kk 



about these quantities over the sea. The small amount of information 

 available seems to indicate that at sea the influence of thermal stratifica- 

 tion on the wind profile can be taken into account in the same manner as 

 this is done over land (Deacon, 1962). 



In this connection the following seems to be important: The stability 

 of the air above the sea does not only depend on the vertical temperature 

 distribution. It is determined by the vertical variation of density, i.e. 

 there may be an effect of the humidity gradient also. Kraus (l96i|-) has 

 drawn our attention to the fact that a strong humidity decrease with height 

 in the lowest layer above the sea may even be sufficient to. reverse the 

 stabilizing effect of a small temperature increase with height. A decrease 

 of water vapor pressure of 5 mt) would compensate a temperature increase of 

 0.5°C. Therefore, according to Kraus, this effect must be taken into 

 account under relevant circumstances, for example, by an additional terra 

 in the Richardson number which is normally used as a measure of stability. 



So far I have been talking about the wind field in the marine boundary 

 layer. The quantity that is of most importance in the field of mechanical 

 interaction air-sea certainly is the wind stress acting on the sea surface. 

 Very little is known as yet about the normal wind stress , its size and 

 spectral distribution associated with the turbulent wind blowing over the 

 water. This lack of knowledge is regrettable, as it seems we may be sure 

 that the atmospheric pressure fluctuations play an important i>art in the ' 

 generation of wind waves at the sea surface. Thus, we have not been able 

 to check the theory advanced by Phillips (1957> 1958> 19^2) who considered 

 the initial waves generated by a resonance mechanism between the surface 

 wave modes and the random pressure fluctuation associated with the turbulent 

 wind blowing over the water and convected by the mean flow. There is, how- 

 ever, some very recent empirical evidence (Snyder, 1965) by which the 

 importance of the resonance mechanism is questioned. 



The tangential wind stress , however, has been the subject of consider- 

 able number of investigations, although one cannot say that all the problems 

 connected with it have been solved. The tangential wind stress is equivalent 

 to the vertical transport of horizontal momentum in a viscid fluid. In the 

 turbulent boundary layer of the atmosphere the shear stress is usually 

 considered as constant with height. Over the sea this may not be true as 

 we know from the theories of Miles and Stewart, but up to now the vertical 

 constancy of the wind stress is the generally accepted practice also for the 

 marine boundary layer. Consequently, the tangential vind stress t observed 

 in the boundary layer is equal to the tangential wind stress t = t 

 exerted by the wind on the sea surface. The latter quantity is of ftnportance 

 for quite a number of air-sea interaction problems, e.g. generation and 

 growth of ocean waves, of drift curirents, and storm surges. 



It is customary to express the surface drag t^ 

 surface in terms of the mean speed u^q a^'t the height 10 ra 



