Doppler Radar Analysis of the Northfield, Texas, Tornado of 25 May 1994

Abstract

A large tornado was observed near Northfield, Texas, on 25 May 1994 during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). An analysis of the tornado and its parent storm is discussed. Doppler wind velocity spectra of the tornado and its parent circulation, which were computed from data collected by a low-power, portable, FM-CW (frequency-modulated continuous-wave), 3-cm-wavelength Doppler radar, are presented at increments in the range of 78 m. The FM-CW radar data from the tornado are the first ever collected of high enough quality to analyze. The CW spectra computed from data collected by the portable radar, a pseudo-dual-Doppler analysis of airborne Doppler radar data collected by a National Oceanic and Atmospheric Administration P-3 aircraft, photogrammetric analysis of a video of the tornado, and a ground-based damage survey are discussed in the context of the FM-CW spectra. This study is unique in that both ground-based and airborne Doppler radar systems probed the tornado and its environment. Wind speeds of 60 m s?1 were indicated in the tornado in a swath 300 m across, with some smaller areas of possible wind speeds up to 75 m s?1. Circumstantial evidence is presented that the tornado originated along an elliptically shaped cyclone/shear zone along the leading edge of a large hook echo in its parent supercell storm. The tornado?s parent vortex (mesocyclone) was approximately 2 km in diameter and contained tangential wind speeds of 45?50 m s?1.