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

Abstract

Hailfall is one of the most dangerous and damaging phenomena associated with severe thunderstorms. Its remote identification is still under investigation, especially by means of polarimetric radar data. The need to use polarimetric measurements is a limit to operational implementation of the techniques developed up till now. The routine sampling of radar allows the computation of the differential hail signal H_DR (Aydin et al., 1986) every 15 minutes. The assessment of the HDR is performed by comparison with the ground truth from a hailpad network. The spatial resolution of this network is quite coarse (one pad per 16 km²) for the collection of all the hail shafts, as they generally have a small horizontal extent and are short lived. Such bounds have been considered in the analysis of four large hailfalls recorded in 1997. This study reveals also the limitation on radar hail detection arising from the frequent occurrence of strong differential attenuation. Nevertheless, to identify regions where a sharp rain-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 probability of correct hail detection as well as an acceptable number of false alarms.