Chapter 4 - Atmospheric Influences

 Chapter 4 Contents (4.1) The Earth in Space (4.2) Atmospheric Wind Systems (4.3) The Planetary Boundary Layer (4.4) Measurement of Wind (4.5) Wind Stress (4.6) Important Concepts

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:

 T = ρ CD U210 (4.1)

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).

 Figure 4.6 Observations of the drag coefficient as a function of wind speed U10 ten meters above the sea. The solid line is 1000 CD = 0.44 + 0.063 U10 proposed by Smith (1980) and the dashed line follows from Charnock (1955). From Smith (1980). Triangles are values measured by Powell, Vickery, and Reinhold (2003).

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:

 1000 CD = 0.29 + 3.1/U10 + 7.7/U102 (3 less than or equal to U10 less than or equal to 6 m/s) (4.2a) 1000 CD = 0.60 + 0.070 U10(6 less than or equal to U10 less than or equal to 26 m/s) (4.2b)

for neutrally stable boundary layer. Other values are listed in their table 1 and in Figure 4.6.

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