The Evolution of a Tornado: Ground-Based Velocity Track Display (GBVTD) Analysis of Mobile, W-Band Doppler Radar Data on 15 May 1999 near Stockton, Kansas

On 15 May 1999, a storm intercept team from the University of Oklahoma collected high-resolution, W-band Doppler radar data in a tornado near Stockton, Kansas. The radar used was a mobile, 3-mm wavelength, pulsed Doppler radar from the University of Massachusetts, having a half-power beamwidth of 0.18 degrees, an azimuthal resolution of 15 m at a range of 5 km, and a pulse resolution of 30 m. Thirty-five sector scans were obtained at a single elevation angle over a period of approximately ten minutes with an average interval of approximately 20 seconds between scans, capturing the tornado life cycle from just after tornadogenesis to the decay stage. A low-reflectivity “eye” – whose diameter changed markedly during the period of observation – was present in the reflectivity scans.

A Ground-Based Velocity Track Display (GBVTD) analysis of the W-band Doppler radar data of the Stockton tornado was conducted; results and interpretations are presented and discussed. It is found from the analysis that the axisymmetric component of the azimuthal wind profile of the tornado was suggestive of a Rankine-combined vortex during the most intense phase of life cycle of the tornado. At the same time, the axisymmetric radial wind profile was suggestive of a two-celled vortex structure at the level of the radar scan, which was approximately 100 m AGL.

The temporal evolution of the axisymmetric components of azimuthal and radial wind, as well as wavenumber-1, -2, and -3 angular harmonics of the azimuthal wind, are also presented. In particular, a quasi-stationary wavenumber-2 feature of the azimuthal wind was present in 25 out of the 35 scans. It is shown that this feature may result from the superposition of a wind field of deformation onto the vortex wind field.

From the GBVTD analysis, it is seen that the maximum azimuthally averaged azimuthal wind speed increased while the radius of maximum wind (RMW) decreased slightly during the intensification phase of the Stockton tornado. In addition, the maximum azimuthally averaged azimuthal wind speed and the RMW both decreased simultaneously as the tornado decayed. The circulation at the RMW increased as the tornado intensified, and decreased as the tornado decayed.