Characterization of hailstone size spectra in hailpad networks in France, Spain, and Argentina

Hailstone size distribution has been an integral part of hailfall measurement in several research projects. The exponential distribution is usually assumed to be an adequate representation of hailstone size spectra. This paper presents the results of the fitting to the exponential distribution of databases obtained from several hailpad networks set up in southern France (3 networks with 1,081 hailpads), Spain (2 networks with 269 hailpads), and Argentina (3 networks with 567 hailpads). These networks use the same type of plate, method of calibration, and semi-automatic measurement procedure. Results show that the characteristic parameters of the fittings are different in each area, even in areas that lie geographically quite close to each other. The values of the slopes obtained in the French networks are higher than the ones found in Spain and Argentina. Statistical techniques were applied to detect “breakpoints” in the hail size distribution, using piece-wise regression models (or “broken-stick” models). These points were used to estimate critical thresholds associated with transitions between two hailstone size distributions. In 4 networks in Spain and Argentina, breakpoints appear in large hail. The results show that, in 2 of the networks, the distribution is exponential (statistically significant) again from the breakpoint on.