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|Title:||Nearshore wind-stress measurements : background preliminary field work and experiment design|
|Authors:||Long, Charles E. (Charles Edward)|
Hubertz, Jon M.
|Publisher:||Coastal Engineering Research Center (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: 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 measure. The experiments were conducted at the Coastal Engineering Research Center's Field Research Facility (FRF) on the coast of the Atlantic Ocean near Duck, North Carolina, where advantage could be taken of low land relief, reasonably uniform alongshore topography and bathymetry, and a pier, 600 m in length, which could ac t as a s t able, over-water instrument platform. The first experiment was conducted in fall 1985 during a larger scale experiment known as DUCK85. Its purpose was to provide an exploratory survey of the horizontal variability of winds at the site and to investigate adequacy of various platforms for detailed wind- stress measurements. Data from that experiment (which includes passage of Hurricane Gloria) indicate that wind differences from the landward end of the pier to the seaward end are finite, varying by up to 40 percent in speed, but highly systematic. Anemometers on the roof of the FRF building and near the pier deck at the seaward end of the pier indicated substantial flow distortion due t o the proximity of wind- blocking structures. From this, it is concluded that extreme caution is necessary when mounting anemometers to avoid platform distortion effects and that careful interpretation is necessary when using beachbased anemometers to characterize winds over water. The second experiment was conducted in fall 1986 during a very large- scale experiment known as SUPERDUCK . Its purpose was to provide detailed wind-stress measurements. Coupled with measurements made by other investigators during SUPERDUCK of other relevant parameters, data from this experiment can be used to develop and test various models of drag-coefficient variability. Experiment rationale and design are described here. Experiment results and analysis will be given in a subsequent report.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Miscellaneous Paper|
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