Atmospheric drivers and modeling challenges of Mediterranean flood-inducing precipitation events: Insights from high-resolution ICON-NWP simulations

The Mediterranean region is one of the key hotspots for recurrent heavy precipitation events (HPEs) that frequently trigger devastating hydrological hazards. Given their severe socio-economic impacts, understanding the atmospheric processes driving these events is of paramount importance. In this context, this study provides a detailed, process-based analysis of the physical mechanisms governing two catastrophic flood-inducing episodes—12–13 November 1999 and 14–15 October 2018—affecting the northwestern Mediterranean region of southern France. The analysis is based on high-resolution ERA5-driven simulations performed using the ICON-NWP modeling framework. Process-based diagnostics reveal that the 1999 event was characterized by cyclonic Rossby wave breaking and an associated PV streamer that resulted in a quasi-stationary cut-off low system, whereas the 2018 event was driven by the remnants of Hurricane Leslie phasing with the downstream flank of a high-amplitude Rossby wave breaking streamer, establishing a quasi-stationary frontal system over the impacted region. Despite their differing synoptic origins, both events featured atmospheric river-like moisture plumes fueled by persistent southeasterly Mediterranean flow, with deep-layer atmospheric ascent and low-level moisture convergence establishing favourable conditions for sustaining extreme precipitation. Evaluation against Météo-France station observations and gridded precipitation datasets such as E-OBS and EMO-1 demonstrates the clear added value of convection-permitting models (CPMs) in representing localized precipitation intensity, spatial structure, and statistical distribution compared with coarse-resolution simulations. However, accurately reproducing the intensity and precise spatial location of observed precipitation extremes remains challenging. Overall, this study underscores the immense value of convection-permitting modeling in understanding the processes driving extreme weather events, while highlighting key areas for improvement in future research.