Dusan S. Zrnic

Bulk Hydrometeor Classification and Quantification Using Polarimetric Radar Data: Synthesis of Relations

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.

Cloud Microphysics Retrieval Using S-Band Dual-Polarization Radar Measurements

Recent studies have shown the utility of polarimetric radar observables and derived fields for discrimination of hydrometeor particle types. Because the values of the radar observables that delineate different particle types overlap and are not sharply defined, the problem is well suited for a fuzzy logic approach. In this preliminary study the authors have developed and implemented a fuzzy logic algorithm for hydrometeor particle identification that is simple and efficient enough to run in real time for operational use. Although there are no in situ measurements available for this particle-type verification, the initial results are encouraging. Plans for further verification and optimization of the algorithm are described.

Polarimetry for Weather Surveillance Radars

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.

THE JOINT POLARIZATION EXPERIMENT Polarimetric Rainfall Measurements and Hydrometeor Classification

As part of the evolution and future enhancement of the Next Generation Weather Radars (NEXRAD), the National Severe Storms Laboratory recently upgraded the KOUN Weather Surveillance Radar-1988 Doppler (WSR-88D) to include a polarimetric capability. The proof of concept was tested in central Oklahoma during a 1-yr demonstration project referred to as the Joint Polarization Experiment (JPOLE). This paper presents an overview of polarimetric algorithms for rainfall estimation and hydrometeor classification and their performance during JPOLE. The quality of rainfall measurements is validated on a large dataset from the Oklahoma Mesonet and Agricultural Research Service Micronet rain gauge networks. The comparison demonstrates that polarimetric rainfall estimates are often dramatically superior to those provided by conventional rainfall algorithms. Using a synthetic R(Z, KDP, ZDR) polarimetric rainfall relation, rms errors are reduced by a factor of 1.7 for point measurements and 3.7 for areal estimates [when ...

Estimation of Spectral Moments for Weather Echoes

Estimators of echo signal power, mean Doppler velocity, and spectrum width are investigated. Maximum likelihood (ML) solutions can improve the estimate variances by an order of magnitude over those by the autocovariance or the Fourier method. However, the required computations are excessive for routine implementations on weather radars. Spectrum power estimation together with two conventional methods of mean Doppler and width estimation are reviewed. Several results previously not available are presented.

An Examination of Propagation Effects in Rainfall on Radar Measurements at Microwave Frequencies

Propagation effects in rainfall are examined at three microwave frequencies corresponding to S (3.0 GHz), C (5.5 GHz), and X ( 10.0 GHz) bands. Attenuation at horizontal polarization, as well a s differential attenuation and differential propagation phase between horizontal (HI and vertical ( V ) polarizations are considered. It is shown that at the three frequencies both attenuation and differential attenuation are nearly linearly related to differential propagation phase ( cbDP). This is shown through simulation using ( a ) gamma raindrop size distributions (RSD) with three parameters (No, DO. m ) that are varied over a very wide range representing a variety of rainfall types, and ( b ) measured raindrop size distributions at a single location using a disdrometer. Measurements of X-band specific attenuation and S-band specific differential phase in convective rainshafts using the National Center for Atmospheric Research CP-2 radar are presented in order to experimentally demonstrate the linear relationship between attenuation and differential propagation phase. Correction procedures for reflectivity and differential reflectivity (&) are developed assuming that differential propagation phase is measured using a radar that alternately transmits H and V polarized waves with copolar reception through the same receiver and processor system. The correction procedures are not dependent on the actual rainrate profile between the radar and the range location of interest. The accuracy of the procedure depends on, (a) RSD fluctuations, (b) variability in the estimate of differential propagation phase due to measurement fluctuations, and ( c ) nonzero values of the backscatter differential phase ( & ) between H and V polarizations. Simulations are used to gauge the accuracy of correction procedures at S- and C-bands assuming 6 is negligible. The correction accuracy for attenuation at S-band is estimated to be -0.05 dB while at C-band it is estimated to be within I dB if (IDP G 60 deg). Simulations further indicate that C-band differential attenuations effects can be corrected to within -35% of the mean value.

Simulation of Weatherlike Doppler Spectra and Signals

Abstract A versatile algorithm to generate weatherlike spectra of any desired shape is described, and applications are briefly discussed.

Agile-Beam Phased Array Radar for Weather Observations

Weather radars with conventional antenna cannot provide desired volume scan updates at intervals of one minute or less, which is essential for significant improvement in warning lead time of impending storm hazards. The agile-beam multimission phased array radar (MPAR) discussed herein is one potential candidate that can provide faster scanning. It also offers a unique potential for multipurpose use to not only sample weather, but support air traffic needs and track noncooperative airplanes, thus making it an affordable option. After introducing the basic idea behind electronic beam steering, the needs for frequent observations of convective weather are explained. Then, advantages of the phased array radar (PAR) for weather monitoring and improving data quality are examined. To explore and develop weather-related applications of the PAR, a National Weather Radar Testbed (NWRT) has been established in Norman, Oklahoma. The NWRTs main purpose is to address the advanced capabilities anticipated within the n...

Spectral Moment Estimates from Correlated Pulse Pairs

Estimates statistics of the first two power spectrum moments from the pulse pair covariance are analyzed. The input signal is assumed to be colored Gaussian and the noise, white Gaussian. Perturbation formulas for the standard deviation of both mean frequency and spectrum width are applied to a Gaussian shaped power spectrum, and so is a perturbation formula for the bias in the width estimate. Mean frequency estimation from interlaced pulse pairs is presented. Throughout this study, estimators from independent, spaced, and contiguous pulse pairs are compared to provide a continuum of statistics from equispaced tightly correlated to statistically independent pulse pairs.

Advantages of Rain Measurements Using Specific Differential Phase

Abstract The following advantages of rain measurements using specific differential phase are examined: 1) immunity to beam blockage, 2) immunity to ground clutter canceling, and 3) case to isolate effects of anomalous propagation. We quantify immunity to beam blockage via examples of measurements that corroborate theoretical expectations. Comparisons of rain accumulations between radar and rain gauges are included. We also contrast beam-filling effects on rain estimates from reflectivity and specific differential phase.