On the limits to near-surface intensification of tornado vortices

How large can near-surface tornado velocities become relative to velocities aloft? There is a physical feedback that tends to limit the magnitude of the near-surface intensification of a vortex: higher swirl velocities and pressure drops near the surface relative to conditions aloft imply a vertical pressure gradient that tends to drive a core downdraft, reducing the intensification. There is also a well known purely fluid-dynamic mechanism that can oppose this feedback: balancing the vertical pressure gradient with core updrafts (either central or annular) that decelerate with height, with the enhanced updraft at low levels supported by a radial overshoot in the vortex corner flow produced by cyclostrophic imbalance in the surface layer. A simple analytical model for a steady supercritical end-wall vortex (Barcilon 1967; Fiedler and Rotunno, 1986) suggests that the level of intensification that can be supported by this mechanism (as measured by the ratio of peak swirl velocities in the corner flow to peak swirl velocities aloft) is limited by the conservation of mass, angular momentum and vertical momentum to a factor ~2, a result supported by many laboratory and numerical studies. Here this model is generalized to consider corner flows besides the supercritical end-wall vortex, more general angular momentum distributions and time dependence. The model predicts the general structure and intensification of quasi-steady vortex corner flows as a function of corner flow swirl ratio in agreement with simulation results, and suggests how larger intensification factors can be achieved for some conditions with more complex surface layers or time evolution. Examples of these are realized using high-resolution large-eddy simulations. Quasi-steady nested inner and outer corner flows with intensification factors approximately twice any previously reported in quasi-steady state were realized by tuning the near-surface inflow layer at the outer boundary of the simulations. Many of the features observed in these simulations, and significant additional intensification, can be realized naturally without fine tuning in a class of unsteady evolutions producing a dynamic corner flow collapse. These scenarios, triggered purely by changes in the far-field near-surface flow, provide an attractive mechanism for naturally achieving an intense near-surface vortex from a much larger-scale less-intense swirling flow. Applied on different scales, this may sometimes play a role in tornadogenesis and/or tornado variability.

This work draws on results from two papers recently submitted to the Journal Of the Atmospheric Sciences and available at http://eiger.mae.wvu.edu/tornado.html.