Large-Eddy Simulations of the tornado-scale wind field are used to study the corner-flow region, where the turbulent interaction of the tornado with the surface produces maximum wind speeds, for boundary conditions which are nonaxisymmetric and/or time varying. This continues previous work (Lewellen et. al., JAS 1997 p.581, JAS 2000 p.527) which concentrated more on boundary conditions which were both quasi-steady and axisymmetric. The introduction of significantly nonaxisymmetric near surface flow, for example due to large translation velocities of the tornado, strongly affects the corner flow structure. In general it tilts and twists the main vortex, reduces the effective corner flow swirl ratio, and when secondary (suction) vortices are present generally decreases their number, strengthening some while weakening others. Time varying near surface conditions lead to a progression of different corner flow dynamics. For example a high swirl vortex with multiple secondary vortices can evolve into a narrow, intense, low swirl vortex before separating from the surface and dramatically weakening, as the volume of low swirl fluid near the surface is increased with time. When the volume of flow is being increased by translation of the tornado, an individual secondary vortex evolves into a highly asymmetric primary vortex as the translation becomes sufficiently strong with respect to its swirl.

We intend to show to what extent rapidly evolving vortices can lead to differing intensities from that obtained from a series of quasi-steady states, and use our simulation results to speculate on whether the asymmetry introduced by the translation, and possibly the rear flank downdraft gust front, into the near surface inflow layer play a role in determining when an existing supercell low level mesocyclone produces a tornado.