Divergent European hail projections from machine learning and physically based models under global warming
摘要
Reliable fine-scale hail projections are needed for robust risk assessments, yet future trends remain uncertain and contradictory across studies. Using 11-year pan-European convection-permitting climate simulations for the present and a + 3 °C pseudo-global-warming climate, we compare an online hail-growth diagnostic (HAILCAST) with an offline machine learning model (XGBoost) trained on ERA5 hail environments. Under present conditions, both approaches provide a plausible European hail climatology. However, XGBoost predicts widespread hail suppression in a warmer climate, mainly driven by increasing freezing-level heights outside the training distribution. HAILCAST instead simulates how enhanced storm updrafts can sustain hail growth despite a warmer atmosphere, projecting increases over central-eastern Europe and larger hailstones. The results show that data-driven approaches alone should be used with caution under global warming, because the relationships learned under present-day conditions are not climate-change invariant, underscoring the need for physically based hail representations in convection-permitting models to robustly assess future convective hazards.