A comprehensive description of first August 2021 hailstorm in Azzano Decimo, NE Italy

On 1 August 2021, a vigorous hailstorm hit Azzano Decimo, in northeastern Italy. The supercell storm produced hailstones up to 10 cm in maximum dimension, which was quite unusual in this area. The storm’s environment registered one of the largest potential instabilities (> 3400 Jkg−1) ever observed by the local operational Udine radiosonde site. The mesoscale environment supporting the storm is analyzed, with particular focus on the distribution of θe. Observations from the nearby operational Fossalon di Grado dual-polarization radar showed the presence of a pronounced Bounded Weak Echo Region and differential reflectivity column, both proxies for intense updrafts. However, Doppler velocities revealed only weaker winds, with the mesocyclone mostly confined to midlevels and largely absent at low levels. A comparison with the recent storm producing the new European hail size record (about 19 cm maximum dimension, which fell on 24 July 2023 also in Azzano Decimo) revealed for the latter a much lower CAPE, but a much more pronounced mesocyclone and higher Bounded Weak Echo Region. Two independent observers in Azzano Decimo collected nine hailstones for the 1. August 2021 event, including one with a maximum dimension of 9 cm. The physical structure of these hailstones was analyzed in the National Center for Atmospheric Research (NCAR, Boulder, USA) cold room, including normal and cross-polarized light photographs of thin sections to identify the different growth layers inside each hailstone. This analysis revealed a large variability in the internal structure even if the hailstones were collected in the same location and at the same time. Additionally, ice samples were taken from 1 cm x 1 cm x 1mm pieces from these thin slices. The analyses of the oxygen and hydrogen isotope composition were performed in Venice (Italy) on these specimens, using a Picarro cavity ring-down spectrometer. Unfortunately, the Jouzel 1975 isotopic model failed to relate the isotopic composition to specific cloud temperatures (estimated from adiabatic analysis), because of the large variability present in the isotopic data of the hailstone samples.