Characterization of the air-sea exchange mechanisms during a Mediterranean heavy precipitation event using realistic sea state modelling

This study investigates the mechanisms acting at the air-sea interface during a heavy precipitation event that occurred between 12 and 14 October 2016 over the northwestern Mediterranean area and led to large amounts of rainfall (up to 300mm in 24 h) over the Herault region (southern France). The study case was characterized by a very strong ( > 20ms 1) easterly to south-easterly wind at low level that generated very rough seas (significant wave height of up to 6 m) along the French Riviera and the Gulf of Lion. In order to investigate the role of the waves on air-sea exchanges during such extreme events, a set of numerical experiments was designed using the Meteo-France kilometre-scale AROME-France numerical weather prediction model - including the WASP (Wave-Age-dependant Stress Parametrization) sea surface turbulent flux parametrization - and the Wave-Watch III wave model. Results from these sensitivity experiments in the forced or coupled modes showed that taking the waves generated by the model into account increases the surface roughness. Thus, the increase in the momentum flux induces a slowdown of the easterly low-level atmospheric flow and a displacement of the convergence line at sea. Despite strong winds and a young sea below the easterly flow, the turbulent heat fluxes upstream of the precipitating system are not significantly modified. The forecast of the heaviest precipitation is finally modified when the sea state is taken into account; notably, in terms of location, this modification is slightly larger in the forced mode than in the coupled mode, as the coupling interactively balances the wind sea, the stress and the wind.