Evidence for the Effects of Swell and Unsteady Winds on Marine Wind Stress

Abstract

Over the past four decades much effort has been directed toward determining a parameterization of the sea surface drag coefficient on readily measurable quantities, such as mean wind speed and atmospheric stability. Although such a parameterization would have obvious operational advantages, the considerable scatter present between experiments, or within any one experiment, indicates that it is not easily achievable. One likely candidate for much of the scatter is the underlying wave field. Unfortunately, few campaigns over the years have included spectral measurements of the waves. Among those that have, the results are inconclusive. Here data are presented from the Surface Wave Dynamics Experiment and High Resolution Remote Sensing Program campaigns in which 3-m discus buoys were instrumented with K-Gill and sonic anemometers and complete motion packages to measure the direct (eddy correlation) stress and, concurrently, the directional ocean wave spectrum. These data are examined for the effects of swell on the drag coefficient. It is found that much of the scatter in the drag coefficient can be attributed to geophysical effects, such as the presence of swells or nonstationary conditions.