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[1] Several authors have reported the correlation between the shape (μ) and slope (Λ) of the gamma distribution of raindrops to reduce the number of parameters required to measure rainfall by remote sensing methods. However, we find that there are no well-defined μ-Λ, or associated relations between reflectivity (Z) and rain rate (R) or differential reflectivity for all storms or portions thereof. Rather, there is a general behavior such that A and b (in the Z = ARb relation) and median volume drop diameter Do all decrease from convective (C) to stratiform (S) to transition (T) rains. The μ-Λ correlation of the investigators in question appears to be limited to rainfall events which do not include convective rain; it is biased toward S and T rains. They miss the narrow (large μ), large Do DSDs of convective rain that are often found to have equilibrium spectra. The dependence of Do on the strength of the updraft and the findings of others concerning the association with the physics, dynamics, and climate regime strongly suggests that it is necessary to characterize the physical and dynamic nature of the storms in order to select the appropriate remote sensing algorithms.

estimated from power measurements. Comparison of the direct estimate of propagation phase (F m ) measurement, which is unaffected by absolute calibration of the radar system, with the estimated propagation phase (F e ) from power measurements is the basis for the calibration method. Polarization measurements such as reflectivity (Z ), differential reflectivity (ZDR), and differential propagation phase (FDP) are sensitive to drop size distribution (DSD) and mean drop shape. It is important to devise a calibration technique relatively unperturbed by changes in DSD and drop shape. Statistical fluctuation in F e is derived to estimate the accuracy of the calibration procedure, raindrop shape, and attenuation. The proposed method is applied for calibrating reflectivity measurements of the National Center for Atmospheric Research (NCAR) S-band polarization radar (S-Pol) in midlatitude, subtropical, and tropical rain events. INDEX TERMS: 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; 6904 Radio Science: Atmospheric propagation; 6969 Radio Science: Remote sensing; KEYWORDS: polarization radar calibration, reflectivity, differential reflectivity, differential phase, raindrop size distribution

Whereas different hydrometeor types cannot be readily distinguished using radar reflectivity alone, this paper shows how the differential reflectivity available from dual-polarization radars can be used as an indicator of hydrometeor type (when supported by reflectivity factor also), and that the spatial variability of differential reflectivity is a useful addition. Various hydrometeor types, ground targets, and burnt chaff from straw fires may be identified, whereas conventional radar reflectivity measurement alone would require considerable pattern recognition to make the same distinctions.

This paper summarizes some of the highlights of the symposium, synthesizes them in the context of prior observations and understanding, and attempts to resolve apparent conflicts. The differential reflectivity ZDR technique has shown excellent results in the quantitative measurement of rainfall. Systematic differences were found to be due to deviations from the originally assumed exponential drop size distribution. The consensus was that the circular depolarization ratio (CDR) method is more versatile and powerful but that ZDR is simpler and cheaper. Precautions in the use of ZDR have been noted. The apparent conflict between CDR and ZDR measurements near the melting layer seems to have been resolved. A spectrum of scientific and practical applications has been identified. Most importantly, the ZDR method is recommended for further development and use wherever accurate rainfall measurements are required.

A novel approach to making differential reflectivity measurements in multiple polarization planes has been proposed. This paper describes a versatile technique using a mechanical polarization switch and the systematic errors in differential reflectivity measurements which result. Initial results of simultaneous X band slant path measurements of attenuation and depolarization at 11.6 GHz and differential reflectivity measurements at 2.8 GHz using a slow polarization switch are presented.

A measurement technique employing the differential reflectivity factor Z(DR) and the horizontal polarization reflectivity factor Z(H) is the basis of the present study of the effects of variations in the shape or breadth of the drop size distribution (DSD) on rainfall parameters. Theoretical expressions are derived for rainfall rate, liquid water content W, and median volume diameter D(O) in terms of Z(DR), Z(H), and size distribution-dependent factors. The latter calculations assume backscattering cross sections for oblate, nonoscillating raindrops falling in still air with equilibrium shapes. These expressions are used to quantitatively assess the effects of changes in DSD breadth on values of R, W, and D(O) deduced from Z(DR) and Z(H).

The differential scattering properties of classes of hydrometeors at linear orthogonal polarizations provide potentially important differences which may be exploited for radar measurements of precipitation. The ratio of the reflectivity at horizontal (ZH) and vertical (Zv) polarizations may be combined with other radar measurements such as absolute reflectivity and differential phase shift to determine drop size distributions. Previous model calculations of differential reflectivity ZDR = 10 log (ZH/Zy) were made using Gans' theory of scattering by spheroids. New calculations, using Waterman's T-matrix method, are in excellent agreement with Gans' theory for raindrops at S-band wavelengths. The ZDR-differential phase shift combination, which need not depend on any absolute measurements, is shown to be a reasonable radar method for measuring two-parameter raindrop size distributions. Finally, calculations of ZDR for oblate hailstones indicate the possibility of uniquely detecting large, dry hail when the hail is falling with preferred orientation.