Simulated tornadic vortex and reflectivity signatures of numerically modeled tornadoes having weak echo holes

A tornadic vortex signature (TVS) is a degraded Doppler velocity signature that occurs when a tornado is smaller than the effective beamwidth of a sampling Doppler radar. Early simulations, which used a uniform reflectivity distribution across the tornado, showed that the extreme Doppler velocity values were separated by about one effective beamwidth. For a scanning radar with the effective beamwidth of approximately 1.0 degree and data collected at 1.0-degree azimuthal intervals, the two extreme Doppler velocity measurements appear at adjacent azimuths (“gate-to-gate”) as expected. However, with the recent beginning of 0.5-degree azimuthal sampling (“super resolution”) by WSR-88D radars, the extreme measurements unexpectedly appear at 0.5-degree instead of 1.0-degree azimuthal intervals.

So, in order to investigate why the super-resolution Doppler velocity measurements do not agree with the earlier simulations that assumed uniform reflectivity across the tornado, we used the results of a numerical tornado model to obtain a more realistic nonuniform reflectivity profile. Radar simulations based on the numerical model results show that the presence of centrifuged radar-detectable particles does have an influence on the character of the TVS even when not obvious in reflectivity measurements made at a distance from a radar. The simulated data indicate that the extreme Doppler velocity values of the TVS should be separated by 0.5 to 1.0 beamwidth, consistent with the WSR-88D super-resolution measurements.