Detection of hail by means of polarimetric radar data and hailpads: results from four storms

Hailfall is one of the most dangerous and damaging phenomena associated wit h severe thunderstorms. Its remote identification is still under investigat ion, especially, by, means of polarimetric radar data. The need to use pola rimetric measurements is a limit to operational implementation of the techn iques developed up till now. The routine sampling of radar allows the compu tation of the differential hail signal H-DR (Aydin et al., 1986) every 15 m inutes. The assessment of the H-DR is performed by comparison with the grou nd truth from a hailpad network. The spatial resolution of this network is quite coarse (one pad per 16 km(2)) for the collection of all the hail shaf ts, as they generally have a small horizontal extent and are short lived. S uch bounds have been considered in the analysis of four large hailfalls rec orded in 1997. This study reveals also the limitation on radar hail detecti on arising from the frequent occurrence of strong differential attenuation. Nevertheless, to identify regions where a sharp min-hail transition occurs , polarimetric measurements work much better than algorithms based only, on horizontal reflectivity. Results show that use of H-DR gives a high probab ility of correct hail detection as well as an acceptable number of false al arms.