Opportunities and challenges of impact-based forecasts for hail damage in Switzerland

Hail is a severe meteorological hazard causing extensive damage to agriculture, buildings, and cars. Within the scClim hail research project, we prototype an application of the open-source risk assessment platform CLIMADA to quantify hail risk. This allows for the provision of impact-based forecasts in Switzerland based on operational radar data, crowdsourced hail reports, and ensemble weather forecasts with the HAILCAST diagnostic built into the operational COSMO model. Combining this with exposure and vulnerability information, we provide hail impact estimates for buildings and different crops in Switzerland. The platform is co-designed with relevant stakeholders from public and private institutions and is running in a pre-operational fashion during the 4-year research project. In addition to forecast products, the platform provides post-event assessments of hail impacts based on radar data and exposure information, which is being used by, e.g., building insurances to have an immediate first estimate for the extent of damages. However, radar-based damage estimates entail considerable uncertainties, which may be constrained locally by using crowdsourced hail reports.Exchanges with stakeholders show that for bespoke preventive actions, such as sheltering cars and animals, or rolling up blinds to avoid hail damage, nowcasting information on expected hazard intensities is often sufficient. Nevertheless, for larger-scale actions such as a preterm crop harvest, preparation of temporary roof covers, or communication to the public, impact forecasts based on ensemble weather prediction may prove highly beneficial, depending on the forecasts accuracy.As a possible expansion, probabilistic hazard event sets were generated using kilometer-scale regional climate model simulations for Europe covering the present-day period from 2011 to 2021 and projecting a future climate using the pseudo global warming approach for a 3 K global warming level. The feasibility and utility of such an expansion are currently being evaluated, not least toassign return periods to predicted hail events.