559 



^ v3o^ 



7° 



_l 



FIGURE 4. The area of observation off Sylt. Active 

 wave monitoring stations and station of observation 

 indicated by dots. Wind direction indicated by arrow. 



whereas the coast near Noordwijk is more continuous 

 and symmetric. For both sites the water is effec- 

 tively deep for waves generated by an off-shore wind. 



The sortie in the area west of Sylt was carried 

 out during the field operations of JONSWAP in 1973, 

 on September 18th, at 17:30 hr (local). Briimmer et 

 al . (1974) describe the large scale weather features 

 during the JONSWAP operations of 1973 and also give 

 results of meteorological observations from ships, 

 buoys, and balloons in the area. According to this 

 information the windspeed and direction prior to the 

 flight had been fairly constant for one day. Since 

 the wind was almost perfectly off-shore the situa- 

 tion was classified as an "ideal" generation case. 

 In the two hours prior to the flight the windspeed 

 and direction at station 8 (see Figure 4) , at 10 m 

 elevation was approximately 13 m/s and 110° respec- 

 tively. The direction is only a few degrees off the 

 "ideal" off-shore direction of 107°. 



The weather during this flight was poor for photo- 

 graphic operations and all pictures which were taken 

 were under-exposed, in spite of the best possible 

 photographic measures. Pictures were taken over 

 six stations of JONSWAP, including active wave mon- 

 itoring stations 5, 7 and 9 (see Figure 4) . The 

 frequency spectra observed at these stations are 

 given in Figure 6 and they may be used for a direct 

 comparison with the results of stereo observations 

 over these stations. But in selecting the pictures 

 for preliminary investigation preference was given 

 to photographic quality rather than availability of 

 ground-true information and it appeared that the best 

 pictures were taken over station 10, which was other- 

 wise inactive during the flight. 



The frequency spectrum at station 10 was estimated 

 with a "hindcast" procedure based on the JONSWAP pa- 

 rameter relationships [Hasselmann et al. (1973)]. 

 The "hindcast" was attempted for stations 5, 7, and 

 9 with the observed windspeed of 13 m/s but the re- 

 sults (Figure 6) were rather poor, although they 

 seemed consistent with the statistical variation in 

 the observations of JONSWAP. The agreement improved 

 when a windspeed of 15 m/s was used (Figure 7) . This 

 was the windspeed estimated just prior to the flight. 

 Since this fictitious windspeed produced more real- 

 istic results, in particular for station 9 which was 

 the nearest to station 10, it was used for the "hind- 

 cast" at station 10. The resulting spectrum is given 

 in Figure 8, the comparison with the stereophoto- 

 graphic results will be discussed in Section 5. 



The second and third set of pictures to be ana- 

 lyzed were chosen from the pictures obtained in the 

 area off Noordwijk. The main reason for selecting 

 these pictures rather than the pictures taken off 

 Sylt was that the results of the sortie just de- 

 scribed indicated that the data were influenced by 

 the asymmetry of the coastline of Sylt. The coast 

 near Noordwijk is more symmetric for off-shore wind 

 directions . The information on the atmospheric con- 

 ditions during these flights was based on standard 

 synoptical observations which were received through 

 the office of the Royal Netherlands Meteorological 

 Institute. In addition a cup-anemometer and a wind- 

 cone were available at an observation tower located 

 9.5 km off-shore from Noordwijk (see Figure 3). 



The second sortie (the sequence refers to the 

 sequence of analysis, not the time sequence of the 

 flights) was flown in off-shore wind conditions on 

 November 12, 1976, at 13:05 hr (local). From the 

 synoptical observations it was found that the wind 

 was rather weak over the entire North Sea and the 

 wind in the area of observation was mainly caused 



Sylt 7309ie 



JONSWAP spedru m 



observed spectrum 



1751 start of re'cord (duration 25min) 



Stat .9 

 17!25hr 



020 0.25 0.30 



frequency ( Hz) 



FIGURE 5. Bottom profiles off Sylt (direction 287 ) 

 and off Noordwijk (direction 300°) . 



FIGURE 6. Observed frequency spectra at stations 5, 7, 

 and 9 and corresponding JONSWAP spectra for U = 13 m/s. 



