Comparison of future hail trends across Europe based on the HAILCAST diagnostic and hail proxies

Thunderstorm-related severe weather, particularly hail, causes extensive damage to life and infrastructure across Europe. However, the effect of a warmer climate on the occurrence of hail is still not fully understood. To date, most projections of hail occurrence under future climate scenarios have relied on hail proxies derived from global and regional climate models that use parameterized representations of convection. Recently, convection-permitting regional climate simulations with a high computational resolution of 2 km, using the COSMO model with the online hail diagnostic HAILCAST, have provided new insights. The simulations revealed spatially contrasting changes in hail frequency under a 3°C global warming scenario, showing a substantial decrease in summer hail frequency in southwestern Europe and an even larger increase in central and eastern Europe. In this study, we leverage these high-resolution model outputs to assess future projections of hail occurrence. Specifically, we compare differences in hail day frequencies between a warmer future climate and the present day climate as derived from (i) traditional hail proxies using environmental variables (e.g. CAPE and wind shear) and (ii) the HAILCAST online diagnostic. Through this comparison, we aim to better understand two key questions: (1) how accurately hail proxies capture the spatial and temporal patterns of hail occurrence in comparison to HAILCAST, and (2) whether the relationship between environmental variables and hail occurrence remains stationary under changing climatic conditions.