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Stephan P. Nelson
and
Rodger A. Brown

Abstract

Actual data are used in one case to investigate the nature and source of vertical velocity errors resulting from analyses of multiple-Doppler radar measurements. Consistent with earlier analytical works, larger errors are found than would be expected from previous theoretical studies. It is shown that the reconstructed maximum updraft speed in strong updrafts (>20 m s−1) is accurate, on the average, to within about 10% (standard deviation of 10%). Storm advection, incomplete sampling of low-altitude divergence caused by the radar horizon, top boundary errors, and uneven terrain are studied and all are dismissed as dominant sources of error in the case considered here. The inability to determine a dominant error source has important consequences for the formulation of vertical velocity adjustment schemes.

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Vincent T. Wood
and
Rodger A. Brown

Abstract

When a thunderstorm mesocyclone changes range relative to a Doppler radar, the deduced core diameter and mean rotational velocity of the Doppler velocity mesocyclone signature oscillate back and forth, even though the radar beam’s physical width changes uniformly with range. The authors investigated the oscillations using a model mesocyclone and a simulated Doppler radar that collected data with an azimuthal sampling interval of 1°. They found that the oscillations are a consequence of changing data point separation with range relative to the Doppler velocity peaks of the mesocyclone signature.

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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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Rodger A. Brown
and
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
and
Vincent T. Wood

Abstract

The National Weather Radar Testbed was established in Norman, Oklahoma, in 2002 to evaluate, in part, the feasibility of eventually replacing mechanically scanned parabolic antennas with electronically scanned phased-array antennas on weather surveillance radars. If a phased-array antenna system is to replace the current antenna, among the important decisions that must be made are the design (flat faces, cylinder, etc.) that will be needed to cover 360° in azimuth and the choice of an acceptable beamwidth. Investigating the flat-face option, four faces seem to be a reasonable choice for providing adequate coverage. To help with the beamwidth decision-making process, the influence of beamwidth on the resolution of various-sized simulated vortices is investigated. It is found that the half-power beamwidth across the antenna should be no more than 1.0° (equating to a broadside beamwidth of 0.75°) in order to provide National Weather Service forecasters with at least the same quality of data resolution that is currently available for making tornado and severe storm warnings.

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Tetsuya Fujita
,
Kenneth A. Styber
, and
Rodger A. Brown

Abstract

During the month of July 1960, a mesometeorological field network was established in an area of 30 by 40 mi centered around San Francisco Mountain, Arizona. Network instrumentation included 33 micro-barographs, 10 hygrothermographs, 10 Beckman-Whitley wind recorders, about 165 nonrecording rain gauges, and 165 hail gauges. Daily precipitation amounts were carefully studied in order to relate them with the characteristics of moisture inflow into the network area. An analysis of the 22 July case over the network area revealed that a very small low-pressure area formed over the heated side of the mountain slope, providing a field of convergence leading to the morning cumulus convection. AS time went on, this low dissipated and cumulonimbus convection occurred. The mesometeorological network was found to be most suitable for the investigation of cumulus to cumulonimbus convection over the San Francisco Mountain area.

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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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