Formation of intense vortical circulations through a vortex sheet roll-up process: applications to surface tornadogenesis

The vortex sheet roll-up process is ubiquitous in atmospheric dynamics across all spatial scales. A significant characteristic of this process is that it focuses a relatively low amplitude vortex sheet into a tube of maximum curvature vorticity. This resulting curvature vorticity subsequently develops a dynamically balanced pressure response.

Vortex sheet roll-ups are only sustainable when flow into, or out of, the vorticity axis of the roll-up is restricted. In the case of a supercell environment, vertically oriented roll-ups are generated along vertically oriented vortex sheets formed by surface deformation processes, such as those along the rear-flank, or forward-flank, gust fronts. Unrestricted roll-up processes of this type are commonly referred to as “gustnadoes”, and do not directly lead to tornadogenesis. However, when occurring under the mesocyclone of a supercell storm, downward acceleration of vertical flow drawn into the roll-ups from above is shown to be restricted by the vertical dynamic pressure gradient produced by the mesocyclone aloft. This allows roll-ups to develop to tornado intensities.

Our numerical study suggests that the roll-up of a vortex sheet along a supercell's rear-flank gust front is an important dynamical process leading to surface tornadogenesis and vortex intensification aloft. This presentation will discuss the dynamics of the roll-up process in more detail and draw comparisons with existing vortex line analyses of pre-tornadogenesis environments.