110 



The saving grace for formulations such as (l) and (2) for the computa- 

 tion of sea-air energy transfers is that, by and large, the higher the 

 transfer, the more accurately these formulae predict it . As will be shown 

 in later sections, the largest exchange occurs at times of strong winds. 

 Here the shearing is unlikely to i)ermit maintenance of large lapse rates 

 over the open ocean and, even if these did occur, the strong wind shear keeps 

 the Richardson number in a reasonable range. When the wind approaches calm 

 and the air-sea temperature difference remains large (lonlikely), the formulae 

 are in trouble and give particularly bad results for sensible heat exchange. 

 But all the exchanges are then relatively small and perhaps even negligible 

 for large scale budget and dynamic considerations. 



The preceding establishes the fact that if the bulk aerodynamic 

 equations can be used at all, they can probeibly be best applied to conditions 

 prevailing over the tropical oceans. However, it is also quite clear that 

 the drag coefficient, Cd, is not a constant but is a function of, at least, 

 height (z); wind speed (u) and stability (Rg) . By applying considerations 

 outlined by Monin and Obukhov (195^) it can be shown that 



^t - ^*ii <=:^>' --KI cf /2 _ ,,3/2 (^lj2^ 



"^ "6 



where C* is the drag coefficient for neutral or adiabatic conditions and 

 the subscripts 6 and 11 refer to heights (in meters) above the sea surface. 

 A linear dependence of the drag coefficient upon height and wind 8i)eed under 

 adiabatic conditions was assumed using values obtained by Deacon and Webb 

 (1962). In practice (6) was approximated and the contribution of the term 

 in brackets on the right hand side was neglected. Figure 1 shows the 

 functional relationship between the remaining terms of the above expression. 

 Based upon these considerations the drag coefficient was then computed from 



C^ = (1.1+6 + 0.07 iL - 4.2 R„) X 10"^ . (7) 



b DC 



Finally, before using equations (l), (2) and (7) to compute the transfer of 

 latent and sensible heat, an evaluation of the accuracy of the measurements 

 must be made. Wet- and dry-bulb temperatures and wind speeds were measured 

 at 6.0 m above the sea surface at a point i+.7 m on a pulpit ahead of the 

 bow of the vessel. Sea surface temperatures were measured 10 cm below the 

 surface and k,3 m ahead of the vessel. The temperature sensors were therm- 

 istors and the wind speed was obtained from a recording cup anemometer. 

 Care was taken to avoid radiation effects. Supplementary temperature 

 measurements were made around the ship (out to 1^0 ft) on three different 

 occasions. These measurements were used to determine any extraneous effects 

 on the pulpit observations. Wind speeds were checked for ship effect in a 

 manner similar to that described by Deacon et al. (1956) . It is felt that 



