Impact of a Tornado's Low–Reflectivity Eye on Distorting the Associated Peak Doppler Velocity Measurements: A Simulation Study

When simulating Doppler velocity measurements in a tornado, one frequently assumes that reflectivity is uniform across the tornado. However, 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.

At the 23rd Conference on Severe Local Storms, we presented simulated mean Doppler velocity measurements made across tornadoes that were generated using the Dowell et al. (2005) axisymmetric numerical model. We used 1.5–mm–diameter raindrops in the model to produce flow and raindrop concentration fields for three different–sized tornadoes. The model output then was scanned with a WSR–88D emulator to produce simulated reflectivity and mean Doppler velocity measurements within the tornadoes. The results show that the extreme mean Doppler velocity values associated with a low-reflectivity eye at relatively close ranges occur at a smaller radius than in the model tornado. The simulations indicate that this anomalous situation is not very evident in small tornadoes, but occurs out to ranges of 15–20 km for large tornadoes and out to 30–40 km for very large tornadoes using legacy WSR–88D sampling. With the new higher–resolution super–resolution sampling, the range to which the anomalies are evident increased by nearly 50%.

In order to explain why the extreme Doppler velocity values do not occur at the radius of maximum tangential flow, we analyze, in this paper, what is occurring within the beam as the radar scans across a nearby tornado. The anomalous situation occurs when (1) the beam center is slightly offset from the tornado center and (2) the only significant reflectivity values are at the far edge on one side of the beam near the location of the peak tangential velocities. When the radar processes the measurements within the beam, it assumes that reflectivity and Doppler velocity values are distributed throughout the beam. Consequently the mean reflectivity–weighted Doppler velocity value is assigned to the center of the beam. With the only significant reflectivity values being located at the edge of the beam where the peak Doppler velocities are found, the mean Doppler velocity based on the peak values is displayed at the azimuth of the beam center. Consequently, the reflectivity–weighted mean of the peak tangential velocities at the edge of the beam is displayed at the azimuth of the center of the beam—moving the peak mean Doppler velocity value much closer to the tornado center than would have occurred had the reflectivity distribution been uniform across the beam. The diameter of the peak tangential velocities in the tornado is thus significantly underestimated.