4.5 Wind Stress
The wind, by itself, is usually not very interesting. Often we are much more interested in the force of the wind, or the work done by the wind. The horizontal force of the wind on the sea surface is called the wind stress. Put another way, it is the vertical transfer of horizontal momentum. Thus momentum is transferred from the atmosphere to the ocean by wind stress. Wind stress T is calculated from:
where ρ= 1.3k g/m3 is the density of air, U10 is wind speed at 10 meters, and CD is the drag coefficient. CD is measured using the techniques described in §5.6. Fast response instruments measure wind fluctuations within 10-20 m of the sea surface, from which T is directly calculated. The correlation of T with U210 gives CD (Figure 4.6).
Various measurements of CD have been published based on careful measurements of turbulence in the marine boundary layer. Trenberth et al., 1989) and Harrison (1989) discuss the accuracy of an effective drag coefficient relating wind stress to wind velocity on a global scale. Perhaps the most recently published value is that of Yelland and Taylor (1996), who give:
for neutrally stable boundary layer. Other values are listed in their table 1 and in Figure 4.6.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
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Updated on September 29, 2008