Exploring uncertainty in future hail trends: A comparison of proxy-based and diagnostic approaches using convection-permitting climate simulations over Europe

Historical and future hail trends over Europe generally point to an increased hail threat. However, these trends often diverge at the regional scale. In particular, Western and Southern Europe show conflicting signals with some models and observations indicating more frequent hail events, while others suggest a decline in a warmer climate. Most existing future projections of hail occurrence rely on hail proxies, estimates based on environmental conditions indicative of hail formation, derived from global and regional climate models that use parameterized convection schemes. Recent developments have introduced more advanced statistical hail models and refined proxies. Despite these advancements, systematic comparisons of different hail proxies - especially when derived from a common dataset - remain limited. In this study, we utilize high-resolution (2 km), convection-permitting regional COSMO climate simulations with the embedded online hail diagnostic HAILCAST to assess the present day and future hail occurrence in a 3°C pseudo global warming scenario. We compare hail frequencies and hail frequency changes derived from (i) established hail proxies based on environmental thresholds and statistical models and (ii) the HAILCAST online diagnostic. Our goal is to evaluate how spatial and temporal patterns of hail occurrence differ between methods and to assess the associated uncertainties in hail trend projections across Europe.