Interpretation of simulated WSR-88D Doppler velocity signatures of tornadoes associated with nonuniform reflectivities

Typically, when simulated Doppler velocity measurements are made across a tornado, one assumes that the associated reflectivity field is uniform. However, recent measurements made by mobile Doppler radars in the immediate vicinity of tornadoes reveal the presence of a low-reflectivity eye centered on the tornado. The eye arises from the centrifuging of debris and hydrometeors within the tornadic circulation.

Dowell et al. (2005) employed an axisymmetric numerical model to study particle motions and concentrations in tornadoes. We used 1.5-mm-diameter raindrops in the model to produce flow and reflectivity patterns for three different sized tornadoes: medium, large, and very large. The model output then was scanned with a WSR-88D emulator to produce simulated reflectivity and Doppler velocity measurements within the tornadoes.

We found that, except for the rare very large tornado, peak Doppler velocity values associated with a low-reflectivity eye at close range occurred at a smaller radius than in the model tornado. These peak Doppler velocity values also were at a smaller radius than peak values associated with a uniform reflectivity pattern. As distance from the radar increased, the widening radar beam smeared the low-reflectivity eye to produce a more uniform distribution of reflectivity. At the same time, the peak Doppler velocity values approached those obtained for a uniform reflectivity distribution. Thus, in a typical tornado near a WSR-88D, we would expect the presence of a low-reflectivity eye to cause the peak Doppler velocity values to appear at a smaller radius than the radius of the true peak tangential velocities.