Climate extremes in the Mediterranean region: the challenges of predicting hailstorms with current climate model simulations

Within the global effort to understand climate extremes and their projected changes, the response of hailstorms to global warming remains unclear. Hailstorms are difficult to model due to their low frequency of occurrence, scarcity and short duration of available observations. Moreover, the spatio-temporal scales involved in hail formation are small, requiring high resolution modelling. In this study, we present a novel approach to model future changes in hailstorm frequencies by combining reanalysis data with climate model projections, in an attempt to overcome the issue of coarse model resolution. We identify a set of 84 storm-related parameters, which are found to be good hail predictors and can be estimated by current global or regional climate model simulations. The hail predictors were retrieved from 3-hourly time scale ERA5 reanalysis data during 1999-2023, and calibrated against a hail dataset (Laviola et al. 2022) derived from passive microwave satellite observations over the whole Mediterranean basin (5W-35E, 25N-50N). A multi-model of 9 CMIP6 simulations was developed, selecting models with daily output, spatial resolution finer than 100 km and availability of variables (relative humidity, geopotential height, temperature and u- and v-component of wind) used to calculate the 84 hail predictors. ERA5-based hail predictors were projected to the end of the century using CMIP6 multi-model trends, adopting a trend-based scaling approach. The proposed technique provides a simple yet robust framework for assessing future changes in the occurrence of hail events. Hail climatologies were also compared to those of the extreme ETCCDI indices. Results of the assessment indicate that the multi-model is able to simulate the observed hail climatology over the Mediterranean with good accuracy (pattern correlations of 0.90), particularly capturing the main maxima over the Po Plain and over the central Mediterranean sea between Sicily and Greece. Eventually, a strong connection was found between large-scale atmospheric circulation (represented by the Euro-Atlantic weather regimes of Grams et al. 2017) and hailstorm occurrence, as also highlighted by ten exceptional hailstorms (>10 cm) occurring during summer 2023.