Storylines of extreme precipitation events and flood impacts in alpine and pre-alpine environments under various global warming levels.

Exploring the impacts of extreme weather events has gained increased attention in recent years after repeated record-breaking events, such as unprecedented river flood events in central Europe in summer 2021. After the flood event, risk management institutions, such as for example, fire brigades, civil protection units, or natural hazard experts asked if it was possible to predict the impacts of such an unprecedented event ex-ante and if similar events could occur elsewhere or if climatic changes can even worsen extreme flood events. Here, we present an approach to explore storylines of physically plausible extreme river flood events in a warming climate. The simulated flood events are based on extreme precipitation events selected from re-forecast archives. River discharge, flood processes, and their impacts on people and infrastructure are simulated for nine storylines under five global warming levels for the alpine and pre-alpine headwater catchments of the main rivers in northern Switzerland. The precipitation intensity was increased linearly with global warming according to the Clausius-Clapeyron relation. The coupled hydrological-hydraulic simulations show that the increase in peak discharge is higher than the increase in precipitation. The flood impacts increase non-linearly with higher global warming levels. The results show that record-breaking, high-impact river floods are possible under the current atmospheric conditions, and climate change substantially aggravates flood impacts. The simulations show a broad range of potential outcomes of flood impact storylines depending on the simulated scenario and local conditions. This leads to the conclusion that storylines of extreme flood events are a valuable tool to explore, describe, and communicate extreme events. Still, the inherent challenge is the communication of the representativeness of a particular storyline and, hence, the practical consequences that should or should not be derived. We, therefore, suggest considering storylines of events with a range of magnitudes and different spatiotemporal precipitation patterns to comprehend the possible range of outcomes and select appropriate storylines out of the set for risk communication.