Comparisons of Doppler velocity tornadic vortex signatures with signatures from model vortices

Doppler velocity tornadic vortex signatures (TVSs) produced when WSR-88Ds scan through tornadoes are compared with three axisymmetric vortex models of simulated TVSs to determine which model best represents the measurements. The three vortex models are the Rankine combined vortex (RCV), a modified Rankine combined vortex (MRCV), and the Burgers-Rott vortex (BRV). For all three models, the tangential velocity is zero at the center of the vortex, increases with increasing radius (R), reaches a maximum value at the core radius, and then decreases as the radial distance continues to increase. The difference between the RCV and MRCV models is that tangential velocity decreases beyond the core radius as 1/R for RCV and as 1/(R power 0.6) for MRCV. The (M)RVC and BRV have different shapes of the tangential velocity profiles in the vicinity of the core radius. The RCV and MRCV profiles are pointed where two different tangential velocity distributions meet, while the BRV profile is rounded owing to a balance between inward advection and outward diffusion of angular momentum.

A simulated WSR-88D radar was used to produce simulated TVS profiles for the three types of vortices at the same ranges as actual TVS measurements. Since a TVS is produced when the radar beamwidth is larger than the tornado, past studies have shown that the peak values of a TVS are separated by about one beamwidth. The smeared Doppler velocity measurements do not represent the size or strength of the measured tornado, so a variety of simulated tornado sizes and strengths can produce essentially identical TVS profiles having the same peak Doppler velocity value. This coincidence permits one to fit measured TVS data points to a simulated TVS curve without having to know the size or strength of the actual tornado. The procedure used in this study was to produce a set of simulated TVS profiles having different peak Doppler velocity values. A series of data points resulting from an azimuthal scan through a TVS were then compared to each simulated profile for each vortex model and the best fit was noted. The results of these comparisons will be presented at the conference.