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  • Author or Editor: Stephen D. Hrebenach x
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Howard B. Bluestein and Stephen D. Hrebenach

Abstract

Small-scale cyclonic vortices, which were embedded within a larger, mesoscale area of cyclonically curved flow, are documented over the mountains of Taiwan during the Taiwan Area Mesoscale Experiment. The vortices are visualized in dual-Doppler radar analyses of the wind field in the region of stratiform precipitation of a decaying mesoscale convective system. Each vortex was approximately 20 km across and had a lifetime on the order of an hour. The vortices formed and decayed during a 4-h period over Taiwan's Central Mountain Range and propagated with the flow associated with the mesoscale area of cyclonically curved flow in which they were embedded. The vortices extended through a deep layer in the troposphere, but were strongest near the mountaintop level, and decayed in intensity with height. Circumstantial evidence is presented that the vortices may have formed through the interaction of the mesoscale area of cyclonic flow with a mountain ridge.

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Howard B. Bluestien, Stephen D. Hrebenach, Chee-Foong Chang, and Edward A. Brandes

Abstract

The synthetic dual-Doppler (SDD) analysis technique is applied to the 6–7 May 1985 mesoscale convective system (MCS) that occurred during the Oklahoma-Kansas Preliminary Regional Experiment-STORM Central. This system had a cyclonic mesoscale circulation in its stratiform precipitation region. The SDD analyses are compared to corresponding actual dual-Doppler analyses. The sensitivity of the former to various parameters is discussed.

The SDD analysis technique is also applied to an MCS that passed by the NEXRAD (Next Generation Weather Radar) facility in Norman, Oklahoma, on 13 June 1989. Analyses of both the leading line of convective cells and the trailing stratiform precipitation area we presented; the salient features were similar to those found in dual-Doppler analyses of other systems. There was a mesoscale cyclonic circulation present at low and midlevels in the stratiform precipitation area. Vertical wind profiles obtained from the SDD analysis are compared to those obtained front velocity-azimuth display (VAD) analyses and from a conventional sounding. Vertical profiles of divergence and vertical velocity that were determined from VAD analyses indicated upward motion ahead of the convective line, and sinking motion and rising motion in the lower and upper troposphere, respectively, in the stratiform precipitation area behind the leading line. Rising motion and sinking motion in the lower and upper troposphere, respectively, were indicated in the vicinity of the mesoscale cyclonic circulation.

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Kenneth D. Lapenta, Barbara J. McNaught, Steven J. Capriola, Louis A. Giordano, Charles D. Little, Stephen D. Hrebenach, Gary M. Carter, Mario D. Valverde, and Derek S. Frey

Abstract

Heavy rain and flooding are of particular concern to forecasters throughout the eastern and southeastern United States. In this paper, the unique combination of synoptic-scale and mesoscale weather conditions and topographic factors that contribute to and enhance convectively produced flash flooding and river flooding are described and categorized. A classification system for synoptic-scale flooding events is developed, which is then used to identify the distribution (by month and synoptic-scale weather system) of major floods in the region of interest. Also, examples of flooding caused by synoptic-scale weather systems are presented.

Characteristics of mesoscale heavy-rain events that result in flooding are discussed. Topographical and geographical factors in the region, which play a role in these events, are described, along with examples of mesoscale, flash flood events. Finally, floods produced by warm-top thunderstorms, which pose a unique and challenging forecast problem throughout the east and southeast, are described and an example is provided.

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