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

In Europe, hailstorms are a major source of weather-related damages, and Northern Italy represents one of the areas at the greatest risk. Although several works have analysed severe hailstorms in the Mediterranean area, they are largely case-based and do not provide a systematic classification of the synoptic circulation regimes that favour them. In this study, we analysed summer hail events in Northern Italy over the period 2014-2023 using the satellite-based Microwave Cloud Classification dataset, which provides detection of hail ≥2 cm. A total of 122 events were identified and classified through Principal Component Analysis and k-means clustering, leading to three distinct spatial patterns. ERA5 reanalysis was used to characterize the associated large-scale circulation. Common features across all clusters include south-westerly flow in the mid-troposphere, upper-level divergence at 250 hPa, and enhanced mid-tropospheric moisture transport. Spatial pattern 1 is mainly linked to hail in north-eastern Italy, spatial pattern 2 to the western Po Plain and Ligurian Sea areas, and spatial pattern 3 to north-western Italy and the Alps. The pattern 3 is particularly distinct, showing stronger geopotential and temperature anomalies, with moisture transport between 600 and 800 hPa exceeding the 90th percentile of the decade. Comparing the sub-periods 2014-2018 and 2019-2023, we found both an increase in hail frequency and a larger contribution of Cluster 3. These results highlight the role of synoptic-scale variability in shaping hail risk in Northern Italy and suggest a shift toward circulation types more favourable for Alpine hail over the past years. The methodology and findings provide a framework for future analyses linking hail occurrence to large-scale climate variability and projections.