the local wind has been established, the problem of wave 

 prediction for a given wind field reduces to the numerical 

 integration of the radiative transfer equation under appro- 

 priate initial and boundary conditions for the wave field. 

 Until recently, however, forecasting methods based on this 

 approach ([2] [3] [18] [^5] [^7]) have been severely handi- 

 capped by lack of quantitative measurements of the wave 

 energy balance, which have made it difficult to decide 

 between a number of strongly differing proposed source- 

 function models, cf [22]. 



3. THE JOINT NORTH SEA WAVE PROJECT 



One of the purposes of JONSWAP was to obtain informa- 

 tion about the source function suitable for the needs of 

 practical wave forecasting. A second goal was to gain 

 insight into the relative significance of the various inter- 

 action processes contributing to the overall energy balance 

 of the wave field. 



To determine rates of change of the wave spectrum, 

 simultaneous wave measurements were made at 13 stations along 

 a l60 km profile extending westward from the island of Sylt 

 in North Germany (Figs. 1 and 2). Half-hourly recordings 

 were made six to twelve times daily for a period of 10 weeks 

 in July-August of 1969; additionally, 4 weeks of data were 

 obtained in September 1968 during a pilot experiment with a 



25-9 



