The potential use of differential reflectivity measurements at orthogonal polarizations to determine rainfall 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-attenuating wavelength radar are possible under ideal conditions.

No abstract

No abstract

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.

No abstract