Collaborative Research: Concentrating Vorticity Near the Ground: Investigation of Supercell Rear-Flank Precipitation, Vorticity Generation, and Transport Processes

Recent research has indicated the possibility of a critical role of the supercell rear flank downdraft (RFD) in the tornadogenesis process. A multi-sensor analysis of the Dimmitt tornadic storm observed during the Verification of Origins of Rotation in Tornadoes Experiment (VORTEX) has revealed vorticity patterns and evolution consistent with processes first described theoretically by prior researchers. Further, the observed patterns were very similar to those in most of the supercell case studies in the literature. In particular, the prior research of the Principal Investigators supported the conceptual model that vorticity is generated baroclinically as air passes through the RFD from rear to front. This baroclinically generated quasi-horizontal vorticity develops in rings about the buoyancy minimum present at the RFD owing to precipitation loading and/or melting/evaporation of hydrometeors. As the rings pass forward through the RFD, their leading sides are tilted upward in the gradient of vertical velocity present just ahead of the RFD, giving them an arch-like configuration. They are subsequently strongly stretched in the low-level updraft, leading to an intensification of a counter-rotating vortex pair (shown, but not elaborated upon, in most historical case studies). In the Dimmitt storm, the cyclonic member of the counter-rotating vortex pair (connected by arches of vortex lines) was augmented by intense low-level tilting of streamwise inflow vorticity. This member subsequently intensified into a tornado through stretching in the low-level updraft.

In this research, the Principal Investigators seek to advance the understanding of supercell RFD processes. They will concentrate on the following topics:

How is the development of the near-ground, counter-rotating vortex pair governed by downdraft strength, buoyancy, and the ambient shear profile? It is hypothesized that certain shear profiles lead to augmentation of one or both vortices, and/or partial or complete cancellation of one or both.

What factors allow for the development of neutral or positively-buoyant (near the ground) RFDs? It is hypothesized that the rate of inflow of potentially cold air aloft into the rear flank, and the distributions and types of hydrometeors, play dominant roles.

What factors govern the likelihood of baroclinically generated RFD vorticity being concentrated into a tornadic vortex?

Recently, it has been demonstrated that a measurable jump in tornado warning skill by the National Weather Service can be attributed to new scientific knowledge coming from the VORTEX experiment. Prior research findings of these Principal Investigators have been cited in real-time forecast products. The likely societal impact of this research is a continued measurable improvement in tornado forecasts and warnings, leading to continued reductions in losses of property, health, and lives. Further, the Principal Investigators will continue to emphasize direct outreach to the public (web site, public lectures, television documentaries, email queries) as well as to forecasters.