Sensitivity of Storm Boris rainfall intensification to wind nudging strength in event-based climate-change storyline simulations

Understanding how climate change influences environmental extremes is vital for developing effective adaptation and mitigation strategies. In this study, we apply an event-based storyline approach to assess changes in accumulated precipitation associated with Storm Boris, which impacted Central Europe in September 2024. We examine both historical changes (attribution) and future projections and extend previous work by investigating the sensitivity of results to the degree of imposed dynamical constraint. Using the global CMIP6 coupled climate model AWI-CM1, we nudge simulations toward observed ERA5 winds—including the jet stream—across a range of climate backgrounds: preindustrial, present-day, and possible future states with 2, 3, and 4 °C global warming relative to preindustrial conditions. Two nudging configurations are compared: (1) a “weak constraint” configuration, in which only synoptic- and planetary-scale winds in the free troposphere are nudged, permitting some dynamical adjustment with warming; and (2) a “strong constraint” configuration, in which winds at all vertical levels and scales are imposed, thereby completely suppressing dynamical changes.

Both configurations capture the event, with stronger present-day rainfall in the strongly constrained configuration. The observed climate change between pre-industrial and present day is robust, with increases of 7% (4%) in accumulated rainfall under the weak (strong) constraint. Projections up to a 3ºC warmer climate show linear increases in the accumulated rainfall for both configurations. Beyond +3ºC, the response strongly diverges. Under weak constraint, rainfall changes at +4ºC are marginal or even mildly reduced relative to present-day, whereas the strongly constrained configuration continues to show linear increases. This divergence is linked to thermally-driven dynamical adjustments permitted under weak constraint. Whether these adjustments reflect a realistic response or methodological artifacts, and whether similar behaviour occurs in other events, remains to be explored. Our results highlight remaining uncertainties in storyline-based extreme precipitation projections, and demonstrate the importance of considering multiple possibilities.