Atmospheric conditions favouring extreme precipitation and flash floods in temperate regions of Europe

In recent years, flash floods have repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past (especially in the 1990s), floods have been characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes. Here, we explore potential links and causes between the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions. We hypothesize that a change in atmospheric conditions has led to more frequent extreme precipitation events that have subsequently triggered flash flood events in central western Europe. To test this hypothesis, we compiled data on flash floods in central western Europe and selected precipitation events above 40 mm h$^{−1}$ from radar data (the RADOLAN “Radar-Online-Adjustment” dataset from the German Weather Service). Moreover, we identified proxy parameters representative of extreme precipitation favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity (q) in the lower troposphere (q≥0.004 kg kg$^{−1}$), sufficient latent instability (convective available potential energy (CAPE) ≥ 327 J kg$^{−1}$), and weak wind speeds between 10 m a.g.l. ... mehrand 500 hPa (WS$_{10 m−500 hPa}$ ≤ 6 m s$^{−1}$) proved to be characteristic of intense rainfall that can potentially trigger flash floods. We relied on linear models to analyse 40 years worth (1981–2020) of atmospheric parameters as well as related precipitation events. We found significant increases in the atmospheric moisture content and increases in atmospheric instability. Parameters representing the motion and organization of convective systems remained largely unchanged in the considered period (1981–2020); however, the number of precipitation events, their maximum 5 min intensities, and their hourly sums were characterized by large interannual variations, and no trends could be identified between 2002 and 2020. Our study shows that there is no single mechanistic path leading from atmospheric conditions to extreme precipitation and subsequently to flash floods. The interactions between the processes involved are so intricate that more analyses which consider other potentially relevant factors, such as intra-annual precipitation patterns or catchment-specific parameters, are required.