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Rodger A. Brown

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Rodger A. Brown

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A technique is described whereby an investigator can obtain a more representative peak velocity for a tornadic vortex signature (TVS) than can be obtained by using the extreme measured Doppler velocities associated with the signature. This technique, which is based on theoretical curves derived by scanning a simulated radar through Rankine combined vortices, is applied to WSR-88D Doppler velocity data collected in the Garden City, Kansas, tornado of 16 May 1995. The technique produces peak TVS velocities that are more consistent in time and height than those computed directly from the extreme measured values.

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Rodger A. Brown

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Composite profiles of thermodynamic and kinematic variables are prepared to represent the characteristics of the environment within which a particular atmospheric phenomenon occurs. During the averaging process, it is desirable to retain the dominant features and associated gradients found in the individual profiles so that representative values of quantities such as flux parameters, energy budgets, convective available potential energy, and various stability indices can be computed from the composite profiles. The conventional compositing approach, where averages are computed at common heights, reduces or even smooths out a significant feature when the height and vertical extent of the feature differ from one individual profile to the next.

To retain a desirable feature in the composite profile, it is necessary to compute averages at the heights where the feature occurs and to compute the average height of the feature itself. As an example of the capabilities of this scaling or feature-preserving approach, the technique was applied to a set of 33 hodographs from supercell thunderstorm environments as documented in the literature. The feature-preserving technique retained realistic wind-shear values, including a midlatitude minimum-shear layer that disappeared when the conventional compositing technique was used.

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RODGER A. BROWN

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Rodger A. Brown

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Rodger A. Brown

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One of the distinguishing characteristics of a supercell thunderstorm is the presence of a rotating updraft. During the past 30 years, various hypotheses have been proposed to explain the initiation and maintenance of rotation. However, attempts to verify the initiation process have been frustrated by the lack of multiple-Doppler radar measurements at the time that the first rotating updraft appears. Discussed in this paper are dual-Doppler radar measurements that successfully captured the initiation and evolution of rotation in the Agawam, Oklahoma, storm of 6 June 1979, which occurred during the storm-scale phase of the Severe Environmental Storms and Mesoscale Experiment (SESAME). The process leading to updraft rotation appears to follow that proposed in 1968 by Fujita and Grandoso, whereby a middle-altitude vorticity couplet formed on the downwind flanks of a strong nonrotating updraft, with cyclonic vertical vorticity on the right-forward flank and anticyclonic vertical vorticity on the left-forward flank. With low-altitude flow approaching the storm from the right, a new updraft developed on the rightward-propagating gust front located along the right edge of the storm beneath the cyclonic vorticity region. The growing updraft acquired cyclonic rotation at middle altitudes by entraining and stretching the ambient vertical vorticity. Subsequent right-flank updrafts in the Agawam storm appear to have developed middle-altitude rotation in the same manner. Based on observations made within the Agawam storm and its immediate environment, the conventional hypothesis that employs low-altitude vertical shear of the horizontal wind as the vorticity source did not likely play a significant role in establishing updraft rotation.

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Rodger A. Brown
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Vincent T. Wood

Abstract

A tornadic vortex signature (TVS) is a degraded Doppler velocity signature of a tornado that occurs when the core region of a tornado is smaller than the half-power beamwidth of the sampling Doppler radar. Soon after the TVS was discovered in the mid-1970s, simulations were conducted to verify that the signature did indeed represent a tornado. The simulations, which used a uniform reflectivity distribution across a Rankine vortex model, indicated that the extreme positive and negative Doppler velocity values of the signature should be separated by about one half-power beamwidth regardless of tornado size or strength. For a Weather Surveillance Radar-1988 Doppler (WSR-88D) with an effective half-power beamwidth of approximately 1.4° and data collected at 1.0° azimuthal intervals, the two extreme Doppler velocity values should be separated by 1.0°. However, with the recent advent of 0.5° azimuthal sampling (“superresolution”) by WSR-88Ds at lower elevation angles, some of the extreme Doppler velocity values unexpectedly were found to be separated by 0.5° instead of 1.0° azimuthal intervals. To understand this dilemma, the choice of vortex model and reflectivity profile is investigated. It is found that the choice of vortex model does not have a significant effect on the simulation results. However, using a reflectivity profile with a minimum at the vortex center does make a difference. The revised simulations indicate that it is possible for the distance between the peak Doppler velocity values of a TVS to be separated by 0.5° with superresolution data collection.

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Rodger A. Brown
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Vincent T. Wood

Abstract

Simulations were conducted to investigate the detection of the Doppler velocity tornado signature (TS) and tornadic vortex signature (TVS) when a tornado is located at the center of the parent mesocyclone. Whether the signature is a TS or TVS depends on whether the tornado’s core diameter is greater than or less than the radar’s effective beamwidth, respectively. The investigation included three radar effective beamwidths, two mesocyclones, and six different-sized tornadoes, each of which had 10 different maximum tangential velocities assigned to it to represent a variety of strengths. The concentric tornadoes and mesocyclones were positioned 10–150 km from the radar. The results indicate that 1) azimuthal shear at the center of the mesocyclone increases as the associated tornado gains strength before a TS or TVS appears, 2) smaller tornadoes need to be much stronger than larger tornadoes at a given range for a signature to appear within the mesocyclone, and 3) when the tornado diameter is wider than about one-quarter of the mesocyclone diameter, the TS or TVS associated with a given mesocyclone appears when the tornado has attained about the same strength regardless of range.

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Rodger A. Brown
and
John M. Lewis

In this historical paper, we trace the scientific-and engineering-based steps at the National Severe Storms Laboratory (NSSL) and in the larger weather radar community that led to the development of NSSL's first 10-cm-wavelength pulsed Doppler radar. This radar was the prototype for the current Next Generation Weather Radar (NEXRAD), or Weather Surveillance Radar-1998 Doppler (WSR-88D) network.

We track events, both political and scientific, that led to the establishment of NSSL in 1964. The vision of NSSL's first director, Edwin Kessler, is reconstructed through access to historical documents and oral histories. This vision included the development of Doppler radar, where research was to be meshed with the operational needs of the U.S. Weather Bureau and its successor—the National Weather Service.

Realization of the vision came through steps that were often fitful, where complications arose due to personnel concerns, and where there were always financial concerns. The historical research indicates that 1) the engineering prowess and mentorship of Roger Lhermitte was at the heart of Doppler radar development at NSSL; 2) key decisions by Kessler in the wake of Lhermitte's sudden departure in 1967 proved crucial to the ultimate success of the project; 3) research results indicated that Doppler velocity signatures of mesocyclones are a precursor of damaging thunderstorms and tornadoes; and 4) results from field testing of the Doppler-derived products during the 1977-79 Joint Doppler Operational Project—especially the noticeable increase in the verification of tornado warnings and an associated marked decrease in false alarms—led to the government decision to establish the NEXRAD network.

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Rodger A. Brown
and
Rebecca J. Meitín

Abstract

During the late afternoon and early evening of 27 June 1989. Three splitting thunderstorms formed over Standing Rock Indian Reservation in the southern portion of the North Dakota Thunderstorm Project area. The first two storms are the subject of this study. The entire life cycles of both storms were documented using a single ground-based Doppler radar. Radar reflectivity signatures of updraft summits and Doppler velocity signatures of divergence near storm top were used to deduce updraft evolution within the storms. Dual-Doppler radar observations from a ground-based radar and an airborne Doppler radar provided fragmentary documentation of the storms’ life cycles.

The splitting storms on that day were unusual in two distinct ways: (a) the left members of the splitting storms were the dominant and longer-lasting ones, and (b) none of the deduced updrafts were collocated with centers of vorticity signatures that would have indicated updraft rotation. Both of the left-moving storms had 10 sequential primary updrafts, whereas their right-hand counterparts had 3 or 4 primary updrafts. Initial formation of the right-flank updrafts lagged behind the initial formation of the left-flank updrafts by 40–70 min. All the individual updraft summits moved in the general direction of the mean wind. Sequential updraft development on the left and right flanks of the storms suggested that expanding gust fronts provided the propagational component of storm motion.

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