Supercell rear flank downdrafts (RFD) are associated with large horizontal vorticity, with vortex lines hypothesized to form rings around the downdraft. Under certain conditions, these rings can be drawn upward in an adjacent updraft, forming arches of vortex lines on one side of the downdraft. This is a compact way of describing a pair of counter-rotating vortices that straddle a region of horizontal vorticity associated with large gradients of vertical velocity. Tornadogenesis occasionally occurs in the cyclonic member of the vortex pair. The arching process is thought to depend on the RFD in at least two ways: 1) the negative buoyancy in the RFD generates the horizontal vorticity that is eventually tilted, forming the arches, and 2) the negative buoyancy of the RFD can be large enough to cause the vortex rings to simply spread at the ground, precluding the arching process. In this research, complementary observational and idealized numerical modeling, theoretical methods will be employed to study this problem.
Intellectual Merit: Supercell tornadogenesis apparently is the result of a complex series of processes. Evidence suggests that the vorticity in a tornado originates as horizontal vorticity between the supercell updraft and a trailing RFD. The RFD, in turn, appears to be partially the result of small-scale precipitation structures unique to supercells: the hook echo and/or a narrow descending reflectivity core. In some supercells, the vorticity generated between the major vertical drafts is drawn upward in the updraft, leading to arched vortex lines and associated counter-rotating vortices in the rear flank gust front convergence zone. Under certain conditions, that appear to be governed by the degree of negative buoyancy in the RFD, tornadogenesis can occur in the vicinity of the cyclonic member of the counter-rotating pair. Utilizing a state-of-the-art observational analysis tool suite and a cloud model, this research will further understanding of rear flank downdraft buoyancy, tornado cyclone genesis, and tornadogenesis.
Broader Impacts: This research has considerable value in the area of public safety. New understanding of RFD buoyancy and tornado cyclone genesis can be used directly in the tornado warning process by diagnosing rear-flank precipitation thickness and hydrometeor structure using Doppler radars with dual-polarization diversity capability. Further, it is anticipated that RFD buoyancy can be estimated through knowledge of the low-level thermodynamic stratification. Eventually, it is likely that operational meteorologists can make much better discriminations between potentially tornadic and non-tornadic supercells. Traditionally, the lead PI (Rasmussen) has been active in transferring new knowledge directly to National Weather Service and other forecasting organizations via seminars. Also, all of the co-PIs participate in conferences, and invited seminars to communicate new knowledge. These types of outreach work are planned to continue. Funding will support a female researcher and a female student, which should help facilitate the retention of women in science, as well as promote diversity.
