Drag of the Water Surface at Very Short Fetches: Observations and Modeling

International audienceThe specific properties of the turbulent wind stress and the related wind wave field are investigated in a dedicated laboratory experiment for a wide range of wind speeds and fetches, and the results are analysed using the Wind Over Wave Coupling model. Compared to long-fetch ocean wave fields, wind wave fields observed at very short fetches are characterized by higher significant dominant wave steepness, but much smaller macroscale wave breaking rate. The surface drag dependence on fetch and wind then follows closely the dominant wave steepness dependence. It is found that the dimensionless roughness length z∗0 varies not only with wind forcing (or inverse wave age) but also with fetch. At a fixed fetch, when gravity waves develop, z∗0 decreases with wind forcing according to a -1/2 power law. Taking into account the peculiarities of laboratory wave fields, the WOWC model predicts rather well the measured wind stress values. The relative contributions to surface drag of the equilibrium range wave-induced stress and the air flow separation stress due to wave breaking remain small, even at high wind speeds. At moderate to strong winds, the form drag due to dominant waves represents the major wind stress component