A storyline climate-change attribution study of the extreme hail event in Switzerland on 28 June 2021

We employed a “storyline” approach to explore possible anthropogenic climate change influences on the extreme hail event in Switzerland on 28 June 2021, with a particular focus on hailfall near Zurich, Switzerland. The event was successfully simulated using the weather research and forecasting model configured over a European regional domain, with initial and boundary conditions from ERA5. An ensemble of factual 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 initial and boundary conditions. This signal was computed using differences between global climate model (GCM) data averaged over current-day and pre-industrial time intervals; data from six different GCMs yielded a range of climate change signals, thus contributing to the counter-factual ensemble.  Relative to factual simulations, counter-factual simulations exhibited overall less hail, particularly for diameters ≥ 3 cm. This tendency is consistent with relatively lower convective available potential energy but comparable melting depths in the counter-factual environments. We quantified the fraction of attributable risk and concluded that the geographical area covered by hail of diameter larger than 3 cm and 5 cm appears to have been increased by the meteorological changes attributable to climate change.