Hailstone Characteristics in Northeast Italy from 29 Years of Hailpad Data

Abstract

Although hail is a well-known meteorological hazard, it is hard to find long records of hail observed at the ground with high spatial resolution. Most hail climatologies are based on remote sensing observations or inhomogeneous networks of human observers. In the plain of Friuli Venezia Giulia (northeast Italy), a hailpad network of 367 stations has operated since 1988. During the 1988–2016 warm seasons, 7782 hailpads were impacted by hailstones and more than one million dents were observed and automatically analyzed, even though only 63% of them were associated with valid hailstone dents. In this work, this large quantity of direct hail observations is used to build a hail climatology in terms of hailstone size, areal density, and flux of kinetic energy. From the empirical distributions of data collected, it is possible to fit statistical distributions to the different hailstone/hailpad behaviors. In particular, it is also possible to find an approximate estimation of the flux of kinetic energy based only on the largest hail diameter observed on the hailpad. Last, temporal and spatial distributions of these characteristics are investigated. Hailstones are larger along a southwestern-to-northeastern alley, which is parallel to the main pre-Alpine crest, with the very largest sizes being more frequent on the southwestern corner. The only hail climate change signal that one can infer from the analysis of these multidecadal trends is that, in more recent years, hailstorms seem to produce fewer and larger hailstones, on average.