Multi‐wavelength radar reflectivity of hailstorms

Computations are made of the radar reflectivity Z e of particles present in hailstorm updraughts in exponential size spectra. The reflectivity varies substantially according to the radar wavelength and according to whether the particles are wet or dry. In severe storms a very strong echo may be observed aloft. Partly by reference to a particular example it is shown that its magnitude at the shorter wavelengths ( Z e ± 10 7 mm 6 m –3 ), and the variation of its intensity with wavelength, can reasonably be explained only if the responsible particles are large dry hailstones of almost uniform size. The dryness and sorting of the large stones is attributed to fall outside the updraught through a wind shear, and the decrease of echo intensity lower down to their re‐entry into the updraught and wetting. It appears that the likelihood and size of hail at the ground are indicated by echo intensity aloft at 3 cm, and less sensitively at 4·7 cm, but not at 10 cm. Measurements of echo intensity at several wavelengths are helpful in the study of severe storms.