Cross-scale characteristics of extreme precipitation events under climate change

Abstract. Extreme precipitation is intensifying globally, with Central Europe emerging as a hotspot for accelerating hydroclimatic risks. Traditional hazard metrics often fail to capture the spatial extent and compound temporal characteristics of these events. This study employs the cross-scale Weather Extremity Index (xWEI) – a metric integrating intensity, duration, and area – to systematically analyze the 200 most extreme precipitation events over Germany. Using high-resolution (3 km) convection-permitting COSMO-CLM simulations under the RCP8.5 scenario, we compare event characteristics across historical (1971–2000), present (1990–2019), and future (2031–2060; 2071–2100) climate states. Our results reveal a profound intensification: averaged over the top 200 events and relative to the historical baseline, the xWEI is projected to increase by 27 % and 45 % until the near and far futures, respectively. This is driven by increasing peak rainfall intensity, accompanied by a fundamental structural shift in the extreme-event population of the top 200 events. The composition transitions from being dominated by short-lived (1–4 h), small-scale (< 5,000 km²) events toward more persistent (12–24 h) and spatially extensive rainfall systems. Furthermore, we demonstrate that the perceived severity of these changes depends on the statistical frame of reference. When return periods are re-calibrated ("adapted") to each specific climate period, adjusting for the dominant effect of peak intensification, they reveal a second-order intensification that remains hidden in traditional assessments. This effect, diagnosed primarily between the historical and subsequent climates, manifests as a rise in the cross-duration and cross-space characteristics: events are becoming more extreme across a broader range of durations and areas simultaneously, significantly expanding the total volume of extreme precipitation. These findings highlight the emergence of more complex, high-impact hazard structures that necessitate the multi-scale assessment capabilities of the xWEI framework.