Chapter II 

 The growth of waves under the action of wind 



1) Introduction and general remarks 



Any attempt to calculate the growth of the waves under the 

 action of wind requires the knowledge of both the energy transfer 

 from wind to waves and the rate of dissipation of wave energy in 

 different phases of wave development. The sea may grow only in 

 the case where the supply of energy by wind exceeds the loss of 

 energy by friction and turbulence. At a given wind velocity the 

 waves attain their fully developed state when the energy transfer 

 by wind A equals the energy dissipation by frictional forces D, 

 which are connected with turbulent wave motion. This means that 

 the sea is "mature" when the energy balance A - D = 0. 



As to the question of energy transfer by wind it seems to be 

 necessary to take into account the complex pattern of sea surface 

 roughness, because the stress or the total drag of the wind blow- 

 ing over a rough and wavy air-sea interface depends to a large ex- 

 tent on the "short-wavy" roughness, which is always superimposed on 

 the main profile of larger, striking waves. The energy transfer 

 from wind to waves is due partly to normal pressure components and 

 partly to tangential pressure components (tangential stress, drag), 

 and to do a net amount of work both of these wind force components 

 shoiild be regarded as acting on the particle velocity of water due 

 to the wave motion. Therefore, the actual velocity difference be- 

 tween the particle speed and the wind speed immediately at the sea 

 surface must be considered. But at the present state of our know- 



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