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Rodger A. Brown

Abstract

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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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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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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Rodger A. Brown
,
Vincent T. Wood
, and
Timothy W. Barker

Abstract

KMSX, near Missoula, Montana, is one of the Weather Surveillance Radars-1988 Doppler (WSR-88Ds) that are located on the top of a mountain. Because all WSR-88Ds employ scanning strategies that were developed for flatland radars, mountaintop radars send signals well above the populated valleys and terrain surrounding the radars. Forecasters who use mountaintop WSR-88Ds are at a distinct disadvantage in not being able to detect crucial weather phenomena near the earth's surface. The use of negative elevation angles has been proposed as a solution to this problem. This type of radar operation poses no public radiation hazard, because the microwave radiation exposure level is about two orders of magnitude below the acceptable guideline near the radar and rapidly decreases with increasing distance. The feasibility of KMSX using negative elevation angles is simulated using several different weather situations. The simulations show the potential for improved detections of low-altitude weather conditions in the surrounding valleys and improved estimates of precipitation amounts throughout the coverage area. For example, using the lowest elevation angle (+0.5°) of the current WSR-88D scanning strategies, simulated rainfall rates detected in the valleys progressively decrease from about 80% of the surface value near the radar to only 1% of the surface value at 220 km. However, using an elevation angle of −0.8°, simulated rainfall rates detected at all ranges out to 220 km are about 80%–95% of the surface values.

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Rodger A. Brown
,
Vincent T. Wood
, and
Dale Sirmans

Abstract

The Weather Surveillance Radar-1988 Doppler (WSR-88D) is an important operational and research tool for detecting and monitoring convective storms. Two scanning strategies, or volume coverage patterns, VCP 11 and 21, are used in storm situations. Users find that these original VCPs do not always provide the vertical or temporal resolution that is desired. To help solve these resolution problems, a procedure is proposed for developing optimized and flexible VCPs. A VCP is optimized when the maximum height uncertainty (expressed in percent of true height) is essentially the same at all ranges and for all heights of storm features. A VCP becomes flexible when the volume scan terminates and recycles after it tilts above all radar return or reaches a specified elevation angle. Two sample VCPs, which are optimized and flexible, are presented, and simulated radar data show that they perform better than the current VCPs.

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Rodger A. Brown
,
Janelle M. Janish
, and
Vincent T. Wood

Abstract

The operational meteorological community generally recognizes that the greater spatial and temporal resolution of WSR-88D volume coverage pattern (VCP) 11 is preferable to VCP 21 when using algorithms to identify severe storm characteristics. The coarser vertical sampling of storms with VCP 21 likely produces less accurate results. An experiment was conducted to investigate the comparative effects of VCPs 11 and 21. Since VCP 21 is nearly a subset of VCP 11, the appropriate elevation angles were deleted from two VCP 11 datasets to produce proxy datasets for VCP 21 (called VCP 22). Various WSR-88D operational algorithms and National Severe Storms Laboratory prototype severe storm algorithms were run on the VCP 11 and VCP 21/22 datasets. At heights above 5° elevation angle, where VCP 21/22 had missing elevation angles relative to VCP 11, the majority of algorithm parameters had different values at least 50%–70% of the time. Therefore, this study confirms that VCP 11—not VCP 21—should be used in those convective storm situations where there is a contribution to critical warning parameters from elevation angles greater than 5°.

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Vincent T. Wood
,
Rodger A. Brown
, and
Dale Sirmans

Abstract

The Doppler velocity signature of a thunderstorm mesocyclone becomes increasingly degraded as distance from the radar increases. Degradation is due to the broadening of the radar beam with range relative to the size of the mesocyclone. Using a model mesocyclone and a simulated WSR-88D Doppler radar, a potential approach for improving the detection of mesocyclones is investigated. The approach involves decreasing the azimuthal sampling interval from the conventional 1.0° to 0.5°. Using model mesocyclones that cover the spread of expected mesocyclone sizes and strengths, simulations show that stronger mesocyclone signatures consistently are produced when radar data are collected at 0.5° azimuthal increments. Consequently, the distance from the radar at which a mesocyclone of a given strength and size can be detected increases by an average of at least 50% when data are collected using 0.5° azimuthal increments.

The simulated findings are tested using Archive Level I (time series) data collected by the WSR-88D Operational Support Facility’s KCRI radar during the Oklahoma–Kansas tornado outbreak of 3 May 1999. With the availability of time series data, an Archive Level II dataset was produced for both 1.0° and 0.5° azimuthal intervals. One-third of the mesocyclone signatures collected using 0.5° azimuthal intervals were 10% to over 50% stronger than their 1.0° azimuthal interval counterparts.

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