Vincent T. Wood

Effects of Radar Sampling on Single-Doppler Velocity Signatures of Mesocyclones and Tornadoes

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.

Improved Tornado Detection Using Simulated and Actual WSR-88D Data with Enhanced Resolution

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 Services Radar Operations Centers 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 ...

Effects of radar proximity on single-Doppler velocity signatures of axisymmetric rotation and divergence

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.

On the interpretation of single-doppler velocity patterns within severe thunderstorms

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.

New WSR-88D Volume Coverage Pattern 12: Results of Field Tests

Abstract For the first time since the installation of the national network of Weather Surveillance Radar-1988 Doppler (WSR-88D), a new scanning strategy—Volume Coverage Pattern 12 (VCP 12)—has been added to the suite of scanning strategies. VCP 12 is a faster version of VCP 11 and has denser vertical sampling at lower elevation angles. This note discusses results of field tests in Oklahoma and Mississippi during 2001–03 that led to the decision to implement VCP 12. Output from meteorological algorithms for a test-bed radar using an experimental VCP were compared with output for a nearby operational WSR-88D using VCP 11 or 21. These comparisons were made for severe storms that were at comparable distances from both radars. Findings indicate that denser vertical sampling at lower elevation angles leads to earlier and longer algorithm identifications of storm cells and mesocyclones, especially those more distant from a radar.

Improved WSR-88D Scanning Strategies for Convective Storms

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.

Technique for Improving Detection of WSR-88D Mesocyclone Signatures by Increasing Angular Sampling

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 ...

A New Parametric Tropical Cyclone Tangential Wind Profile Model

AbstractA new parametric tropical cyclone (TC) wind profile model is presented for depicting representative surface pressure profiles corresponding to multiple-maxima wind profiles that exhibit single-, dual-, and triple-maximum concentric-eyewall wind peaks associated with the primary (inner), secondary (first outer), and tertiary (second outer) complete rings of enhanced radar reflectivity. One profile employs five key parameters: tangential velocity maximum, radius of the maximum, and three different shape velocity parameters related to the shape of the profile. After tailoring the model for TC applications, a gradient wind is computed from a cyclostrophic wind formulated in terms of the cyclostrophic Rossby number. A pressure, via cyclostrophic balance, was partitioned into separate pressure components that corresponded to multiple-maxima cyclostrophic wind profiles in order to quantitatively evaluate the significant fluctuations in central pressure deficits. The model TC intensity in terms of varying...

A Technique for Detecting a Tropical Cyclone Center Using a Doppler Radar

Abstract A ground-based Doppler radar technique is developed for detecting a tropical cyclone center position. Accurate determination of the cyclone center position, based on Doppler velocity measurements, will become essential for the issuance of hurricane advisories, forecasts, and warnings once a network of WSR-88D Doppler radars is deployed on the United States coastlines, islands, and military bases during the 1990s. This will allow high-resolution detection and tracking of hurricanes nearing land for the first time. Simulated Doppler velocity data, which were reconstructed from wind field data collected by reconnaissance aircraft during Hurricanes Alicia (1983) and Gloria (1985), were used to test the concept of using ground-based Doppler radar data to estimate cyclone center location. The center range and azimuth estimates of a hurricane signature were calculated from the simulated coastal Doppler radar velocity data. Preliminary results indicate that the technique performed well for estimating cen...

Simulated WSR-88D Velocity and Reflectivity Signatures of Numerically Modeled Tornadoes

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 ve...