The Tornadic Vortex Signature (TVS)

A Tornadic Vortex Signature (TVS) is a degraded Doppler velocity signature that occurs when a tornado is smaller than the effective beamwidth of the sampling Doppler radar. Soon after the TVS was discovered in the mid–1970s, simulations were conducted to verify that the signature did indeed represent a tornado. The simulations, which used a uniform reflectivity distribution across the tornado, indicated that the extreme Doppler velocity values should be separated by about one effective beamwidth. For a radar with an effective beamwidth of approximately 1.0 degree and data collected at 1.0–degree azimuthal intervals, the two extreme Doppler velocity measurements should appear at adjacent azimuths (“gate–to–gate”)—as observed. 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.

With the advent, during the 1990s, of mobile Doppler radars that make measurements very close to tornadoes, it was confirmed that the centrifuging of radar–detectable particles produces a reflectivity minimum (or hole) at the center of tornadoes. However, the coarser reflectivity measurements made by radars at greater distances rarely detect the presence of the hole, which is why the uniform reflectivity assumption was made in the early simulations. To study what effect the centrifugally–induced hole might have on the TVS, we produced numerically-modeled tornadoes and then sampled them with a simulated WSR–88D. We found that the presence of centrifuged radar–detectable particles—even when not obvious in the reflectivity measurements—does have an influence on the character of the TVS. 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.