33 



INTRODUCTION 



I hope that the title chosen for this talk has already indicated with, 

 sufficient clearness that I am going to deal with processes of small scale . 

 We all are very well aware of the fact that air- sea interaction is not 

 restricted to such small-scale processes but extends through the whole scale 

 of motions comprising mesoscale and synoptic processes and reaching even the 

 planetary scale by affecting the atmospheric circulation and the energy 

 balance of the earth. Nevertheless, the limitation imposed pn this lecture 

 helps to focus our attention on the crucial region of air-sea interchange. 

 This comparatively shallow layer with a thickness of only ia few meters in 

 air and water and characterized by vertical fluxes and energy transforma- 

 tions of different kinds apparently holds a key position in the interaction 

 between the atmosphere and the ocean. All the motions and processes of 

 other scales and related to air-sea interaction are in some way predetermined 

 by the small-scale exchange occurring in the boundary layer air-sea. 

 Therefore, any progress in our understanding of the interaction between ocean 

 and atmosphere on the whole and in all its different i)arts cannot be 

 accomplished without a simultaneous or preceding progress of our knowledge 

 about the physics of this interchange in this shallow boundary layer. 



This can be stated much more easily than it can be translated into 

 action. The sea surface is distinguished from the atmospheric boundary 

 conditions prevailing over land by very peculiar properties. On the con - 

 tinents, shape and size of the elements of surface roughness are clearly 

 defined and comparatively easy to determine . Generally, their nature and 

 locality are fixed and they neither vary with time nor do they dei)end 

 strongly on atmospheric conditions. Their aerodynamics are relatively well 

 defined and known. 



Contrary to the boundary conditions found over land the surface rough- 

 ness encountered at sea is composed of a great variety of moving elevations 

 which are different in size, shape, and velocity as well as subject to 

 continuous and irregular changes. The dimensions, the spatial distribution, 

 and the temporal variations of the ocean waves are governed by statistical 

 laws wherein the character and speed of the air flow play a decisive role. 

 Moreover, the wind generates orbital motion and drift current in the sea 

 and it is quite obvious that these water movements will react on the air 

 flow. With increasing wind speed, the formation of foam and spray, which 

 implies a disintegration of the sea surface, affects large areas and extends 

 to a certain height, thus creating a transition zone between air and sea. 

 Therefore we must realize that the boundary region between air and sea is 

 an extremely variable, ill-defined, and hardly accessible zone where the 

 coupling between atmosphere and ocean occurs in a very complicated manner. 



These dynamic properties of the sea surface considerably Increase the 

 difficulties inherent in any investigation concerned with the mechanism of 

 the air- sea boundary layer. 



