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

TLDR

Differential reflectivity (ZDR) results from raindrop distortion during terminal fall and can be theoretically determined. The study examines using orthogonal‑polarization differential reflectivity to estimate rainfall rate. The authors use ZH and ZV measurements, apply Gans’ scattering theory to distorted spheroidal drops, derive equations linking size‑distribution parameters to the data, and assess sensitivity to radar errors while proposing implementation strategies. The results show that accurate rainfall rate estimates can be obtained with a single non‑attenuating‑wavelength radar under ideal conditions.

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-attenuating wavelength radar are possible under ideal conditions.