Near-Surface Vortex Intensification through Corner Flow Collapse

Abstract

Results are presented from a large set of large-eddy simulations of a class of unsteady vortex evolution that may sometimes play a role in tornadogenesis or tornado variability. Beginning with a high-swirl parent vortex with an excess of low-swirl flow through the surface/corner/core region, perturbation of the low-swirl near-surface inflow at a large radius can trigger a subsequent dynamic ?corner flow collapse? producing dramatic near-surface intensification relative to conditions aloft of an order of magnitude or more in velocity scale. This paper presents a more detailed treatment of the physics and simulation of corner flow collapse, expanding upon the presentation given in a companion paper treating near-surface vortex intensification more generally for both steady and unsteady conditions. The basic scaling of the onset, intensification, structure, and duration of the phenomenon is explored as a function of some of the dominant physical parameters involved. A dimensionless rate of change of the low-swirl flux through the surface/corner flow during the process is identified as a critical governing parameter. Given the mode of triggering near the surface at large radii, the large intensification that can result, and the sensitivity to some of the parameters involved, corner flow collapse may provide a mechanism by which the rear-flank downdraft can promote tornadogenesis and help explain why seemingly similar conditions sometimes produce intense tornadoes and other times do not.