On 10-11 September 2023, Storm Daniel, an exceptionally deep, early-season Mediterranean cyclone triggered an extreme flood in the city of Derna, Libya. The flood destroyed two dams that were built to protect the city, resulting in more than 5,000 casualties, making it one of the deadliest hydrometeorological disasters on record. Here, we aim to unravel (a) whether the catastrophic impact was inevitable, caused by an extremely rare storm, or was it foredoomed because of inadequate dam design, (b) how pivotal were the dam bursts in the tragic outcome, and (c) what insights this event provides for risk management and mitigation strategies elsewhere. Based on atmospheric reanalysis, satellite-based precipitation data, and rainfall-runoff modelling, we estimate the return period of both rainfall and flood to be only a few decades, meaning that the event was not as extreme as could have been implied by its disastrous consequences. Hydraulic simulations of the flood's stream power reveal that the damage caused by the collapse of the dams was approximately 20 times higher than would have been if dams were not built in the Wadi. Our methodological framework, which is based on globally available data, is applicable to risk assessment in other regions 1 as well. It underscores the importance of uncertainty-aware frequency analysis and suggests that distributed flood prevention and warning techniques should be favourable in mitigating flood risks.

Water volume estimates of shallow desert lakes are the basis for water balance calculations, important both for water resource management and paleohydrology/climatology. Water volumes are typically inferred from bathymetry mapping; however, being shallow, ephemeral and remote, bathymetric surveys are scarce in such lakes. We propose a new, remote-sensing based, method to derive the bathymetry of such lakes using the relation between water occurrence, during >30-yr of optical satellite data, and accurate elevation measurements from the new Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). We demonstrate our method at three locations where we map bathymetries with ~0.3 m error. This method complements other remotely sensed, bathymetry-mapping methods as it can be applied to: (a) complex lake systems with sub-basins, (b) remote lakes with no in-situ records, and (c) flooded lakes. The proposed method can be easily implemented in other shallow lakes as it builds on publically accessible global data sets.

Heavy precipitation events (HPEs) can lead to deadly and costly natural disasters and are critical to the hydrological budget in regions where rainfall variability is high and water resources depend on individual storms. Thus, reliable projections of such events in the future are needed. To provide high-resolution projections under the RCP8.5 scenario for HPEs at the end of the 21st century and to understand the changes in sub-hourly to daily rainfall patterns, weather research and forecasting (WRF) model simulations of 41 historic HPEs in the eastern Mediterranean are compared with “pseudo global warming” simulations of the same events. This paper presents the changes in rainfall patterns in future storms, decomposed into storms’ mean conditional rain rate, duration, and area. A major decrease in rainfall accumulation (-30% averaged across events) is found throughout future HPEs. This decrease results from a substantial reduction of the rain area of storms (-40%) and occurs despite an increase in the mean conditional rain intensity (+15%). The duration of the HPEs decreases (-9%) in future simulations. Regionally maximal 10-min rain rates increase (+22%), whereas over most of the region, long-duration rain rates decrease. The consistency of results across events, driven by varying synoptic conditions, suggests that these changes have low sensitivity to the specific large-scale flow during the events. Future HPEs in the eastern Mediterranean will therefore likely be drier and more spatiotemporally concentrated, with substantial implications on hydrological outcomes of storms.

Projections of extreme precipitation based on modern climate models suffer from large uncertainties. Specifically, unresolved physics and natural variability limit the ability of climate models to provide actionable information on impacts and risks at the regional, watershed and city scales relevant for practical applications. Here we show that the interaction of precipitating systems with local features can constrain the statistical description of extreme precipitation. These observational constraints can be used to project local extremes of low yearly exceedance probability (e.g., 100-year events) using synoptic-scale information from climate models, which is generally represented more accurately than the local-scales, and without requiring climate models to explicitly resolve extremes. The novel approach offers a path for improving the predictability of local statistics of extremes in a changing climate, independent of pending improvements in climate models at regional and local scales.

The weathering of continental surfaces and the transport of sediments via rivers into the oceans is an integral part of the dynamic processes that shape the Earth’s surface. To understand how tectonic and climatic forcings control regional rates of weathering, we must be able to identify their effects on sedimentary archives over geologic timescales. Cosmogenic nuclides are a valuable tool to study rates of surface processes and have long been applied in fluvial systems to quantify basin-wide erosion rates. However, in large rivers, continual processes of erosion and deposition during sediment transport make it difficult to constrain how long sediments spend within the fluvial system. In this study, we examine the role of rivers in transmitting and buffering perturbations to the continental erosional signal by constraining the timescales of fluvial transport in large rivers across the world. We apply a stochastic numerical model based on measurements of cosmogenic nuclides concentrations and calculate sediment residence times of 10^4-10^5 years in large rivers. These timescales are equal to or longer than climatic cycles, entailing that changes to rates of weathering brought on by climatic variations are buffered during transport in large rivers and are not manifested in the sedimentary record.

Orographic impact on extreme sub-daily precipitation is critical for risk management but remains insufficiently understood due to complicated atmosphere-orography interactions and large uncertainties. We investigate the problem adopting a framework able to reduce uncertainties and isolate the systematic interaction of Mediterranean cyclones with a regular orographic barrier. The average decrease with elevation reported for hourly extremes is found enhanced at sub-hourly durations. Tail heaviness of 10-minute intensities is negligibly affected by orography, suggesting self-similarity of the distributions at the convective scale. Orography decreases the tail heaviness at longer durations, with a maximum impact around hourly scales. These observations are explained by an orographically-induced redistribution of precipitation towards stratiform-like processes, and by the succession of convective cores in multi-hour extremes. Our results imply a breaking of scale-invariance at sub-hourly durations, with important implications for natural hazards management in mountainous areas.