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- Author or Editor: Dúsan S. Zrnić x
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Abstract
We have developed a procedure that detects and tracks gust fronts automatically. It does not rely on a single method but requires simultaneous operation of two related algorithms. The convergence algorithm measures radial convergence and hence only gusts propagating along radials can be readily detected. The mesocyclone-shear algorithm measures azimuthal shear and is suitable for detecting gusts parallel with radials as well as low-level vortices. Long shear lines that these algorithms detect are classified as gusts whereas symmetric shear features are rejected if their shear and flux or “momentum” are insignificant; otherwise they are classified as low-level vortices. To locate gusts we use second-order polynomials in the range-azimuth plane. It is shown that predicted gust locations from simple linear projections of the least square fitted curves agree very well with actual gust locations.
Abstract
We have developed a procedure that detects and tracks gust fronts automatically. It does not rely on a single method but requires simultaneous operation of two related algorithms. The convergence algorithm measures radial convergence and hence only gusts propagating along radials can be readily detected. The mesocyclone-shear algorithm measures azimuthal shear and is suitable for detecting gusts parallel with radials as well as low-level vortices. Long shear lines that these algorithms detect are classified as gusts whereas symmetric shear features are rejected if their shear and flux or “momentum” are insignificant; otherwise they are classified as low-level vortices. To locate gusts we use second-order polynomials in the range-azimuth plane. It is shown that predicted gust locations from simple linear projections of the least square fitted curves agree very well with actual gust locations.
Abstract
Pulse-to-pulse switching of polarizations (alternate transmission mode) is considered for polarimetric phased array radar (PAR). It is argued that the performance of the radar in terms of data quality should match or exceed the achieved standards of the Weather Surveillance Radar-1988 Doppler (WSR-88D). It turns out that the most stringent demand on the radar concerns the surveillance scan at the lowest elevations wherein the polarimetric variables are free of overlaid echoes, while ground clutter is significantly reduced. The scan uses a long pulse repetition time that has repercussion on the standard errors of the polarimetric variables and hence the choice of polarimetric mode. Herein the dwell time of this scan serves as a benchmark for comparisons of the accuracy of estimates. Because weather PAR should provide useful information at low signal-to-noise ratios (SNR) as low as those measured by the WSR-88D, the statistics of polarimetric variables, known at high SNR, is extended to low SNRs. It follows that the alternate mode would not match the performance of the simultaneous mode in the surveillance scans on the WSR-88D. Quasi-simultaneous transmission and reception of horizontally polarized and vertically polarized waves is discussed as a cost-effective alternative.
Abstract
Pulse-to-pulse switching of polarizations (alternate transmission mode) is considered for polarimetric phased array radar (PAR). It is argued that the performance of the radar in terms of data quality should match or exceed the achieved standards of the Weather Surveillance Radar-1988 Doppler (WSR-88D). It turns out that the most stringent demand on the radar concerns the surveillance scan at the lowest elevations wherein the polarimetric variables are free of overlaid echoes, while ground clutter is significantly reduced. The scan uses a long pulse repetition time that has repercussion on the standard errors of the polarimetric variables and hence the choice of polarimetric mode. Herein the dwell time of this scan serves as a benchmark for comparisons of the accuracy of estimates. Because weather PAR should provide useful information at low signal-to-noise ratios (SNR) as low as those measured by the WSR-88D, the statistics of polarimetric variables, known at high SNR, is extended to low SNRs. It follows that the alternate mode would not match the performance of the simultaneous mode in the surveillance scans on the WSR-88D. Quasi-simultaneous transmission and reception of horizontally polarized and vertically polarized waves is discussed as a cost-effective alternative.
This paper is an overview of weather radar polarimetry emphasizing surveillance applications. The following potential benefits to operations are identified: improvement of quantitative precipitation measurements, discrimination of hail from rain with possible determination of sizes, identification of precipitation in winter storms, identification of electrically active storms, and distinction of biological scatterers (birds vs insects). Success in rainfall measurements is attributed to unique properties of differential phase. Referrals to fields of various polarimetric variables illustrate the signatures associated with different phenomena. It is argued that classifying hydrometeors is a necessary step prior to proper quantification of the water substance. The promise of polarimetry to accomplish classification is illustrated with an application to a hailstorm.
This paper is an overview of weather radar polarimetry emphasizing surveillance applications. The following potential benefits to operations are identified: improvement of quantitative precipitation measurements, discrimination of hail from rain with possible determination of sizes, identification of precipitation in winter storms, identification of electrically active storms, and distinction of biological scatterers (birds vs insects). Success in rainfall measurements is attributed to unique properties of differential phase. Referrals to fields of various polarimetric variables illustrate the signatures associated with different phenomena. It is argued that classifying hydrometeors is a necessary step prior to proper quantification of the water substance. The promise of polarimetry to accomplish classification is illustrated with an application to a hailstorm.
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.
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.
Abstract
When spectral moments in the azimuth are spaced by less than a beamwidth, it is called oversampling. Superresolution is a type of oversampling that refers to sampling at half a beamwidth on the national network of Doppler weather radars [Weather Surveillance Radar-1988 Doppler (WSR-88D)]. Such close spacing is desirable because it extends the range at which small severe weather features, such as tornadoes or microbursts, can be resolved. This study examines oversampling for phased array radars. The goal of the study is to preserve the same effective beamwidth as on the WSR-88D while obtaining smaller spectral moment estimate errors at the same or faster volume update times. To that effect, a weighted average of autocorrelations of radar signals from three consecutive radials is proposed. Errors in three spectral moments obtained from these autocorrelations are evaluated theoretically. Methodologies on how to choose weights that preserve the desirable effective beamwidth are presented. The results are demonstrated on the fields of spectral moments obtained with the National Weather Radar Testbed (NWRT), a phased array weather radar at NOAA’s National Severe Storms Laboratory (NSSL).
Abstract
When spectral moments in the azimuth are spaced by less than a beamwidth, it is called oversampling. Superresolution is a type of oversampling that refers to sampling at half a beamwidth on the national network of Doppler weather radars [Weather Surveillance Radar-1988 Doppler (WSR-88D)]. Such close spacing is desirable because it extends the range at which small severe weather features, such as tornadoes or microbursts, can be resolved. This study examines oversampling for phased array radars. The goal of the study is to preserve the same effective beamwidth as on the WSR-88D while obtaining smaller spectral moment estimate errors at the same or faster volume update times. To that effect, a weighted average of autocorrelations of radar signals from three consecutive radials is proposed. Errors in three spectral moments obtained from these autocorrelations are evaluated theoretically. Methodologies on how to choose weights that preserve the desirable effective beamwidth are presented. The results are demonstrated on the fields of spectral moments obtained with the National Weather Radar Testbed (NWRT), a phased array weather radar at NOAA’s National Severe Storms Laboratory (NSSL).
Abstract
A new synthesis of information forming the foundation for rule-based systems to deduce dominant bulk hydrometeor types and amounts using polarimetric radar data is presented. The information is valid for a 10-cm wavelength and consists of relations that are based on an extensive list of previous and recent observational and modeling studies of polarimetric signatures of hydrometeors. The relations are expressed as boundaries and thresholds in a space of polarimetric radar variables. Thus, the foundation is laid out for identification of hydrometeor types (species), estimation of characteristics of hydrometeor species (size, concentrations, etc.), and quantification of bulk hydrometeor contents (amounts). A fuzzy classification algorithm that builds upon this foundation will be discussed in a forthcoming paper.
Abstract
A new synthesis of information forming the foundation for rule-based systems to deduce dominant bulk hydrometeor types and amounts using polarimetric radar data is presented. The information is valid for a 10-cm wavelength and consists of relations that are based on an extensive list of previous and recent observational and modeling studies of polarimetric signatures of hydrometeors. The relations are expressed as boundaries and thresholds in a space of polarimetric radar variables. Thus, the foundation is laid out for identification of hydrometeor types (species), estimation of characteristics of hydrometeor species (size, concentrations, etc.), and quantification of bulk hydrometeor contents (amounts). A fuzzy classification algorithm that builds upon this foundation will be discussed in a forthcoming paper.
Abstract
Disdrometer data collected during three spring days, with moderate to heavy rain in the Norman, Oklahoma region are used with various polarimetric radar algorithms to simulate rain rates. It is assumed that available measurables are 1) reflectivity at horizontal polarization, Z H , 2) differential reflectivity, Z DR (ratio of horizontal to vertical reflectivity factors in dB), and 3) differential propagation constant, K DP . The accuracies of the simulated rain rates from Z H , Z DR , and K DP are evaluated and compared. A new algorithm that utilizes both reflectivity factor and differential propagation constant is also examined. In comparing the relative accuracies, the disdrometer-derived rain rates are assumed to be the “truth” measurements.
Abstract
Disdrometer data collected during three spring days, with moderate to heavy rain in the Norman, Oklahoma region are used with various polarimetric radar algorithms to simulate rain rates. It is assumed that available measurables are 1) reflectivity at horizontal polarization, Z H , 2) differential reflectivity, Z DR (ratio of horizontal to vertical reflectivity factors in dB), and 3) differential propagation constant, K DP . The accuracies of the simulated rain rates from Z H , Z DR , and K DP are evaluated and compared. A new algorithm that utilizes both reflectivity factor and differential propagation constant is also examined. In comparing the relative accuracies, the disdrometer-derived rain rates are assumed to be the “truth” measurements.
Abstract
This paper describes the implementation of the staggered pulse repetition time (PRT) technique on NOAA's research and development WSR-88D in Norman, Oklahoma. The prototype algorithm incorporates a novel rule for the correct assignment of Doppler mean velocity that is needed to accommodate arbitrary stagger ratios. Description of the rule, consideration of errors, and choice of appropriate stagger ratios are presented. The staggered PRT algorithm is integrated with the standard processing on the WSR-88D, some details of which are included in the paper. A simple ground clutter canceller removes the pure complex time series mean (DC) component from autocovariance estimates; censoring of overlaid echoes and thresholding are equivalent to those used on the WSR-88D. Further, a cursory verification of statistical errors indicates good agreement with theoretical expectations. Although the staggered PRT algorithm operates in real time, it was advantageous to collect several events of staggered PRT time series data for further scrutiny. Results presented from one of the events demonstrate the potency of the staggered PRT to mitigate range and velocity ambiguities.
Abstract
This paper describes the implementation of the staggered pulse repetition time (PRT) technique on NOAA's research and development WSR-88D in Norman, Oklahoma. The prototype algorithm incorporates a novel rule for the correct assignment of Doppler mean velocity that is needed to accommodate arbitrary stagger ratios. Description of the rule, consideration of errors, and choice of appropriate stagger ratios are presented. The staggered PRT algorithm is integrated with the standard processing on the WSR-88D, some details of which are included in the paper. A simple ground clutter canceller removes the pure complex time series mean (DC) component from autocovariance estimates; censoring of overlaid echoes and thresholding are equivalent to those used on the WSR-88D. Further, a cursory verification of statistical errors indicates good agreement with theoretical expectations. Although the staggered PRT algorithm operates in real time, it was advantageous to collect several events of staggered PRT time series data for further scrutiny. Results presented from one of the events demonstrate the potency of the staggered PRT to mitigate range and velocity ambiguities.
Abstract
Currently, signal detection and censoring in operational weather radars is performed by using thresholds of the estimated signal-to-noise ratio (SNR) and/or the magnitude of the autocorrelation coefficient at the first temporal lag. The growing popularity of polarimetric radars prompts the quest for improved detection schemes that take advantage of the signals from the two orthogonally polarized electric fields. A hybrid approach is developed based on the sum of the cross-correlation estimates as well as the powers and autocorrelations from each of the dual-polarization returns. The hypothesis that “signal is present” is accepted if the sum exceeds a predetermined threshold; otherwise, the data are considered to represent noise and are censored. The threshold is determined by the acceptable rate of false detections that is less than or equal to a preset value. The scheme is evaluated both in simulations and through implementation on time series data collected by the research weather surveillance radar (KOUN) in Norman, Oklahoma.
Abstract
Currently, signal detection and censoring in operational weather radars is performed by using thresholds of the estimated signal-to-noise ratio (SNR) and/or the magnitude of the autocorrelation coefficient at the first temporal lag. The growing popularity of polarimetric radars prompts the quest for improved detection schemes that take advantage of the signals from the two orthogonally polarized electric fields. A hybrid approach is developed based on the sum of the cross-correlation estimates as well as the powers and autocorrelations from each of the dual-polarization returns. The hypothesis that “signal is present” is accepted if the sum exceeds a predetermined threshold; otherwise, the data are considered to represent noise and are censored. The threshold is determined by the acceptable rate of false detections that is less than or equal to a preset value. The scheme is evaluated both in simulations and through implementation on time series data collected by the research weather surveillance radar (KOUN) in Norman, Oklahoma.
