Detection of hail by dual-polarization radar

Abstract

Hail causes considerable damage to property and agriculture and a simple, unambiguous method of determining which convective storms contain hail would be of great use for short-term weather forecasting, and would also find application in the study of the mechanism of hailstone growth. Previous radar methods of hail detection^1–3 have not proved reliable, but here we report results of a new technique based on the measurement of differential reflectivity using dual polarization radar and suggest an additional criterion for hail recognition. Hail is known to occur in vigorous convective storms in which the effective radar reflectivity, Z, measured by conventional radar is high. If the hail reaches the ground it is usually restricted to long narrow swathes⁴, often about 1 km wide but up to 50 km long, accompanied by surrounding precipitation in the form of heavy rain. The differential reflectivity technique involves comparing the radar reflectivity factor for horizontally (Z_H) and vertically (Z_V) polarized radiation; the comparison provides a measure of the mean shape of the precipitation particles. Close to the ground the values of Z are high for both hail and heavy rain, but because the large raindrops are oblate and fall with their horizontal diameter greater than the vertical, the value of Z_H in the rain exceeds that of Z_V. The smaller regions where hail is present can be clearly distinguished from the neighbouring heavy rain because, although individual hailstones may not be spherical, they tumble as they fall and so result in equal values of Z_H and Z_V.