Identification of the atmospheric types that promote summer hail events in the Northern Italy

Hailstorms are among the most damaging weather phenomena impacting people's life and human activities. Climate change is expected to modify the environmental conditions in which hailstorms typically develop, affecting low-level moisture and convective instability, microphysical processes, and vertical wind shear. This is particularly true in a climatic hotspot such as the Mediterranean area, which is one of the most hail-prone areas of the Earth. Despite their relevance and associated risk in the Mediterranean, open issues remain concerning the identification of the synoptic-scale meteorological conditions favourable to hailstorms’ occurrence.

Using the high-resolution (1°x1°) daily satellite-based hail climatology developed in Laviola et al. (2022), this work aims to identify the main summer large hail (>2 cm) spatial patterns in the Northern Italy (44.0-47.0°N, 6.0-14.0°E) and the associated atmospheric types. To pursue such goals, Principal Component Analysis and Cluster Analysis have been employed. Several atmospheric fields retrieved from ERA5 have been considered to characterize the large-scale atmospheric patterns. To mitigate possible inconsistencies in the available dataset, the period 2014-2023 has been selected for this analysis.

Main results shows that the spatial distribution of large hail events in Northern Italy follows three main clusters that appear to be relatively well separated among them. In the first cluster, the hail episodes mainly affect the northeastern Italy; in the second cluster, the hail events are concentrated over the Po Plain, whereas in the third cluster the hailstorm generally affect the northwestern sector. The atmospheric circulation schemes associated with the first two spatial clusters (ACS  1 and ACS 2) are linked by the presence of a trough over the western Mediterranean basin, which advects a warm and moist southwestern flow over northern Italy, and by a thermal boundary at 850-hPa level. The ACS related to the third cluster (ACS 3) is characterized by a cyclonic area over western Europe, associated with a strong thermal boundary over France with a hot subtropical southwestern flow over Italy. Another key meteorological feature of the identified synoptic patterns lies in the anomalous intense water-vapor transport in the 2-5 km atmospheric layer, favouring large liquid water content. From a comparison between 2014-2018 and 2019-2023 sub-periods, it emerged an increase of the relative contribution to the total number of hail events provided by ACS 3 (from 19.6 to 31.8%). This means that hail events are increasing in the northwestern Italy in terms of frequency of occurrence, both in absolute and relative terms.

 

References

Laviola, S.; Monte, G.; Cattani, E.; Levizzani, V. Hail Climatology in the Mediterranean Basin Using the GPM Constellation (1999–2021). Remote Sens. 2022, 14, 4320. https://doi.org/10.3390/rs14174320.