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Vincent T. Wood
,
Rodger A. Brown
, and
David C. Dowell

Abstract

Low-altitude radar reflectivity measurements of tornadoes sometimes reveal a donut-shaped signature (low-reflectivity eye surrounded by a high-reflectivity annulus) and at other times reveal a high-reflectivity knob associated with the tornado. The differences appear to be due to such factors as (i) the radar’s sampling resolution, (ii) the presence or absence of lofted debris and a low-reflectivity eye, (iii) whether measurements were made within the lowest few hundred meters where centrifuged hydrometeors and smaller debris particles were recycled back into the tornadic circulation, and (iv) the presence or absence of multiple vortices in the parent tornado.

To explore the influences of some of these various factors on radar reflectivity and Doppler velocity signatures, a high-resolution tornado numerical model was used that incorporated the centrifuging of hydrometeors. A model reflectivity field was computed from the resulting concentration of hydrometeors. Then, the model reflectivity and velocity fields were scanned by a simulated Weather Surveillance Radar-1988 Doppler (WSR-88D) using both the legacy resolution and the new super-resolution sampling. Super-resolution reflectivity and Doppler velocity data are displayed at 0.5° instead of 1.0° azimuthal sampling intervals and reflectivity data are displayed at 0.25-km instead of 1.0-km range intervals.

Since a mean Doppler velocity value is the reflectivity-weighted mean of the radial motion of all the radar scatterers within a radar beam, a nonuniform distribution of scatterers produces a different mean Doppler velocity value than does a uniform distribution of scatterers. Nonuniform reflectivities within the effective resolution volume of the radar beam can bias the indicated size and strength of the tornado’s core region within the radius of the peak tangential velocities. As shown in the simulation results, the Doppler-indicated radius of the peak wind underestimates the true radius and true peak tangential velocity when the effective beamwidth is less than the tornado’s core diameter and there is a weak-reflectivity eye at the center of the tornado. As the beam becomes significantly wider than the tornado’s core diameter with increasing range, the peaks of the Doppler velocity profiles continue to decrease in magnitude but overestimate the tornado’s true radius. With increasing range from the radar, the prominence of the weak-reflectivity eye at the center of the tornado is progressively lessened until it finally disappears. As to be expected, the Doppler velocity signatures and reflectivity eye signatures were more prominent and stronger with super-resolution sampling than those with legacy-resolution sampling.

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

Abstract

The magnitude of the Doppler velocity signature of a tornado depends on the effective width of the radar beam relative to the size of the tornado. The effective beamwidth is controlled by the antenna pattern beamwidth and the azimuthal sampling interval. Simulations of Weather Surveillance Radar-1988 Doppler (WSR-88D) velocity signatures of tornadoes, presented in this paper, show that signature resolution is greatly improved when the effective beamwidth of the radar is reduced. Improved signature resolution means that stronger signatures can be resolved at greater ranges from the radar.

Using a special recording device on the National Weather Service's Radar Operations Center's KCRI test bed radar, Archive Level I time series data were collected during the Oklahoma–Kansas tornado outbreak of 3 May 1999. Two Archive Level II meteorological datasets, each having a different effective beamwidth, were created from the Archive Level I dataset. Since the rotation rate and time interval between pulses are common for both Archive Level II datasets, the only parameter that could be changed to reduce the effective beamwidth of the KCRI data was the number of pulses, which also changed the azimuthal sampling interval. By cutting the conventional number of pulses in half for one of the Archive Level II datasets, the effective beamwidth was decreased by about a quarter and the azimuthal sampling interval was decreased from 1.0° to 0.5°. The 3 May 1999 data confirm the simulation results that stronger Doppler velocity signatures of tornadoes typically are produced when the azimuthal sampling interval, and thus the effective beamwidth, is decreased.

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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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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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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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Vincent T. Wood
,
Rodger A. Brown
, and
Steven V. Vasiloff

Abstract

About one-third of the Weather Surveillance Radar-1988 Doppler (WSR-88D) radars located in the western third of the United States are on the tops of mountains. These mountaintop radars employ scanning strategies that were designed for flatland radars, with the lowest elevation angle being +0.5°. Consequently, the radar signals are sent well above the populace and terrain surrounding the radar. The inability to adequately detect low-altitude weather events results in missed warnings of severe weather and in underestimates of the amount and areal extent of precipitation. Mountaintop radars could be utilized much more effectively if the scanning strategies included negative elevation angles. The state of Utah has the disadvantage that all three of the WSR-88Ds used by the National Weather Service to monitor weather events in the state are located on the tops of mountains. To determine the extent to which negative elevation angles would improve the detection capabilities of these radars over Utah and portions of the adjacent states, a WSR-88D simulation model is used to compare the existing scanning strategies with those that incorporate negative elevation angles. As might be expected, the use of negative elevation angles enhances low- to midaltitude detection of weather events over a much larger area than is possible using the existing scanning strategies. For example, the area where the centers of the beams from the three radars currently are within 1 km of the ground encompasses only 2% of the area within 230 km of the radars. Using negative elevation angles, the areal coverage within 1 km of the ground increases to over 30%.

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Rodger A. Brown
,
Thomas A. Niziol
,
Norman R. Donaldson
,
Paul I. Joe
, and
Vincent T. Wood

Abstract

During the winter, lake-effect snowstorms that form over Lake Ontario represent a significant weather hazard for the populace around the lake. These storms, which typically are only 2 km deep, frequently can produce narrow swaths (20–50 km wide) of heavy snowfall (2–5 cm h−1 or more) that extend 50–75 km inland over populated areas. Subtle changes in the low-altitude flow direction can mean the difference between accumulations that last for 1–2 h and accumulations that last 24 h or more at a given location. Therefore, it is vital that radars surrounding the lake are able to detect the presence and strength of these shallow storms. Starting in 2002, the Canadian operational radars on the northern side of the lake at King City, Ontario, and Franktown, Ontario, began using elevation angles of as low as −0.1° and 0.0°, respectively, during the winter to more accurately estimate snowfall rates at the surface. Meanwhile, Weather Surveillance Radars-1988 Doppler in New York State on the southern and eastern sides of the lake—Buffalo (KBUF), Binghamton (KBGM), and Montague (KTYX)—all operate at 0.5° and above. KTYX is located on a plateau that overlooks the lake from the east at a height of 0.5 km. With its upward-pointing radar beams, KTYX’s detection of shallow lake-effect snowstorms is limited to the eastern quarter of the lake and surrounding terrain. The purpose of this paper is to show—through simulations—the dramatic increase in snowstorm coverage that would be possible if KTYX were able to scan downward toward the lake’s surface. Furthermore, if KBUF and KBGM were to scan as low as 0.2°, detection of at least the upper portions of lake-effect storms over Lake Ontario and all of the surrounding land area by the five radars would be complete. Overlake coverage in the lower half (0–1 km) of the typical lake-effect snowstorm would increase from about 40% to about 85%, resulting in better estimates of snowfall rates in landfalling snowbands over a much broader area.

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