560 



Sylt 73091B 



1 stot 9 



I 17 . 25 hr 



JONSWAP spectrum 



observed spectrum 



17 51 stort of record (duration 25 mm ) 



020 25 30 



frequency (Hz) 



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

 and 9 and corresponding JONSWAP spectra for U = 15 m/s. 



by a weak and fairly large low pressure area over 

 central France. Synoptical observations in the 

 coastal region 25 km North and 8 km South of Noord- 

 wijk indicated windspeeds of 4.5 m/s and 4.0 m/s 

 respectively and the wind directions of 100° and 

 150° respectively. The wind observation at the 

 platform was carried out at 23 m above mean sea 

 level. Averaged over the duration of the photo- 

 graphic operations (about 40 min.), the observed 

 windspeed was 6.4 m/s and the directions just prior 

 and just after the flight were approximately 140°. 

 The "ideal" off-shore direction would have been 120° 

 To estimate the windspeed at 10 m elevation, the 

 observed value was corrected. The correction for 

 the bulk of the tower, is known from v;ind- tunnel 



S.0 



• zol. 

 E 



Sylt 730918 



JONSWAP spectrum 

 observed spectrum 

 (from stereo doto) 



tests, and the windspeed was extrapolated using a 

 logarithmic wind-profile with a drag coefficient, 

 Cio ~ ^-5 ^ 10"^. The resulting windspeed is 6.0 

 m/s. The corrections for the wind direction are 

 marginal and well within the error of observation. 

 During this flight pictures were taken over the 

 observation tower and at locations 30 km and 50 km 

 from the coast (see Figure 3) . The pictures taken 

 30 km off-shore seemed to contain sufficient stereo 

 information to obtain a relatively high directional 

 resolution and these were chosen for preliminary 

 investigation. 



Wave observations at sea level were available 

 from a wave gauge at the observation tower and from 

 an accelerometer buoy at the location 30 km off- 

 shore. The spectrum of the buoy is given in Figure 

 9. It will be used for comparison with the stereo- 

 photographic results. During the flight some swell 

 coming from south-westerly directions was observed 

 from the helicopters. 



The third sortie was flown off Noordwijk on 23 

 March 1976 at 12:20 hr (local). The wind was rather 

 weak over the entire North Sea and the direction 

 varied from ENE off the Dutch coast to SSW off the 

 Norwegian coast. This windfield was caused mainly 

 by a fairly weak high pressure ridge over the North 

 Sea and a low pressure area over central France. 

 Synoptical observations at the same coastal stations 

 as mentioned above indicated windspeeds of 11.0 m/s 

 and 8.0 m/s respectively and wind directions of 80° 

 and 70° respectively. The corrected wind speed and 

 direction at the observation tower (averaged over 

 20 min.) were 8.3 m/s and 70°. Since the "ideal" 

 off-shore wind direction would have been 120° the 

 wind is slanting across the coast line at an angle 

 of approximately 50°. Obviously this implies a 

 strong asymmetry of the coast line with respect to 

 the wind direction. Pictures were taken over the 

 observation tower and at locations 17 km and 30 km 

 off-shore. Since the pictures taken 17 km off-shore 

 seemed to be the best, they were analyzed. Unfor- 

 tunately no simultaneous wave observations in the 

 area were available. 



Noordwijk 761112 



OtO - 



o a20 



0.10 



buoy spectrum 

 observed spectrum 

 (from stereo dato ) 



035 f(Hz) 



30 40 050 



frequency ( Hz ) 



FIGURE 8. Spectrum inferred from stereo data and cor- 

 responding JONSWAP spectrum for U = 15 m/s. 



FIGURE 9. Spectrum inferred from stereo data and 

 spectrum from buoy measurement. 



