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

Abstract

Abstract. 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. and 500 hPa (WS10m-500hPa≤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.

Altered activities of extracellular soil enzymes by the interacting global environmental changes

“…The episodes of water stress, which represent an important disturbance for microbial communities of the soil, are predicted to increase in intensity, frequency and duration (de Vries et al, 2018; Pachauri et al, 2014), while precipitation events are predicted to become more sporadic and violent so that certain areas will experience higher eve if irregular water availability at the expense of others (Allen et al, 2019). Temperate zones are particularly affected by altered precipitation patterns and higher intensity and frequency of drought episodes (Ciais et al, 2005; Meyer et al, 2021). Furthermore, arid and semi‐arid land areas have increased their global surface in the last decades, and they are expected to increase even more in the coming decades (Feng & Fu, 2013; Huang et al, 2016; Spinoni et al, 2021).…”

Compounding Heatwave‐Extreme Rainfall Events Driven by Fronts, High Moisture, and Atmospheric Instability

“…We therefore analyze if these differences in rainfall behavior after heatwaves can be explained by variations in atmospheric conditions. Specifically, we analyze the atmospheric instability and availability of moisture before and after the heatwave termination as these are important requirements for extreme rainfall in thunderstorms and their mechanistic roles in contributing to extreme rainfall is well known (Meyer et al., 2022).…”

“…All variables are aggregated from hourly to daily timescales with respect to their local time zones (maximum and minimum daily temperatures; T max , T min , mean daily TCWV and CAPE). Other variables describing moisture (e.g., absolute humidity) and atmospheric instability (e.g., convective inhibition) are available from reanalysis as well; however, TCWV and CAPE are analyzed as they have been found to serve well as proxy parameters for conditions associated with extreme precipitation (Meyer et al., 2022).…”

“…Several studies therefore point toward moist convection being the main driver of compounding heatwave‐extreme rainfall events, as atmospheric instability tends to be high during the event (Chen et al., 2022; Wu et al., 2021; You & Wang, 2021; Zhang & Villarini, 2020). Indeed, from a theoretical standpoint, high temperatures related to the heatwave have the potential to enhance convective extreme rainfall, provided there is ample moisture supply, as extreme rainfall associated with convection is usually linked to high atmospheric instability and moisture (Brooks, 2013; Groenemeijer & Van Delden, 2007; Meyer et al., 2022; Púčik et al., 2015). However, uncertainties remain as to how other synoptic‐scale drivers contribute; cyclones or fronts could be the causal mechanism of both the reduction in ambient temperature during the heatwave termination as well as the associated rainfall.…”

Changing Water Cycle under a Warming Climate: Tendencies in the Carpathian Basin

“…The recharge rate into still water bodies and infiltration into the ground decreases; thus, water levels of lakes and groundwater tables are expected to decrease [85,88]. • Changes in the temporal distribution of precipitation trigger more extreme rainfall events during shorter periods, resulting in much larger variability of hydrological parameters and an increasing number of flash floods [130].…”