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Rodger A. Brown

Abstract

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

Abstract

No Abstract Available.

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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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Vincent T. Wood
and
Rodger A. Brown

Abstract

Simulated WSR-88D (Weather Surveillance Radar-1988 Doppler) radar data were used to investigate the effects of discrete azimuthal sampling on Doppler velocity signatures of modeled mesocyclones and tornadoes at various ranges from the radar and for various random positions of the radar beam with respect to the vortices. Results show that the random position of the beam can change the magnitudes and locations of peak Doppler velocity values. The important implication presented in this study is that short-term variations in tornado and far-range mesocyclone intensity observed by a WSR-88D radar may be due to evolution or due to the chance positions of the radar beam relative to the vortex’s maximum rotational velocities or due to some combination of both.

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Rodger A. Brown
and
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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Vincent T. Wood
and
Rodger A. Brown

Abstract

A tornadic vortex signature (TVS) is a degraded Doppler velocity signature that occurs when the tangential velocity core region of a tornado is smaller than the effective beamwidth of a sampling Doppler radar. Early Doppler radar simulations, which used a uniform reflectivity distribution across an idealized Rankine vortex, showed that the extreme Doppler velocity peaks of a TVS profile are separated by approximately one beamwidth. The simulations also indicated that neither the size nor the strength of the tornado is recoverable from a TVS. The current study was undertaken to investigate how the TVS might change if vortices having more realistic tangential velocity profiles were considered. The one-celled (axial updraft only) Burgers–Rott vortex model and the two-celled (annular updraft with axial downdraft) Sullivan vortex model were selected. Results of the simulations show that the TVS peaks still are separated by approximately one beamwidth—signifying that the TVS not only is unaffected by the size or strength of a tornado but also is unaffected by whether the tornado structure consists of one or two cells.

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Vincent T. Wood
and
Rodger A. Brown

Abstract

A variety of single Doppler velocity patterns that simulate those observed in a nondivergent environment is presented. Measurements in optically clear air and/or widespread precipitation are simulated, using horizontally uniform wind fields that vary with height. Vertical profiles of wind speed and direction indicated by the simulated Doppler velocity fields agree well with Doppler radar measurements. Horizontally uniform winds veering with height produce a striking S-shaped pattern, indicative of warm air advection; winds backing with height produce a backward S, indicative of cold air advection. A maximum in the vertical profile of wind speed is indicated by a pair of concentric contours, one upwind and one downwind of the radar. The presence of a frontal discontinuity is indicated by rapid variation of wind direction within the frontal zone. The wind speed profile controls the overall pattern including the spacing between contours, whereas the vertical profile of wind direction controls contour curvature.

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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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Vincent T. Wood
and
Rodger A. Brown

Abstract

Geometrical and mathematical relationships are developed to explain the variation with radar range of idealized single-Doppler velocity patterns of axisymmetric rotation and divergence regions. The velocity patterns become distorted as they approach a Doppler radar site. As a consequence, the apparent core diameters and locations of the centers of the features depart from the true values. Equations are derived to estimate the true values from the distorted Doppler velocity fields.

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Rodger A. Brown
and
Vincent T. Wood

Abstract

Although the flow field within a severe thunderstorm is complex, it is possible to simulate the basic features using simple analytical flow models (such as uniform flow, axisymmetric rotation, axisymmetric divergence). Combinations of such flow models are used to produce simulated Doppler velocity patterns that can be used as “signatures” for identifying quasi-horizontal flow features within severe thunderstorms. Some of these flow features are: convergence in the lower portions of a storm and divergence in the upper portions associated with a strong updraft, surface divergence associated with a wet or dry downdraft, mesocyclone (rotating updraft), flow around an updraft obstacle, and tornado. Recognition of the associated Doppler velocity patterns can aid in the interpretation of single-Doppler radar measurements that include only the radial component of flow in the radar viewing direction.

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