Observations of tornadogenesis with a 3-mm-wavelength mobile Doppler radar

In the spring of 1999 a field experiment was conducted in the Southern Plains of the U. S., during which a mobile, mm-wavelength pulsed Doppler radar from the University of Massachusetts was used by a storm-intercept team from the University of Oklahoma to collect data in tornadoes and developing tornadoes. The antenna has a half-power beamwidth of only 0.18 deg, so that the azimuthal resolution is very high. This presentation gives an overview of datasets collected on 15 May and 5 June. Most of the life history of tornadoes on these days was documented. Evidence is presented that the tornadoes formed at an inflection point where the hook echo met the rear-flank downdraft. Also discussed is a boundary-layer VAD taken minutes before the tornado on 15 May formed. Evidence is presented that boundary-layer horizontal vorticity advected into the storm may have been important in tornadogenesis. A very thin band of reflectivity curling up into the tornado, the "umbilical cord," bears resemblance to a feature exhibited by the vortex roll-up process.