Radar reflectivity of storms containing spongy hail

Experimental measurements at 5-cm wavelength show that the backscattering cross sections of ice spheres approximately 2 cm in diameter which are coated with a mixture of water and ice (‘spongy’ coating) are 3 to 4 db above that of the equivalent all-water spheres and at least 10 db above that of the equivalent solid ice spheres. Thus it is shown that the extremely large reflectivity factor at 3-cm wavelength (Ze = 107 mm6 m−3) observed for hailstones can be accounted for with concentrations of only about 3 g m−3 of 1-cm-diameter hailstones which have a spongy coating. Both the falling away of the spongy coating upon melting and its freezing upon being lifted in the updraft will cause reductions in reflectivity, thereby producing the maximum reflectivity observed at an intermediate level of the storm. The existence of spongy ice allows the surface and radar observations of hailstorms to be explained without any of the physically implausible assumptions of previous hypotheses.