Thomas A. Seliga

Potential Use of Radar Differential Reflectivity Measurements at Orthogonal Polarizations for Measuring Precipitation

Abstract The potential use of differential reflectivity measurements at orthogonal polarizations to determine rain-fall rate is examined. The method involves measurements of ZH and ZV, the radar reflectivity factors due to horizontally and vertically polarized incident waves respectively. The differential reflectivity, ZDR = 10 log (ZH/ZV), which should be precisely determinate, occurs as a result of the distortion of raindrops as they fall at terminal velocity. The approximate theory of Gans for electromagnetic scattering by spheroids is applied to the distorted raindrops. Assuming a general exponential form for the raindrop size distribution, equations are derived relating the distribution parameters to the measurements. The determination of rainfall rate follows directly. Finally, the sensitivity of the distribution parameters to radar inaccuracies is examined, and several methods of implementing the measurements are suggested. It is concluded that good estimates of rainfall rate using a single non-att...

A Preliminary Study of Comparative Measurements of Rainfall Rate Using the Differential Reflectivity Radar Technique and a Raingage Network

Abstract Radar measurements of average rainfall rate over two separate 550 km2 areas am compared with raingage measurements in the same areas over time intervals of ∼1 h. The measurements were performed to test the differential reflectivity (ZDR) technique of Soliga and Bringi (1976) which provides a means of estimating rainfall rate (R)by combining measurements of radar reflectivity factors at horizontal (ZH) and vertical (ZV) polarizations; ZDR(dB) is defined as the ratio of these reflectivities, i.e., ZDR = 10 log(ZH/ZV). Results from an experiment performed near Chicago, Illinois, on 9 August 1978, using the University of Chicagco-Illinois State Water Survey (CHILL) radar and the Illinois State Water Surveys Chicago Hydrometeorological Area Project (CHAP) raingage network are presented. ZDR estimates of rainfall rate compared very favorably with the raingages measurements and were significantly better than estimates obtained from two Z-R relationships, one of which was obtained by raingage calibratio...

Hail Detection with a Differential Reflectivity Radar

A major objective in the remote sensing of convective storms by radar is the clear and reliable differentiation between regions of hail and regions of rainfall. This report describes the application of the differential reflectivity (ZDR) radar technique to the problem of hail detection. The procedure is based on the markedly different polarization-dependent backscatter characteristics of rain and hail. Field experiments conducted in Colorado during the spring of 1983 provide significant evidence of hail detection with this new radar technique.

Statistical Properties of the Dual-Polarization Differential Reflectivity (ZDR) Radar Signal

The statistical characteristics of three estimators of the differential reflectivity (ZDR) radar signal are examined theoretically and compared with pulse-to-pulse measurements of reflectivity at horizontal and vertical polarizations. The square-law and log-ratio estimators provide satisfactory means for measuring ZDR while the log-ratio estimator does not. In addition, the theory is consistent with radar data, clearly showing that ZDR could be measured accurately from a sequence of alternately polarized waves.

Differential Reflectivity Measurements in Rain: First Experiments

Results of the first measurements of differential reflectivity (ZDR) in rain are reported. ZDR = 10 log (ZH/ZV) dB involves copolar measurements of ZH and ZV, the radar reflectivity factors at horizontal and vertical polarizations, respectively. The data were obtained with the University of Chicago-Illinois State Water Survey (CHILL) radar facility in Oklahoma during Spring 1977 and confirmed theoretical expectations that ZDR should be positive, ranging between around 0-4 dB. By combining ZDR measurements with ZH, it is shown how estimates of N0 and D0, the parameters of an exponential raindrop size distribution, can be obtained. These estimates were subsequently used to compute rainfall rates as a function of range along a radar ray. These results illustrate that ZDR can have important applications in the quantitative, remote measurement of drop-size distributions and rainfall.

Scattering from axisymmetric dielectrics or perfect conductors imbedded in an axisymmetric dielectric

Watermans T -matrix formulation of classical electromagnetic scattering from a single, homogeneous scatterer has been extended by Peterson and Strom to include scattering from multilayered scatterers. The T -matrix refers to expansions in spherical wave solutions of the vector Helmholtz equation. This theory and the computational method for calculating backscattering cross sections for axisymmetric scatterers are developed. This approach, using the equivalence principle, yields a more concise and systematic development to the T -matrix formulation than that of Peterson and Strom which relies upon the conceptually similar Poincare-Huygens principle. The method is realistically applicable to scatterers of sizes up to ka \sim 2\pi and for a wide range of dielectric constants. Sample computations are compared with extended Mie theory calculations of scattering by concentric shells of varying size and with measured backscattering cross section obtained from a displaced spherical perfect conductor imbedded in a lossy, spherical dielectric.

First Comparisons of Rainfall Rates Derived from Radar Differential Reflectivity And Disdrometer Measurements

A comparison between radar-derived rainfall rates and those obtained from a ground-based disdrometer located at the radar site is presented. The radar measurements were made in the vicinity of Chicago, IL, on August 2, 1978, with the CHILL radar of the Illinois State Water Survey. Three radar-derived profiles of rainfall rate versus distance from the radar were obtained for a single storm in the direction of the storm path toward the radar. These profiles indicated the storm motion and the variability of rainfall as the storm progressed toward the radar. The last two profiles support the hypothesis that the storm was in steady state as it passed over the radar site. This provided an opportunity to compare the radar-derived rates with an apparent rate versus distance profile derived from a disdrometer time record of rainfall rate obtained at the radar site. Excellent agreement between the radar and disdrometer measurements of rainfall resulted.

Scattering from non-spherical hydrometeors

A new theoretical formulation for scattering from a wide class of non-spherical hydrometeors has been developed recently using Waterman’s extended integral equation technique. The transition or T-matrix formulation is an exact solution to the scattering problem and computer programs have been developed to handle both homogeneous and imbedded bodies (e.g., a dielectric imbedded within another dielectric body). This theory will be briefly reviewed followed by sample computations of backscattering calculations of (a)oblate spheroidal icestones of varying sizes and eccentricities, (b)ice-stones with surface perturbations to model roughness, and (c)rough ice-stones covered with a thin coating of water. The method is realistically applicable for sizes up to D ≈3 λand for a wide range of dielectric constants. Comparisons with measurements of backscattering from a non-concentric dielectric-clad spherical perfect conductor is included.AnalyseRécemment, en utilisant la technique de l’équation intégrale étendue de Waterman, on a développé une nouvelle formulation théorique de la diffusion par une classe d’hydrométéore non sphérique. La formulation de la transition ou matrice en T est une solution exacte des problèmes de la diffusion, aussi a- t- il été possible d’élaborer des programmes pour calculateurs s’appliquant à la fois aux corps homogènes et aux corps inhomogènes (par exemple un diélectrique enrobé dans un autre). Cette théorie sera brièvement exposée et suivie d’un exemple de calcul de rétrodiffusion de grêlons (a)en forme de sphéroïde aplati de différentes dimensions et excentricités, (b)avec perturbations de surface par rapport à la rugosité du modèle, (c)recouverts d’une fine pellicule d’eau. La méthode est parfaitement applicable à des grêlons dont le diamètre atteint 3 λet pour une gamme étendue de valeur de la constante de diélectrique. On établit des comparaisons avec des mesures de rétrodiffusion provoquée par une sphère parfaitement conductrice revêtue d’une couche non concentrique de diélectrique.

First international symposium on acid precipitation and the forest ecosystem

These Proceedings report on the results of The First International Symposium on Acid Precipitation and the Forest Ecosystem which was held at The Ohio State University, Columbus, Ohio, U.S.A., on May 12-15, 1975. The Symposium focused on four related topics: (1) atmospheric chemistry, transport and precipitation; and effects of acidic precipitation on (2) aquatic ecosystems, (3) forest soils, and (4) forest vegetation.

Report from the First International Symposium

The First International Symposium on Acid Precipitation and the Forest Ecosystem sponsored by the United States Department of Agriculture, Forest Service, Northeastern Forest Experiment Station, and by the Atmospheric Sciences Program of The Ohio State University was held in Columbus, Ohio, May 12-15, 1975. Approximately 300 people from 12 countries attended the Symposium.