Dynamics-informed attribution of a record-shattering heavy precipitation event over Central Europe during Storm Boris (2024)

Statistics-based extreme event attribution is often limited by the scarce availability of data and by the potentially inadequate representation of relevant physical processes in climate models. Storyline approaches, such as the ones involving large-scale flow analogs, can be used to constrain the impact of anthropogenic climate change on extreme events in a physically robust manner, complementing the information gained from statistics-based approaches.

In this work, we employ operational ECMWF analysis data and simulations from the CESM large ensemble (providing up to 1000 years of data) to characterize the dynamical evolution of Storm Boris, that brought a record-shattering precipitation event over central Europe between the 13^th and the 16^th of September 2024. Leveraging on the available large ensemble, we perform an analog-based attribution of the associated extreme precipitation informed by the peculiar atmospheric dynamics of the event.

The analysis is articulated in two parts. The first concerns a description of the salient dynamical features that made Storm Boris so extreme. Such features are: 1) a deep upper-level cut-off cyclone over the Mediterranean; 2) a slow-moving surface cyclone over eastern Europe; 3) a strong high-latitude blocking anticyclone building up during the event; and 4) moisture contributions from several sources across storm lifetime, rotating from the North Atlantic to the central and the eastern Mediterranean/Black Sea.

The second part is an analog-based attribution of the extreme precipitation that takes into account the pinpointed dynamical features. We show that a correct representation of the upper-level cut-off cyclone (using potential vorticity as a target field to determine analogs) and of the surface cyclone position at the time of the extreme precipitation (using a cyclone detection algorithm) drastically improves the quality of the detected large-scale flow analogs. Those two adjustments, informed by the knowledge of the dynamics of the event, allow to isolate the thermodynamical effect of climate change in a consistent manner and indicate a robust enhancement of extreme precipitation over central Europe for Boris-like storms occurring in a warmer climate.