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Brian A. Klimowski

Abstract

Five dual-Doppler analyses spanning a period of 1.5 h are used to document the initiation and development of rear inflow within a High Plains squall line. The squall line developed as part of the 28-29 June 1989 mesoconvective system that passed through the North Dakota Thunderstorm Project observational network. This storm was explosively initiated along a preexisting, isolated gust front in an environment of much instability and weak upper-air flow, and moved within dual-Doppler coverage soon after its initiation. The kinematic analyses were performed over a 90-min period as the squall line continued to develop and move to the east-northeast.

The Doppler analyses reveal that the rear inflow was initiated near the high-reflectivity cores of the squall line, within 20 min of the formation of the system. With time, the, rear inflow expanded rearward, increased in intensity, and descended to near the surface behind the northern section of the squall line. In regions where the squall line dissipated, the rear inflow also dissipated near the leading edge of the system but remained near the, base of the trailing anvil. Line-parallel analyses indicate that the rear inflow exhibited significant variance in both elevation and magnitude along the length of the squall line. The observations herein suggest that the primary forcing for the rear inflow in this case was the result of processes associated with the strong convention at the leading edge of the squall line and that secondary processes within the trailing anvil-stratiform region may also have contributed to the forcing of rear inflow.

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Brian A. Klimowski and John D. Marwitz

Abstract

Synthetic dual-Doppler (SDD) is a single-Doppler analysis technique that combines measurements from two different times, provided the viewing angle changes significantly. In this study, the viability of the SDD technique is investigated through comparisons with dual-Doppler analyses. Three case studies are used for the comparisons: a mature gust front, a supercell thunderstorm, and a developing squall line. An attempt to internally establish the efficacy of SDD analyses is made by examining the temporal correlation of the single-Doppler reflectivity fields and by comparing wind fields derived from similar synthetic analyses.

Results indicate that the SDD technique may be practical for the estimation of mean velocity fields of certain quasi-steady phenomena. The similarity of the SDD and conventional dual-Doppler results were found to be sensitive to the time between the two volume scans used in the SDD analyses, the angle subtended by the event, and the radii of influence of the Cressman weighting function used for the interpolation of the data.

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Brian A. Klimowski, Mark R. Hjelmfelt, and Matthew J. Bunkers

Abstract

The evolution of 273 bow echoes that occurred over the United States from 1996 to 2002 was examined, especially with regard to the radar reflectivity characteristics during the prebowing stage. It was found that bow echoes develop from the following three primary initial modes: (i) weakly organized (initially noninteracting) cells, (ii) squall lines, and (iii) supercells. Forty-five percent of the observed bow echoes evolved from weakly organized cells, 40% from squall lines, while 15% of the bow echoes were observed to evolve from supercells. Thunderstorm mergers were associated with the formation of bow echoes 50%–55% of the time, with the development of the bow echo proceeding quite rapidly after the merger in these cases. Similarly, it was found that bow echoes formed near, and moved generally along, synoptic-scale or mesoscale boundaries in about half of the cases (where data were available).

The observed bow-echo evolutions demonstrated considerable regional variability, with squall line-to-bow-echo transitions most frequent over the eastern United States. Conversely, bow echoes typically developed from a group of weakly organized storms over the central United States. Bow-echo life spans were also longest, on average, over the southern plains; however, the modal life span was longest over the eastern United States. Finally, the supercell-to-bow-echo evolution was most common across the northern plains, but the data sample is too small for this result to be considered significant.

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Brian A. Klimowski, Mark R. Hjelmfelt, Matthew J. Bunkers, Don Sedlacek, and L. Ronald Johnson

Abstract

No abstract available.

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Brian A. Klimowski, Matthew J. Bunkers, Mark R. Hjelmfelt, and Josiah N. Covert

Abstract

During the warm seasons (May–September) of 1996–99, Weather Surveillance Radar-1988 Doppler (WSR-88D) data and severe wind reports (either gusts >25 m s–1, or damage-related reports) over the northern High Plains (NHP) of the United States were analyzed in order to document the primary modes of convection responsible for severe winds. It was found that two-thirds of the convectively generated severe wind reports over the NHP were identified as being produced by organized convective structures rather than by isolated downburst or microburst activity. Specifically, at least 29% of all severe wind reports were produced by bow echoes, 20% by squall lines, 9% by supercell thunderstorms, and 7% by other convective systems not organized in a linear fashion. The occurrence of linear convective storm types that typically produce high winds (i.e., squall lines and bow echoes) were also documented over the NHP during the period of study. It was found that 51% of all squall lines and 86% of all bow echoes were associated with severe surface winds. There was a preference for these storms to initiate near the interface of the Rocky Mountains and the plains [∼66% formed within 120 km (75 miles) of significant topography], and their typical lifetime was 2–4 h. Also of interest, bow echoes had 3 times the number of severe wind reports as severe hail reports, while this ratio was 1.6 for squall lines, and only 0.6 for supercells. The results from these analyses indicate that the nature and evolution of squall lines and bow echoes over the NHP illustrate some differences from similar storms over other regions. Trailing areas of stratiform precipitation were observed to be less common with squall lines over the NHP than other areas. Back-building squall lines were observed less frequently over the NHP, when compared with the southern plains. It was found that storm mergers were associated with the initiation of 41% of the bow echoes and that significant severe wind events occasionally occurred without any linear organization.

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Matthew J. Bunkers, Brian A. Klimowski, Jon W. Zeitler, Richard L. Thompson, and Morris L. Weisman

Abstract

A physically based, shear-relative, and Galilean invariant method for predicting supercell motion using a hodograph is presented. It is founded on numerous observational and modeling studies since the 1940s, which suggest a consistent pattern to supercell motion exists. Two components are assumed to be largely responsible for supercell motion: (i) advection of the storm by a representative mean wind, and (ii) propagation away from the mean wind either toward the right or toward the left of the vertical wind shear—due to internal supercell dynamics. Using 290 supercell hodographs, this new method is shown to be statistically superior to existing methods in predicting supercell motion for both right- and left-moving storms. Other external factors such as interaction with atmospheric boundaries and orography can have a pronounced effect on supercell motion, but these are difficult to quantify prior to storm development using only a hodograph.

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Matthew J. Bunkers, Jeffrey S. Johnson, Lee J. Czepyha, Jason M. Grzywacz, Brian A. Klimowski, and Mark R. Hjelmfelt

Abstract

The local and larger-scale environments of 184 long-lived supercell events (containing one or more supercells with lifetimes ≥4 h; see Part I of this paper) are investigated and subsequently compared with those from 137 moderate-lived events (average supercell lifetime 2–4 h) and 119 short-lived events (average supercell lifetime ≤2 h) to better anticipate supercell longevity in the operational setting. Consistent with many previous studies, long-lived supercells occur in environments with much stronger 0–8-km bulk wind shear than what is observed for short-lived supercells; this strong shear leads to significant storm-relative winds in the mid- to upper levels for the longest-lived supercells. Additionally, the bulk Richardson number falls into a relatively narrow range for the longest-lived supercells—ranging mostly from 5 to 45. The mesoscale to synoptic-scale environment can also predispose a supercell to be long or short lived, somewhat independent of the local environment. For example, long-lived supercells may occur when supercells travel within a broad warm sector or else in close proximity to mesoscale or larger-scale boundaries (e.g., along or near a warm front, an old outflow boundary, or a moisture/buoyancy axis), even if the deep-layer shear is suboptimal. By way of contrast, strong atmospheric forcing can result in linear convection (and thus shorter-lived supercells) in a strongly sheared environment that would otherwise favor discrete, long-lived supercells.

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Brian A. Klimowski, Robert Becker, Eric A. Betterton, Roelof Bruintjes, Terry L. Clark, William D. Hall, Brad W. Orr, Robert A. Kropfli, Paivi Piironen, Roger F. Reinking, Dennis Sundie, and Taneil Uttal

The 1995 Arizona Program was a field experiment aimed at advancing the understanding of winter storm development in a mountainous region of central Arizona. From 15 January through 15 March 1995, a wide variety of instrumentation was operated in and around the Verde Valley southwest of Flagstaff, Arizona. These instruments included two Doppler dual-polarization radars, an instrumented airplane, a lidar, microwave and infrared radiometers, an acoustic sounder, and other surface-based facilities. Twenty-nine scientists from eight institutions took part in the program. Of special interest was the interaction of topographically induced, storm-embedded gravity waves with ambient upslope flow. It is hypothesized that these waves serve to augment the upslope-forced precipitation that falls on the mountain ridges. A major thrust of the program was to compare the observations of these winter storms to those predicted with the Clark-NCAR 3D, nonhydrostatic numerical model.

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