High-resolution atmosphere-ocean coupling impact on North-Western Mediterranean severe event forecasts with the AROBASE prediction system

The North-Western Mediterranean region, which is a semi-enclosed sea surrounded by mountains, is prone to severe events. The region is notably known as a key spot for strong wind, like mistral and tramontane, channelled and accelerated in the surrounding valleys, that induces sometimes damaging sea states. Heavy precipitation events also often occur when a moist and rapid marine low-level flow converges and/or encounters mountainous area, triggering and feeding stationary deep convective systems that lead to very localized, large amounts of rainfall in only some hours (typically more than 100 mm in less than 24 hours), generating flash-floods. Furthermore, the region is more and more frequently affected by (marine) heatwaves that intensify with climate change and have major impacts on public health, agriculture/fishery and biodiversity. All these extreme events rely on air-sea interactions during their onset and lifecycle, with complex responses and feedbacks in terms of ocean as atmospheric circulation and processes.

To better represent the mesoscale air-sea environment and the exchanges in numerical modelling and weather prediction systems, fine horizontal resolution and coupling are crucial. The AROBASE system assembles kilometer-scale limited-area models of the atmosphere, the ocean, and waves. Since summer 2024, a first AROBASE forecast demonstrator is applied daily over the Metropolitan France region. It couples the AROME numerical weather prediction model at 1.3 km resolution and the NEMO ocean model with a 1/36° resolution. A second version was deployed in autumn 2025 and adds a new coupling to take into account the impacts of waves on the air-sea exchanges. This study will present some comparisons of the atmosphere-ocean(-waves) AROBASE forecast with the (uncoupled) AROME operational forecast during recent severe meteorological situations that affected the North-Western Mediterranean region.