Rear flank downdrafts (RFD), tornado cyclones (TC), and tornadoes are some of the most distinctive dynamic and kinematic phenomena associated with supercell thunderstorms. Tornadoes, and less frequently RFDs and tornado cyclones, have been rigorously studied since the advent of various Doppler, and more recently polarimetric, radar technologies employing mobile platforms. In addition, a few studies using numerical model simulations with advanced microphysics and high-order numerics at very small grid spacings (25 to 125 m) have been presented in the literature. However, little has been revealed about the origins and evolution of the RFD and TC, and the possible role the RFD plays in the life cycle of the TC and tornado. The main objectives of this research are: a) To examine the dynamic forces and thermodynamic processes governing the origins, and temporal and spatial evolution of the RFD. b) To examine the dynamic forces and thermodynamic processes governing the origins, and temporal and spatial evolution of the TC. c) Based on this examination, develop a conceptual model that elucidates the dynamical differences between mesocyclones, tornado cyclones, and tornadoes, if these differences exist.
It is hoped that new knowledge might be gained regarding the evolution of RFDs, their association with the life cycles of TCs, and in what ways TCs are related to tornadoes, by using observational data from the Verifications of the Origins of Rotation in Tornadoes Experiments VORTEX and VORTEX2, as well as numerically modeled information using the Straka Atmospheric Model.
Intellectual Merit: Supercell RFDs, the genesis of TCs, and tornadoes are the result of a complex series of nonlinear processes. Evidence suggests that the vorticity in a tornado originates as horizontal vorticity between the supercell updraft and an associated 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 initially horizontal vortex lines generated between the major vertical drafts are drawn upward in the updraft, which leads to arched vortex lines and associated low-level counter-rotating vortices in the rear flank gust front convergence zone. Under certain conditions, this process appears to be governed by the degree of negative buoyancy in the RFD, and tornadogenesis can occur in the vicinity of the cyclonic member of the counter-rotating vortex pair. This research work, which utilizes state-of-the-art observations, observational analysis tool suites and cloud models, will hopefully help elucidate the physical and dynamical relationships between RFDs, TC genesis, and tornadogenesis.
Broader Impacts: The project has considerable value in the area of public safety, especially with two upgrades forthcoming to the WSR-88D radars. The first of these radar upgrades will include over sampling to 1/2 deg in azimuth and removal of the current radial smoothing of reflectivity. The second upgrade will be the introduction of dual polarimetric radar capabilities across the nation sometime during late 2010. New understanding of RFD buoyancy and tornado cyclone genesis can be used directly in the tornado warning process by diagnosing rear-flank precipitation depth and hydrometeor structure using Doppler radars with dual-polarization diversity capability. Eventually, it is likely that operational meteorologists will be able to make much better distinctions between potentially tornadic and non-tornadic supercells. One of the lead PIs has been active in transferring new knowledge directly to NWS 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. The co-PIs are contemplating a K-12 oriented website, which describes various forms of severe weather associated with tornadoes. Funding will support a faculty member at OU, two senior research scientists (one of which is female), and two female Ph.D. students to work on interpretation of VORTEX, VORTEX2 and other observational analyses as well as numerical simulation results.
