A Storyline Climate-Change Attribution Study of a High-Impact Hailstorm in Switzerland

We employed a “storyline” approach to explore possible anthropogenic climate change influences on the extreme hail event in Switzerland on 28 June 2021. An ensemble of factual WRF simulations with randomly perturbed initial and boundary conditions was compared to an ensemble of counter-factual simulations in which a mean climate change signal was removed from the model conditions. Using data from six GCMs, this signal was computed using differences between data averaged over current-day and pre-industrial time intervals. Relative to counter-factual simulations, factual simulations exhibited overall more hail, particularly for diameters ≥3 cm. This is consistent with increased CAPE but minimal changes in melting depth over this region in the current day. We quantified the fraction of attributable risk and concluded that the geographical areas covered by hail of diameters ≥3 and 5 cm appear to have been increased by the meteorological changes attributable to climate change.