51 



where the subscripts axid a refer to the sea surface and the level 

 z = a, respectively. 



We see that the validity of this approach depends partly on the 

 applicability of the drag coefficient Cg^ and partly on how well the 

 vertical gradients can be approximated by the vertical differences air-sea. 

 The first condition has already been dealt with and we are very well aware 

 of the difficulties and limitations inherent in the Ca concept. But what 

 is less well known, is the validity of the second assumption. 



Therefore, I am going to examine this case more in detail. Some 

 suitable diagrams have been published by Brocks (1963) • Profile measure- 

 ments of wind speed, air temperature, and water vapor pressure were made 

 by means of a buoy well away from land and also without any disturbance 

 from the accompanying vessel. The data used are averages each over a 

 period of 15 minutes. 



Figure 8 shows the vertical wind shear between the 10 m and the 1 m 

 levels as a function of the wind speed measured at 10 m. On the whole 



Figure 8. Vertical wind shear u^^q-U]^ between the 10 ai and 1 m levels as 

 a function of wind speed Uhq and grouped according to the temperature 

 difference air-sea, AT • (from Brocks, I963) 



