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John Marwitz
and
Donald W. Burgess

Abstract

Inbound and outbound passes were made with an instrumented aircraft at a variety of altitudes in the inflow region of a severe thunderstorm containing a mesoscale vortex. The streamwise vorticity, helicity, and geostrophic thermal advection were estimated from these data. The increased helicity in the inflow region may explain the marked decrease in turbulence observed in this region.

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Donald R. MacGorman
and
Donald W. Burgess

Abstract

Although negative ground flashes usually dominate cloud-to-ground lightning activity, positive ground flashes can dominate in some severe storms for periods ranging from 30 min to several hours. Unlike most other types of storms in which positive ground flashes occur, severe storms can have positive flash rates and densities of strike points comparable to those usually observed for negative ground flashes in active thunderstorms. Fifteen such storms are analyzed in this paper to examine relationships of positive ground flashes to various storm characteristics, especially reports of large hail and tornadoes.

In 4 of the 15 storms, ground flash activity was dominated by positive cloud-to-ground lightning throughout most of the life of the storm. In 11 storms, the dominant polarity of ground flashes switched from positive to negative sometime during the mature stage of the storm. In all cases observed by Doppler radar, storms dominated by positive flashes had at least some rotation, and in most cases they were low-precipitation or classic supercell storms. If negative ground flashes subsequently became frequent and dominated positive ground flashes in a storm, it usually remained strong and often became a classic or heavy-precipitation supercell storm.

In all cases for which hail verification efforts were vigorous, large hail was reported during the period when positive ground flashes dominated. Usually, the frequency and reported diameter of hail decreased after the dominant polarity of ground flashes switched to negative. In the 11 storms that were tornadic, tornadoes occurred either during or after the period when positive ground flashes dominated. The strongest tornado usually began after the positive ground flash rate decreased from its maximum value; this was always true when the maximum rate exceeded 1.5 min−1. Although many hailstorms and tornadic storms are dominated by negative flashes, tornadic storms and hailstorms constitute a small fraction of storms dominated by frequent negative flashes, but appear to constitute an overwhelming majority of storms dominated by frequent positive flashes.

The geographic region in which positive or negative flashes dominated on a given day appeared consistent from storm to storm; the dominant polarity switched in roughly the same region for sequential storms following similar tracks. It is inferred that the dominant polarity of lightning is strongly influenced by mesoscale properties of the atmosphere, possibly through systematic effects on other storm properties related to severe weather.

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Dusan Zrnić
,
Donald W. Burgess
, and
Larry Hennington

Abstract

Presented in this Paper are Doppler spectra of a very large tornado that occurred on 22 May 1981 near Binger, Oklahoma. Tracking of the tornado was accomplished with the help of a novel “polar spectra display.” Bimodal tornado spectral signatures (TSS) were observed in about 40 scans. Direct measurements of maximum velocities from spectral skirts yielded a maximum tangential speed of 80 m s−1 (90 m s−1 relative to ground). A diameter of 1 km at 200 m above ground was deduced from a simplified model. Radial centrifuging of radar targets was estimated to be about 20 m s−1. With simple assumptions for radar target sizes and summation of forces, a beamwidth average convergence value of about 2.5 × 10−2 s−1 was calculated for the tornado boundary layer.

Tornado damage to trees and structures was subjectively rated on the Fujita damage scale. The windspeed range associated with the damage scale agreed well with the Doppler-estimated maximum windspeed when the tornado was large (1 km diameter). However, as the tornado diameter decreased, the Doppler-derived windspeed considerably underestimated that associated with the damage scale.

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Charles A. Doswell III
and
Donald W. Burgess

Abstract

A continuing problem in dealing with climatology data concerning tornadoes in the United States is the validity of the quantitative information contained in the various available data bases. Two aspects of tornado data are discussed: the F-scale rating and the occurrence of very long path length events. The argument is advanced that the F-scale is more properly thought of as a damage scale than as an intensity scale. Failing to recognize this leads to confusion and controversy regarding the F-scale ratings assigned to events in the data base.

Changing perceptions of tornadoes have led to some questions concerning the actual frequency of very long path lengths, on the order of 100 statute miles (160.9 km) or more. Evidence is presented that at least some of the events classified as having long tracks are most likely the result of misinterpreting the results of a series of short-path tornadoes, produced by a single supercell thunderstorm.

Some discussion is presented concerning the implications of the problems with the data. Since the climatological record is of both meteorological and societal concern, some alternatives are considered, but no hard conclusions can be drawn without considerable further effort.

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Rita D. Roberts
,
Donald Burgess
, and
Matthew Meister

Abstract

A conceptual model is presented for developing a new tool for nowcasting severe thunderstorms using existing operational data. Selected output from two operational, automated, weather detection and forecasting systems have been combined together within a fuzzy logic–based, data fusion system to test the concept and produce 15-min nowcasts of severe weather. The NCAR Auto-Nowcast System provides information and nowcasts on the evolving boundary layer and storm initiation, growth, and decay. The National Severe Storms Laboratory Warning Decision Support System (WDSS) identifies severe weather attributes within storms and provides storm-centric and specific detections of strong winds, mesocyclones, tornadoes, and probabilities of hail and severe hail. A modified version of the Auto-Nowcast System is employed as the engine for combining the Auto-Nowcast gridded output with the object-based WDSS output. Severe thunderstorm nowcasts are compared with available spotter reports for a multicellular, hail-producing squall-line event and a tornadic supercell event. Proof of concept is demonstrated and the results are encouraging as some skill is observed with the 15-min nowcasts. Many challenges still exist in producing a robust tool and these challenges are discussed.

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Rodger A. Brown
,
Leslie R. Lemon
, and
Donald W. Burgess

Abstract

Doppler radar measurements in the Union City, Okla., tornadic storm of 24 May 1973 led to discovery of a unique tornadic vortex signature (TVS) in the field of mean Doppler velocity data. The distinct character of this signature and its association with the tornado are verified using a model that simulates Doppler velocity measurements through a tornado. Temporal and spatial variations of the TVS reveal previously unknown tornado characteristics. The TVS originates at storm mid-levels within a parent mesocyclone, descends to the ground with the tornado (extending vertically at least 10 km), and finally dissipates at all heights when the tornado dissipates. NSSL Doppler radar data from 1973 through 1976 reveal 10 signatures; eight were associated with tornadoes or funnel clouds, while no reports are available for the other two. Since the TVS first appears aloft tens of minutes before tornado touchdown, the signature has decided potential for real-time warning.

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Vincent T. Wood
,
Rodger A. Brown
, and
Donald W. Burgess

Abstract

Examination of 320 mesocyclones recorded by the National Severe Storms Laboratory's Doppler radars over Oklahoma and adjacent portions of Texas during 20 spring tornado seasons of 1971–90 shows that tornado-producing mesocyclones in this region typically travel farther and live longer than mesocyclones that do not produce tornadoes.

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Leslie R. Lemon
,
Donald W. Burgess
, and
Rodger A. Brown

Abstract

Single-Doppler Velocity data reveal that a dominant feature in the Union City, Okla., tornadic thunderstorm is a core mesocyclonic circulation, 2–6 km in diameter, extending to at least 9 km above ground. There is an apparent flow through the precipitation echo at low levels and divergence at high levels. Considerable similarity appears between mid-level flow structure around the mesocyclone core and that observed around a solid rotating cyclinder embedded in classical potential flow. As tornado time approaches, core circulation tangential velocities increase while diameter decreases. Simultaneously, the collapse of storm top and extensive echo overhang suggest updraft weakening.

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Timothy D. Crum
,
Ron L. Alberty
, and
Donald W. Burgess

In order to support NEXRAD program requirements, WSR-88D systems have the capability to record data and products at four levels. Of these, level II (base data) and level III (products) will be most commonly available for various applications by a wide range of users. This paper overviews the data-recording capabilities of the WSR-88D system, plans for recording and archiving these data, and some uses for these data.

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Glen S. Romine
,
Donald W. Burgess
, and
Robert B. Wilhelmson

Abstract

On 8 May 2003, a tornadic supercell tracked through portions of the Oklahoma City, Oklahoma, metropolitan area and produced violent damage along portions of its path. This storm passed through the dense in situ radar network in central Oklahoma and provided close-range operational, prototype polarimetric and terminal Doppler weather radar observations of the storm as it made the transition into the tornadic phase. The time-evolving polarimetric features were scrutinized with regard to storm morphology, particularly as related to the development of a rear-flank downdraft pulse within the storm immediately preceding the long-track tornado event. Two new polarimetric terms are introduced, the Z dr shield and K dp foot, along with a discussion of the orientation of the Z dr and K dp columns relative to midlevel rotation signatures. Storm downdraft and gust front characteristics are discussed relative to polarimetric fields and background environment characteristics. Highlighted for this event are a “warm” forward-flank downdraft and a particularly cold rear-flank downdraft. Emphasis is also placed on demonstrating key polarimetric field characteristics relative to traditional features at low and midlevels defined in familiar conceptual models of severe storms.

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