Laboratory studies of wind stress over surface waves

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Simultaneous laboratory observations of wind speed, wind stress, and surface wind-wave spectra are made under a variety of wind forcing patterns using clean water as well a water containing an artificial surfactant. Under typical experimental conditions, more than half of the total stress is supported by the wave-induced stress rather than by the surface viscous stress. When the surfactant reduces the short wind-wave spectra, the wind stress also decreases by as much as 20-30% at a given wind speed. When the wind forcing is modulated in time, the wind stress tends to be higher under decreasing wind than under increasing wind at a given wind speed, mainly because the response of short wind-wave spectra to varying wind forcing is delayed in time. These examples clearly demonstrate that the relationship between the wind speed and the wind stress can be significantly modified if the surface wave field is not in equilibrium with the wind forcing. Next, we examine whether the wind stress is estimated accurately if the wave-induced stress by all surface wave components is explicity evaluated by linear superposition and is added to the surface viscous stress. It is assumed that the surface viscous stress is uniquely related to the wind speed, and that the wind input rate is determined by the local, reduced turbulent stress rather than the total stress. Our wind stress estimates including the wave contributions agree well with observed wind stress values, even if the surface wave field is away from its equilibrium with the wind in the presence of surface films and/or under time-transient wind forcing. These observations strongly suggest that the wind stress is accurately evaluated as a sum of the wave-induced stress and the surface viscous stress. At very high winds, our stress estimates tend to be lower than the observations. We suspect that this is because of the enhancement of wind stress over very steep (or breaking) short wind-waves.

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Boundary-Layer Meteorology