564 



Noordwijk 761112 



k=1 1km 



FIGURE 14. Normalized directional distribution 

 functions of the k-spectrum off Noordwijk, 

 Nov. 12th, 1976. Directions are relative to 

 true North. The peak wave number k^ is related 

 to the locally generated wind sea. 



likely because the differences between the wind ob- 

 servations at the coast and at the tower are fairly 

 large and the wind may have varied between the point 

 of observation and the coast. In particular for the 

 slanting wind conditions it is obvious that a JONSWAP- 

 type spectrum would not be found, due to the asym- 

 metry in the coastline around the wind direction. On 

 the other hand, non-linear interactions in the spec- 

 trum may produce a JONSWAP-type spectrum, in spite 

 of the asymmetry and the variations in the windfield 

 [Hasselmann, et al. (1976)]. From an inspection of 

 Figure 8 it can be concluded that the frequency 

 spectrum in the sortie off Sylt is indeed JONSWAP- 

 like. The correspondence of the frequency spectra 

 off Noordwijk with a JONSWAP-type spectrum has not 

 yet been investigated. 



For the k-spectra of the first two sorties one 

 would expect to find directional distribution func- 

 tions having some kind of standard shape, symmetrical 

 about the mesrn direction although some skewness may 

 be expected in the observation off Noordwijk because 

 the wind direction was not perfectly off-shore. For 



the third spectrum strong skewness may be anticipated 

 due to the slanting position of the coastline. 



These expectations seem to be far from reality 

 in the k-spectriom off Sylt. The directional distri- 

 bution near the peak of the spectrum (see Figure 13) 

 is distinctly asymmetric with respect to the wind 

 direction with the highest peak at + 45° off the 

 wind direction (155° from true North) . It is highly 

 improbable that the wave generation mechanism would 

 build a directional distribution as strongly asym- 

 metrical as this . An explanation for this unexpected 

 observation can perhaps be found through a detailed 

 study of the wind and wave fields, possibly using 

 "hindcasting" procedures. But in the context of this 

 paper one can only speculate on some possible causes. 

 The source function is symmetrical, as is the radia- 

 tive energy transfer, since bottom and current re- 

 fraction is virtually non-existent. It seems then 

 that the asymmetry stems from asymmetry in the wind 

 field or in the boundary conditions . As for the 

 wind field, a cursory inspection of the large scale 

 weather maps revealed no asymmetry. As for the 



