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Howard B. Bluestein, James G. Ladue, Herbert Stein, Douglas Speheger, and Wesley F. Unruh

Abstract

A storm-intercept team from the University of Oklahoma, using the Los Alamos National Laboratory portable, continuous wave/frequency modulated–continuous wave, 3-cm Doppler radar, collected close-range data at and below cloud base in six supercell tornadoes in the southern plains during the springs of 1990 and 1991. Data collection and analysis techniques are described. Wind spectra from five weak-to-strong tornadoes and from one violent tornado are presented and discussed in conjunction with simultaneous boresighted video documentation, photogrammetric analysis, and damage surveys.

Maximum Doppler wind speeds of 55–105 m s−1 were found in five of the tornadoes; wind speeds as high as 120–125 m s were found in a large tornado during an outbreak on 26 April 1991. These may be the highest wind speeds ever measured by Doppler radar and the first radar measurements of F-5 intensity wind speeds. The variation in the spectrum across the 26 April 1991 tornado is presented. Standard and mobile soundings, and surface data, used to determine the “thermodynamic speed limit” indicate that it was usually exceeded by 50%–100%. A comparison of actual Doppler spectra with simulated spectra suggests that the maximum in radar reflectivity in supercell tornadoes lies well outside the core.

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Howard B. Bluestein, Stephen G. Gaddy, David C. Dowell, Andrew L. Pazmany, John C. Galloway, Robert E. McIntosh, and Herbert Stein

Abstract

Counterrotating 500-m-scale vortices in the boundary layer are documented in the right-moving member of a splitting supercell thunderstorm in northeastern Oklahoma on 17 May 1995 during the Verification of the Origins of Rotation in Tornadoes Experiment. A description is given of these vortices based upon data collected at close range by a mobile, 3-mm wavelength (95 GHz), pulsed Doppler radar. The vortices are related to a storm-scale, pseudo-dual-Doppler analysis of airborne data collected by the Electra Doppler radar (ELDORA) using the fore–aft scanning technique and to a boresighted video of the cloud features with which the vortices were associated. The behavior of the storm is also documented from an analysis of WSR-88D Doppler radar data.

The counterrotating vortices, which were associated with nearly mirror image hook echoes in reflectivity, were separated by 1 km. The cyclonic member was associated with a cyclonically swirling cloud base. The vortices were located along the edge of a rear-flank downdraft gust front, southeast of a kink in the gust front boundary, a location previously found to be a secondary region for tornado formation. The kink was coincident with a notch in the radar echo reflectivity. A gust front located north of the kink, along the edge of the forward-flank downdraft, was characterized mainly by convergence and density current–like flow, while the rear-flank downdraft boundary was characterized mainly by cyclonic vorticity.

Previously documented vortices along gust fronts have had the same sense of rotation as the others in the group and are thought to have been associated with shearing instabilities. The symmetry of the two vortices suggests that they may have been formed through the tilting of ambient horizontal vorticity. Although the vortices did not develop into tornadoes, it is speculated that similar vortices could be the seeds from which some tornadoes form.

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Howard B. Bluestein, Wesley P. Unruh, David C. Dowell, Todd A. Hutchinson, Todd M. Crawford, Andrew C. Wood, and Herbert Stein

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.

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