While the previous chapters have described the models of drag coefficient and the physics of momentum transfer at the sea surface, it is still necessary to make some direct measurement of the stress. There are unanswered questions about the value of the drag coefficient in unsteady and non-ideal conditions. One needs stress measurement to resolve these issues. In some upper ocean experiments the exact flux of momentum is important enough to justify the added work in making stress measurements rather than the simpler wind measurements needed to use with the drag coefficient. There is also the possibility that from ships and other complex structures that induce large flow distortions it may be easier to measure the stress accurately than to measure the wind speed accurately.
The Measurement of Surface Stress
Over solid surfaces the stress over a portion of the surface can be obtained by relatively straightforward methods such as measuring the force on a drag plate. The sea surface offers no such opportunity. The breaking waves that dominate the surface under conditions of strong forcing make it difficult to imagine a direct measurement technique. Thus we rely on remote observation from which we infer the drag force.
This chapter outlines the three common ways of estimating the surface stress. They are known as the Reynolds stress method, dissipation method and profile method.
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