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

Abstract. This study investigates the mechanisms acting at the air-sea interface during the heavy precipitation event that occurred between the 12–14 October 2016 over the north-western Mediterranean area, and that led to large amounts of rainfall (up to 300 mm in 24 h) over the Herault region (South of France). The study case was characterized by a very strong easterly to southeasterly wind at low level (> 20 m/s) generating a very rough sea (significant wave height up to 6 m) along the French Riviera and the Gulf of Lion. In order to investigate the role of the waves on those air-sea exchanges during such extreme events a set of numerical experiments was designed using the Meteo-France kilometer-scale AROME-France Numerical Weather Prediction model – including the WASP sea surface turbulent fluxes parametrization – and the wave model WaveWatchIII. Results from those 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 easterly low-level atmospheric flow was slowed-down and the turbulent fluxes upstream of the precipitating system were significantly modified. This modified the forecast of the heaviest precipitation, notably in term of location.