Abstract. 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 north-western Mediterranean area and led to large amounts of rainfall (up to 300 mm in 24 h) over the Hérault region (southern France). The study case was characterized by a very strong (>20 m s−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 Météo-France kilometre-scale AROME-France numerical weather prediction model – including the WASP (Wave-Age-dependant Stress Parametrization) sea surface turbulent flux parametrization – and the WaveWatch 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.

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In winter, the Northwest Tropical Atlantic Ocean can be characterized by various wave age‐based interactions among ocean current, surface wind and surface waves, which are critical for accurately describing surface wind stress. In this work, coupled wave‐ocean‐atmosphere model simulations are conducted using two different wave roughness parameterizations within COARE3.5, including one that relies solely on wind speed and another that uses wave age and wave slope as inputs. Comparisons with the directly measured momentum fluxes during the ATOMIC/EUREC4A experiments in winter 2020 show that, for sea states dominated by short wind waves under moderate to strong winds, the wave‐based formulation (WBF) increases the surface roughness length in average by 25% compared to the wind‐speed‐based approach. For sea states dominated by remotely generated swells under moderate to strong wind intensity, the WBF predicts significantly lower roughness length and surface stress (≈15%), resulting in increased near‐surface wind speed above the constant flux layer (≈5%). Further investigation of the mixed sea states in the model and data indicates that the impact of swell on wind stress is over‐emphasized in the COARE3.5 WBF, especially under moderate wind regimes. Various approaches are explored to alleviate this deficiency by either introducing directional alignment between wind and waves or using the mean wave period instead of the wave period corresponding to the spectral peak to compute the wave age. The findings of this study are likely to be site‐dependent, and mostly concern specific regimes of wind and waves where the original parameterization was deficient.

Abstract. The western Mediterranean Sea area is frequently affected in autumn by heavy precipitation events (HPEs). These severe meteorological episodes, characterized by strong offshore low-level winds and heavy rain in a short period of time, can lead to severe flooding and wave-submersion events. This study aims to progress towards an integrated short-range forecast system via coupled modeling for a better representation of the processes at the air–sea interface. In order to identify and quantify the coupling impacts, coupled ocean–atmosphere–wave simulations were performed for a HPE that occurred between 12 and 14 October 2016 in the south of France. The experiment using the coupled AROME-NEMO-WaveWatchIII system was notably compared to atmosphere-only, coupled atmosphere–wave and ocean–atmosphere simulations. The results showed that the HPE fine-scale forecast is sensitive to both couplings: the interactive coupling with the ocean leads to significant changes in the heat and moisture supply of the HPE that intensify the convective systems, while coupling with a wave model mainly leads to changes in the low-level dynamics, affecting the location of the convergence that triggers convection over the sea. Result analysis of this first case study with the AROME-NEMO-WaveWatchIII system does not clearly show major changes in the forecasts with coupling and highlights some attention points to follow (ocean initialization notably). Nonetheless, it illustrates the higher realism and potential benefits of kilometer-scale coupled numerical weather prediction systems, in particular in the case of severe weather events over the sea and/or in coastal areas, and shows their affordability to confidently progress towards operational coupled forecasts.

In winter, the Northwest Tropical Atlantic Ocean can be characterized by various regimes of interactions among ocean current, surface wind, and wind waves, which are critical for accurately describing surface wind stress. In this work, coupled waveocean-atmosphere model simulations are conducted using two different wave roughness parameterizations within COARE3.5, including one that relies solely on wind speed and another that uses wave age and wave slope as inputs. Comparisons with the directly measured momentum fluxes during the ATOMIC/EUREC4A experiments in winter 2020 show that, for sea states dominated by short wind waves under moderate to strong winds, the wave-based formulation increases the surface roughness length by 40\% compared to the wind-speed-based approach. For sea states dominated by remotely generated swells under moderate to strong wind intensity, the wave-based formulation predicts significantly lower roughness length and surface stress (˜20%), resulting in increased near-surface wind speed above the constant flux layer (˜5%). Further investigation of the mixed sea states in the model and data indicates that the impact of swell on wind stress is over-emphasized in the COARE3.5 wavebased formulation, especially under moderate wind regimes. Various approaches are explored to alleviate this deficiency by either introducing directional alignment between wind and waves or using the mean wave period instead of the wave period corresponding to the spectral peak to compute the wave age.