Benefit of hectometric simulations of the 18 August 2022 derecho-producing mesoscale convective system over Corsica

On the night of 17 to 18 August 2022, disorganized thunderstorms located between Catalonia and the Balearic Islands (Spain) gradually strengthened, reaching the size of a mesoscale convective system in the middle of the night. Moving very rapidly over the Mediterranean Sea, this system organized into a squall line which suddenly bowed near Corsica. The system hit the island shortly after 06:00 UTC, accompanied by exceptionally violent winds. An anemometer in Marignana (Corsica) measured an instantaneous wind speed (over 0.5 s) of 225 km h^-1 (62.4 m s^-1), setting a new record under a thunderstorm in mainland France. This system, which lasted around 18 h, produced violent winds on a trajectory crossing 5 European countries from Spain to the Czech Republic, qualifying it as a derecho. Simulations exploring the contribution of increased resolution and testing the latest developments in physical parametrizations were carried out with the Meso-NH numerical research model, using initial and boundary conditions provided by the best AROME-France forecast (18 August 2022 00:00 UTC run). Among the simulations carried out, the one with 1 km horizontal grid spacing and 90 vertical levels, which has an effective resolution 2 to 3 times finer than AROME-France, simulates severe wind corridors more finely than the operational forecast, making it possible to get closer to the extreme wind observations. It also appears that the location of these corridors correlates with areas within the convective system where the vertical component of relative vorticity is very large. Simulations at 250 m horizontal grid spacing show even finer wind corridors and vortices. These results show that very fine scale phenomena occur within a derecho, which can only be reproduced by simulations at hectometric resolution.