Nearshore Wind-Stress Measurements: Background Preliminary Field Work and Experiment Design

摘要

Wind stress, as a forcing term in nearshore circulation and wave generation models, is commonly represented as a product of the square of representative mean wind speed times a closure (or drag) coefficient. Specification of a drag coefficient then becomes a problem of great concern because it contains implicitly all of the physics of air-sea interaction and atmospheric boundary layer flow. A review of recent observations indicates that drag coefficients can vary by a factor of ten or more. In the simple case of steady, uniform winds blowing over the open ocean, the drag coefficient is simply a function of sea surface roughness and total mass flux. However, in the more common cases of interest to the Corps of Engineers, water depths are frequently shallow (in the sense of wind-wave behavior) and conditions are often unsteady and nonuniform. From a survey of recent theoretical considerations and associated field experiments, it is evident that drag coefficients in shallow and nearshore waters depend on a broader suite of parameters than in the open ocean case. Among these are more detailed characterizations of sea state, water surface currents, wind direction, water depth, and distance from shore as well as buoyancy parameters. To help clarify this dependency, a two-part experiment has been conducted wherein all of the hypothetically relevant parameters have been measured. (FR)