A Dual-Wavelength Radar Hail Detector

Abstract

It is proposed that the range derivative of the logarithm of the ratio of average echo powers from two (S- and X-band) synchronized and slaved radars would yield a highly reliable indication of the boundaries of hail shafts. In the presence of rain alone, and ignoring fluctuations, this derivative would always be positive and proportional to the incremental difference in attenuation at the smaller wavelength. In general, the derivative has the same sign as the hail concentration gradient and attains negative values on the far side of a hail shaft. Without hail, signal fluctuations are the only possible source of negative derivatives, and so of false alarms. Thus, a small negative threshold level would avoid the identification of the effect of signal fluctuations at the far side of a hail shaft; similarly a large positive threshold would avoid identifying regions of intense rain as the near side of a hail shaft. This approach is capable of detecting smaller concentrations of hail with greater confidence and in larger backgrounds of non-hail precipitation than the use of the dual-wavelength reflectivity ratio alone because 1) it requires a smaller hail reflectivity ratio, at the two wave-lengths; 2) it is not affected significantly by attenuation, end 3) it is independent of absolute radar calibrations. The limitations of the technique are discussed.