Experimental Investigation on the Wind-driven Suface Flow Associated with Breaking Waves

<p> It is important to characterize the transition process of wind-driven water surface to be closely connected to the momentum and gas exchanges across the air-sea interface. In the present study, the transition of the wind-driven surface flow was investigated by means of laboratory experiments, which were carried out using a wind-water tunnel, 17m long, 0.6m wide and 0.8m high. The velocity of the wind-driven surface flow, which is a Lagrangian surface velocity consisting of the Eulerian flow velocity and the Stokes drift velocity, was evaluated by measuring the velocity of float disk rafting on the water surface. According to the experimental results, we examined the critical conditions under which the micro-scale breaking and bubble-mixed breaking waves appear on the water surface. The relation of the surface flow velocity with the friction velocity was found to be changed around <i>u^*</i> = 0.3m/s like the relations with the drag coefficient and the Stokes drift velocity at the water surface. Our experimental results also showed the behavior of the wind-driven surface flow velocity to be varied depending on the windsea Reynolds number. The results suggest that the wave breaking controls the wind-driven surface flow.</p>