55 



might be compensated by a proper determination of C . Therefore, great 

 care should be taken in assessing a particular value of C separately for 

 each of the two flux equations and for every special situation. Then a 

 rather high accuracy can be expected from this approach as was shown by 

 Webb (i960). Moreover, the concept of the so-called "profile coefficients" 

 (which are simplified forms of the functions f-, , fp, f ) may be used here 

 with some success. 



After this critical treatment of the bulk aerodynamic approach let me 

 pass to some problems occurring immediately at the sea surface . 1-Jhile the 

 molecular viscosity does not play a decisive part, if any, in the exchange 

 of momentum at the sea surface, things seem to be different with the transfer 

 of heat and moisture . The momentum exchange is brought about by pressure 

 forces acting on the roughness elements . However, this model cannot be 

 applied to the exchange of sensible and latent heat, because, in this case, 

 there is no equivalent to the pressure exerted on the waves. 



Thus we are led to believe that the final exchange of heat and 

 moisture between air and sea can only be effected by molecular processes . 

 Consequently, an adequate representation of evaporation and heat transfer 

 must include the molecular coefficients of conductivity and diffusivity. 

 A suitable approach has been described by Sheppard (1958) who, in the basic 

 exchange formulae, simply added the molecular constant v and D to the 

 turbulent coefficients, K-^ and Kj,. ^ 



E = - p (K^ . D) |1 



(12) 



Thus, there is no longer assumed a distinct layer of exclusively molecular 

 transfer; molecular and turbulent exchanges are rather supjKJsed to exist 

 simultaneously. Since the turbulent coefficients decrease when the boundary 

 is approached, the molecular constants will become important very close to 

 the sea surface . I used this model with fair success for the computation of 

 marine evaporation on the basis of profile data measured close to the sea 

 surface . 



Another phenomenon which is connected with the sea surface is the 

 existence of the so-called " cool skin , " j.e., a very shallow surface layer 

 where the water temperature is about 0.5 C lower than measured with con- 

 ventional methods which refer to a deeper layer. This cool skin, which 

 originates from evaporation, may be of importance for the thermal inter- 

 course between air and sea, since it is the actual sea surface temperature 

 that determines the character of the convective and turbulent motions in 

 the marine atmosphere. The observational evidence - given in part by small 

 sensing elements, in part by radiometers which, depending on their optical 

 properties, measure the water temperature of the uppermost layer of, say 



