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Howard B. Bluestein
and
Stephen G. Gaddy

Abstract

On 22 May 1995, numerous thunderstorms, several of which produced large hail and small tornadoes, formed along a dryline in the central and northern Texas Panhandle. The only long-lasting, daytime, severe storms developed later, south of the earlier storms. By late afternoon, new cells merged and evolved into two large supercells. The northern supercell produced three weak tornadoes and hail; the southern supercell did not produce a tornado, but did produce very large hail for an extended period of time.

Verification of the Origins of Rotation in Tornadoes Experiment crews collected data in the southern supercell from its early stage through much of its mature stage. The National Center for Atmospheric Research Electra Doppler Radar collected pseudo-dual-Doppler radar data on this storm continuously for well over an hour. Detailed analyses of this high-resolution dataset revealed several unusual features, most notably an intense elevated rear-inflow jet at midlevels having maximum wind speeds approaching 60 m s−1. Flanking this jet was a strong cyclone–anticyclone couplet. Moreover, a deep convergence zone (DCZ) that extended from the surface up through midlevels was present at the interface between the rear-inflow jet and the strong southeasterly inflow (over 40 m s−1 just above the ground) to the storm. The formation and potential importance of the rear-inflow jet, vorticity couplet, and DCZ are analyzed in detail, with emphasis on comparisons to similar features often found within mesoscale convective systems.

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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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