Concentrating Vorticity Near the Ground: An Investigation of the Interaction of Precipitation Processes and Flow Dynamics in Supercells and Other Severe Thunderstorms

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During the Springs of 1994 and '95, a field program entitled the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) was executed by a combination of NSF and National Oceanic and Atmospheric Administrations sponsored scientists. The primary objective of VORTEX was to evaluate several hypotheses pertaining to low-level mesocyclone intensification, and tornado formation, maintenance, and demise. The observations collected included ground-based (mobile and stationary) and airborne Doppler radars, mobile surface and balloon borne meteorological instruments. Under this award, the Principal Investigators will continue their ongoing research intended to advance state of knowledge of the life cycles of tornadoes. This goal will be accomplished through careful examination of data-rich case studies of the Dimmitt, Friona, and Wheeler (all in Texas) tornadic storms as well as other storms (some tornadic, some not) from the VORTEX and subsequent experiments. As a result of prior research, it has become apparent that future investigations must be expanded to cover a variety of closely related processes to understand tornadogenesis and tornado longevity. Among these are:-factors that govern the amount and type of precipitation that forms/descends in rear flank downdrafts (RFD) at the rear of supercell updrafts;-how this precipitation and its type is related to formation of RFDs;-how RFD generate vorticity and/or barotropically redistribute it:-how outflow from the RFD can be confined to the region below the updraft;-how RFDs lead to the formation of tornado cyclones, and whether the angular momentum of the tornado cyclone is maintained by the RFD or merely "spins down" from some initial distribution;-whether the RFD can lead directly to a tornado cyclone through largely barotropic processes;-whether the tornado cyclone forms from upward tilting of helicity-rich inflow air;-whether only one or both of the above processes is required for tornadogenesis;-whether the depth, size and shape of RFDs are relevant to tornadogenesis.Most of these questions can be, or have been, cast in the form of testable, refutable hypotheses. The Principal Investigators will emphasize the development of theories and models for RFD vorticity processes and the effect of precipitation in redistributing angular momentum in a mesocyclone-scale vortex. The Principal Investigators also will investigate the role of hydrometeors and water phase changes in forcing the kinematic structures.Successful completion of this research could lead to a reduction in tornado warning false alarm rate while maintaining adequate lead times.