z ^ 1,000 m 



z = 1 00 m 



Geostrophic Region 



•J Ekman Region 



V 



y 



7 



p Constant Shear Layer T 



^^^ Surface Roughness (Zq) 



Figure 3-11. Atmospheric boundary layer over waves. 



of the ocean distort the wind field; thus, wind speed and direction become 

 dependent upon elevation above the mean surface, roughness of the surface, 

 air-sea temperature differences, and horizontal temperature gradients. To 

 simplify the discussion, temperature gradients in the horizontal plane will be 

 ignored because their effect can rarely be taken into account in a simplified 

 prediction scheme. 



Below the geostrophic region, the boundary layer may be divided into two 

 sections, a constant stress layer 10 to 100 meters in height and above that an 

 Ekman layer. Emphasis is placed on the constant stress layer. A detailed 

 description of the boundary layer mechanics is given in Resio and Vincent 

 (1977b). 



In the constant stress layer, it is possible to write an equation for the 

 vertical variation in windspeed 



U. 



where 



^o 



L 



U(z) =_[ln[|-j-ntJ] 



(3-25) 



the friction velocity (the shear stress is given by pU^ ) 



the surface roughness 



represents the effects of stability of the air column on the wind 



velocity 



a length scale associated with the mixing process and is dependent 



upon air-sea temperature difference. 



3-25 



