What distinguishes 100-year precipitation extremes over Central European river catchments from more moderate extreme events?

Historical extreme flooding events in Central European river catchments caused high socioeconomic impacts. Previous studies investigated single events in detail but did not focus on an analysis of the underlying extreme precipitation events in general as historical events are too rare for a robust assessment of their generic dynamical causes. This study attempts to fill this gap by analyzing a set of realistic daily 100-year large-scale precipitation events over five major European river catchments with the help of operational ensemble prediction data from the ECMWF. The dynamical conditions during such extreme events are investigated and compared to those of more moderate extreme events (20- to 50-year). 100-year precipitation events are generally associated with an upper-level cut-off low over Central Europe in combination with a surface cyclone southeast of the specific river catchment. The 24 hours before the event are decisive for the exact location of this surface cyclone, depending on the location and velocity of the upper-level low over Western Europe. The differences between 100-year and more moderate extreme events vary from catchment to catchment. Dynamical mechanisms such as an intensified upper-level cut-off low and surface cyclone are the main drivers distinguishing 100-year events in the Oder and Danube catchments, whereas thermodynamic mechanisms such as a higher moisture supply in the lower troposphere east of the specific river catchment are more relevant in the Elbe and Rhine catchments. For the Weser/Ems catchment, differences appear in both dynamical and thermodynamic mechanisms.