Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: Dale Sirmans x
  • Refine by Access: All Content x
Clear All Modify Search
Dale Sirmans
and
Bill Bumgarner

Abstract

Five techniques of estimating power spectrum mean frequency are examined. Performance is given in terms of estimate bias, accuracy, and noise immunity. Techniques examined are: 1) fast Fourier transform, 2) covariance argument approximation, 3) vector phase change, 4) scalar phase change, and 5) time derivative form of covariance.

Estimator evaluation is made from numerical results obtained with a computer-simulated signal having a Gaussian spectral density which serves as the population with known parameters in the statistical analysis, and 2) real data from a pulsed Doppler radar. Both data sets consist of uniformly time-spaced digital samples of a complex signal. Absolute and relative performance of each estimator are noted, and numerical results are compared with theoretical calculations made by other investigators.

Insofar as the pulsed Doppler meteorological return is represented by the signal type examined (narrow, symmetrical spectral densities), the covariance technique of mean frequency estimation is unbiased and superior in terms of accuracy and noise immunity.

Full access
WALTER L. WATTS
and
DALE SIRMANS

Abstract

A digital display of the location and intensity of the most intense radar echo in a selected sector is provided by a digital comparator with shift register storage of digital video signals from the National Severe Storms Laboratory WSR–57 radar. The display provides the radar operator with immediate access to important weather information.

Full access
Dúsan S. Zrnić
,
Dale Sirmans
, and
Edwin Kessler

Abstract

Computer programs for antenna control and data processing were prepared and interfacing was fabricated to enable the Doppler radar at Norman, Oklahoma, to track a reflectorized balloon, and define winds in the layer through which the balloon rises. Differences between wind speeds and directions estimated by this radar and by radiosonde were about ½ m s−1 and 4°. Given a network of Doppler radars such as projected by the NEXRAD program, the incremental cost of hardware for measuring winds by this radar method is small. Subject to refinement of balloon launch and acquisition procedures, and consideration of other possible consraints, the method represents an opportunity to collect wind data by radar during periods of fair weather when natural tracers are weak or absent and the radars are not otherwise dedicated. Winds so acquired could expand the base of data used in weather forecasting.

Full access
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.

Full access
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.

Full access
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.

Full access
Rodger A. Brown
,
Donald W. Burgess
,
John K. Carter
,
Leslie R. Lemon
, and
Dale Sirmans

Some results of the first 10 cm dual-Doppler radar measurements in a tornadic storm are presented. A mesoscale cyclonic circulation confirms proposed single Doppler vortex signature and indicates that the curved reflectivity hook echo is around the periphery of the circulation. The interpolated tornado position is within the mesocyclone where high-variance Doppler velocity spectra suggest strong velocity gradients.

Full access
Rodger A. Brown
,
William C. Bumgarner
,
Kenneth C. Crawford
, and
Dale Sirmans

Single Doppler radar measurements were made in a squall line that formed in southern Kansas during the afternoon of 2 June 1971 and moved south-southeastward through central Oklahoma. During the period of data collection, a pronounced hook echo, having at least one funnel cloud associated with it, developed. Preliminary analyses of these first Doppler velocity measurements within a radar hook echo in the tornado belt are presented.

Full access
Rodger A. Brown
,
Vincent T. Wood
,
Randy M. Steadham
,
Robert R. Lee
,
Bradley A. Flickinger
, and
Dale Sirmans

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.

Full access
Rodger A. Brown
,
Bradley A. Flickinger
,
Eddie Forren
,
David M. Schultz
,
Dale Sirmans
,
Phillip L. Spencer
,
Vincent T. Wood
, and
Conrad L. Ziegler

Abstract

Doppler velocity and reflectivity measurements from Weather Surveillance Radar-1988 Doppler (WSR-88D) radars provide important input to forecasters as they prepare to issue short-term severe storm and tornado warnings. Current-resolution data collected by the radars have an azimuthal spacing of 1.0° and range spacing of 1.0 km for reflectivity and 0.25 km for Doppler velocity and spectrum width. To test the feasibility of improving data resolution, National Severe Storms Laboratory’s test bed WSR-88D (KOUN) collected data in severe thunderstorms using 0.5°-azimuthal spacing and 0.25-km-range spacing, resulting in eight times the resolution for reflectivity and twice the resolution for Doppler velocity and spectrum width. Displays of current-resolution WSR-88D Doppler velocity and reflectivity signatures in severe storms were compared with displays showing finer-resolution signatures. At all ranges, fine-resolution data provided better depiction of severe storm characteristics. Eighty-five percent of mean rotational velocities derived from fine-resolution mesocyclone signatures were stronger than velocities derived from current-resolution signatures. Likewise, about 85% of Doppler velocity differences across tornado and tornadic vortex signatures were stronger than values derived from current-resolution data. In addition, low-altitude boundaries were more readily detected using fine-resolution reflectivity data. At ranges greater than 100 km, fine-resolution reflectivity displays revealed severe storm signatures, such as bounded weak echo regions and hook echoes, which were not readily apparent on current-resolution displays. Thus, the primary advantage of fine-resolution measurements over current-resolution measurements is the ability to detect stronger reflectivity and Doppler velocity signatures at greater ranges from a WSR-88D.

Full access