Abstract
In recent years, flash floods repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past, and especially in the 1990s, floods were characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes (Pfister et al., 2004). The intention of this study is to link the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions to identify the cause for this apparent shift. Therefore, we hypothesise that an increase in extreme precipitation events has subsequently led to an increase in the occurrence of flash flood events in central western Europe and all that being caused by a change in the occurrence of flash flood favouring atmospheric conditions. 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 (RADOLAN, DWD). Moreover, we identified proxy parameters representative for flash flood favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity in the lower troposphere (q ≥ 0.004 kg kg−1), sufficient latent instability (CAPE ≥ 100 J kg−1) and weak deep-layer wind shear (DLS ≤ 10 m s−1) proved to be characteristic for long-lasting intense rainfall that can potentially trigger flash floods. These atmospheric parameters, as well as the flash flood and precipitation events were then analysed using linear models. Thereby we found significant increases in atmospheric moisture contents and increases in atmospheric instability. Parameters representing the motion and organisation of convective systems occurred slightly more often or remained unchanged in the time period from 1981–2020. Moreover, a trend in the occurrence of flash floods was confirmed. The number of precipitation events, their maximum 5-minute intensities as well as their hourly sums were however characterized by large inter-annual variations and no trends could be identified between 2002–2020. This study therefore shows that the link from atmospheric conditions via precipitation to flash floods cannot be traced down in an isolated way. The complexity of interactions is likely higher and future analyses should include other potentially relevant factors such as intra-annual precipitation patterns or catchment specific parameters.
Highlights
Flash floods rank among the most destructive hazards originating from deep moist convection, leading to economic losses, damage to infrastructure, and high mortality rates (Gaume et al, 2009; Hall, 1981; Llasat et al, 2014; WMO, 2017)
We hypothesise that a change in atmospheric conditions led to more frequent extreme precipitation events that subsequently led to more flash flood events in central western Europe
Forecasting potential heavy precipitation based on atmospheric conditions remains a major challenge, as different 125 atmospheric constellations can cause heavy precipitation events, while large hail, for example, is mostly associated with supercells, and less challenging to identify (Púčik et al, 2015). In view of these recent findings, we hypothesise that a change in atmospheric conditions led to more frequent extreme precipitation events that subsequently led to more flash flood events in central western Europe
Summary
Flash floods rank among the most destructive hazards originating from deep moist convection, leading to economic losses, damage to infrastructure, and high mortality rates (Gaume et al, 2009; Hall, 1981; Llasat et al, 2014; WMO, 2017). Varying raindrop sizes and dynamics of merging cells can cause downdrafts producing extremely high precipitation intensities (Doswell et al, 1996; Markowski and Richardson, 2010) This was the case during the flash flood events in Luxembourg in 2016 and 2018 (Mathias, 70 2019, 2021). To ensure a sufficient duration of the rainfall event, a slow storm motion is needed (Van Delden, 2001) This generally occurs in case of very weak 85 pressure/geopotential gradients when the mean wind speed and the bulk shear between the surface and the lower to mid troposphere are weak.
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