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The July 2021 flood in central Europe was one of the five costliest disasters in Europe in the last half century, with an estimated total damage of EUR 32 billion. The aim of this study is to analyze and assess the flood within an interdisciplinary approach along its entire process chain: the synoptic setting of the atmospheric pressure fields, the processes causing the high rainfall totals, the extraordinary streamflows and water levels in the affected catchments, the hydro-morphological effects, and the impacts on infrastructure and society. In addition, we address the question of what measures are possible to generate added value to early response management in the immediate aftermath of a disaster. The superposition of several factors resulted in widespread extreme precipitation totals and water levels well beyond a 100-year event: slow propagation of the low pressure system Bernd, convection embedded in a mesoscale precipitation field, unusually moist air masses associated with a significant positive anomaly in sea surface temperature over the Baltic Sea, wet soils, and steep terrain in the affected catchments. Various hydro-morphodynamic processes as well as changes in valley morphology observed during the event exacerbated the impact of the flood. Relevant effects included, among many others, the occurrence of extreme landscape erosion, rapidly evolving erosion and scour processes in the channel network and urban space, recruitment of debris from the natural and urban landscape, and deposition and clogging of bottlenecks in the channel network with eventual collapse. The estimation of inundation areas as well as the derived damage assessments were carried out during or directly after the flood and show the potential of near-real-time forensic disaster analyses for crisis management, emergency personnel on-site, and the provision of relief supplies. This study is part one of a two-paper series. The second part puts the July 2021 flood into a historical context and into the context of climate change. [ABSTRACT FROM AUTHOR]

Heavy precipitation over western Germany and neighboring countries in July 2021 led to widespread floods, with the Ahr and Erft river catchments being particularly affected. Following the event characterization and process analysis in Part 1, here we put the 2021 event in the historical context regarding precipitation and discharge records, and in terms of the temporal transformation of the valley morphology. Furthermore, we evaluated the role of ongoing and future climate change on the modification of rainfall totals and associated flood hazards as well as implications for flood management. The event was among the five heaviest precipitation events of the past 70 years in Germany. However, considering the large LAERTES-EU regional climate model (RCM) ensemble revealed a substantial underestimation of return values and periods based on extreme value statistics using only observations. An analysis of homogeneous hydrological data of the last 70 years demonstrated that the event discharges exceeded by far the statistical 100-year return values. Nevertheless, the flood peaks at the Ahr River were comparable to the reconstructed major historical events of 1804 and 1910, which were not included in the hazard assessment of flood risk so far. A comparison between the 2021 and past events showed differences in terms of the observed hydro-morphodynamic processes which enhanced the flood risk due to changes in the landscape organization and occupation. The role of climate change and how the 2021 event would unfold under warmer or colder conditions (within a -2K to +4 K range) was analyzed based on pseudo-global-warming (PGW) model experiments. These showed that the spatial mean precipitation scales to first order with the theoretical Clausius-Clapeyron (CC) relation predicting a 7 to 9 % increase per degree warming. Using the PGW rainfall simulations as input to a hydrological model of the Ahr river basin revealed a strong and non-linear effect on flood peaks: For the +2K scenario, the 18 % increase in areal rainfall led to a 39 % increase of the flood peak at gauge Altenahr. The analysis of the high-resolution convection-permitting KIT-KLIWA RCM ensemble confirmed the CC-scaling for moderate spatial mean precipitation but showed a super CC-scaling of up to 10 % for higher intensities. Moreover, also the spatial extent of such precipitation events is expected to increase. [ABSTRACT FROM AUTHOR]

The July 2021 flood in central Europe was one of the five costliest natural disasters in Europe in the last half century with estimated total damage of EUR 32 billion. This study investigates the complex interactions between meteorological, hydrological, and hydro-morphological processes and mechanisms that led to the exceptional flood. Furthermore, we present our estimates of the impacts in terms of inundation areas, traffic disruptions, and economic losses. The estimation of inundation areas as well as the derived damage assessments were carried out during or directly after the flood, and show the potential of near-real-time forensic disaster analyses for crisis management, emergency personnel on-site, and the provision of relief supplies. The superposition of several factors resulted in widespread extreme precipitation totals and water levels well beyond a 100-year event: slow propagation of the low pressure system Bernd, convection embedded in a mesoscale precipitation field, unusually moist air masses associated with a significant positive anomaly in sea surface temperature over the Baltic Sea, wet soils, and steep terrain. Various hydro-morphodynamic processes as well as changes in valley morphology observed during the event exacerbated the impact of the flood. Relevant effects included, among many others, the occurrence of extreme landscape erosion, rapidly evolving erosion and scour processes in the channel network and urban space, recruitment of debris from the natural and urban landscape, deposition and clogging of bottlenecks in the channel network with eventual collapse. This study is part one of a two-paper series. The second part puts the July 2021 flood into a historical context and into the context of climate change. [ABSTRACT FROM AUTHOR]

Heavy Precipitation Events (HPEs) are a challenging atmospheric phenomenon with a high impact on human lives and infrastructures. The achievement of high-resolution simulations for Convection Permitting Modelling (CPM) has brought relevant advancements in the representation of HPEs in climate simulations compared to coarser resolution Regional Climate Models (RCM). However, further insight is needed on the scale-dependency of mesoscale precipitation processes. In this study, we aim at evaluating reanalysis-driven climate simulations of the greater Alpine area in recent climate conditions and assessing the scale-dependency of thermodynamical processes influencing extreme precipitation. We evaluate COSMO-CLM simulations of the period 1971--2015, at resolutions of 25 km (RCM) and 3 km (CPM) downscaled from ERA-40 and ERA- interim. We validate our simulations against high-resolution observations (EOBS, HYRAS, MSWEP, and UWYO). In the methodology, we present a revisited version of the Precipitation Severity Index (PSI) useful for extremes detection. Furthermore, we obtain the main modes of precipitation variance and synoptic Weather Types (WTs) associated with extreme precipitation using Principal Component Analysis (PCA). PCA is also used to derive composites of model variables associated with the thermodynamical processes of heavy precipitation. The results indicate a good detection capability of the PSI for precipitation extremes. We identified four WTs as precursors of extreme precipitation in winter, associated with stationary fronts or a zonal flow regimes. In summer, 5 WTs bring heavy precipitation, associated with upper-level elongated troughs over western Europe, sometimes evolving into cut-off lows, or by winter-like situations of strong zonal circulation. The model evaluation showed that CPM (3 km) represents higher precipitation intensities, better rank correlation, better hit rates for extremes detection, and an improved representation of heavy precipitation amount and structure for selected HPEs compared to RCM (25 km). CPM overestimates grid point precipitation rates especially over elevated terrain fostered by the scale-dependency of convective dynamic processes such as stronger updrafts and more triggering of convective cells. However, at low altitudes, precipitation differences due to resolution are explained through the scale-dependency of thermodynamic variables, where the largest impact is caused by differences in surface moisture up to 1 g kg-1 . These differences show a predominant north-south gradient where locations north of the Alps show larger (lower) surface moisture and precipitation in CPM (RCM) and locations south of the Alps show larger (lower) humidity and precipitation in RCM (CPM). The humidity differences are caused by an uneven partition of latent and sensible heat fluxes between RCM and CPM. RCM simulates larger emissions of latent heat flux over the Sea (25 W m-2 more), and CPM emits larger latent heat over land (15 W m-2 more). In turn, RCM emits larger surface sensible heat fluxes over land (30 W m-2 more), showing a warmer surface (0.5 to 1°C) than CPM. These results provide evidence that CPM is a powerful tool for obtaining accurate high-resolution climate information also pointing at the different scale-dependency of dynamic and thermodynamical precipitation processes at high and low terrain. [ABSTRACT FROM AUTHOR]

Gaining insight on the interaction between atmospheric moisture and convection is determinant to improve the model representation of heavy precipitation, a weather phenomenon that every year brings casualties and important monetary losses in the western Mediterranean region. Given the large variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation of moist convective processes. In this study, we assess the sensitivity of precipitating convection and underlying mechanisms during a heavy precipitation event (HyMeX intensive observation period 16) to corrections of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenised data set of GPS-derived Zenith Total Delays (GPS-ZTD) with sub-hourly frequency (10 minutes) in 7 km and 2.8 km simulations with the COSMO-CLM model over the western Mediterranean region. The analysis shows that (a) large atmospheric moisture amounts (Integrated Water Vapour ~ 40 mm) precede heavy precipitation at the affected areas. This occurs 12 h before initiation over southern France and 4 h over Sardinia, north eastern Italy and Corsica (our main study area). (b) We found that the moisture is transported on the one hand, swept by a westerly large-scale front associated with an upper-level low and on the other hand evaporated from the Mediterranean Sea and north Africa. The latter moisture transport occurs in the < 1 km to 4 km layer and has been identified for this event for the first time. (c) COSMO-CLM overestimated the atmospheric humidity and precipitation amount over the study region (Corsica) and this was, to a good extent, corrected by the GPS-ZTD nudging by reducing noticeably both quantities, bringing results closer to observations. (d) The two processes that exerted the largest control on precipitation were the reduction of atmospheric instability over the island (CAPE -35%) and the drying of the lower free troposphere bringing more dry air entrainment. Besides, the 7 km simulation showed a stronger impact for large-scale dynamical lifting at the target area, given a weakening of the represented low-pressure system and the associated wind circulation. This reduced ultimately, the intensity and number of convective updrafts represented over the island. These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of related processes leading to it across scales. Additionally. The modelling experiments demonstrated the benefit of sub-hourly GPS-ZTD nudging to improve the modelling of precipitation. [ABSTRACT FROM AUTHOR]