TM No. 377 



It was hoped that, during the period of observations, some definite 

 variation would occur in the wind field, permitting observation of reflected 

 changes in the waves. Shortly after the helicopter trip to BBELS at 1500 hours 

 on 29 March, a northeast blizzard commenced. Strong ENE winds and snow 

 continued through midnight. Wind speed and direction for the entire sampling 

 period are plotted in figure V-32. The winds from 1200 to 1800 hours on 29 March 

 were from the east at about 9-12 m sec -1 . The winds abated to about 5 m sec~l 

 and shifted to the NE by 2U00 hours. The snow ceased at about 0200 hours on 

 30 March. By 0U00 the wind had shifted rapidly around to WNW at about 9 ni sec"^, 

 and the skies were clear. From 0^00 on, the winds remained from the west to WSW, 

 but the wind speed steadily increased, attaining 17 m sec~l by 1300 hours. 



During the first 12 hours of observations, the wind waves naturally were out 

 of the east and ENE - from Buzzards Bay. The LIMDUM I system was therefore placed 

 on the east side of BBELS and suspended from the end of the catwalk (see figure 

 IV-U). This was done to avoid measuring waves from the leeward side of the tower. 

 As soon as the winds shifted to the WNW after 0^00 on 30 March, the LIMDUM I 

 system was moved to the usual position on the west side of BBELS (see figure 

 17-18). 



Nine observations, all made at 0.5 meter beneath the wave troughs, were 

 chosen as representative of conditions during each phase of weather variation. 

 The auto-spectra <^ w for this sequence are shown in figure V-33. Greater 

 resolution of the lower frequencies was obtained by computing these spectra 

 using M = 100 lags, in lieu of the 50-lag resolution used in the plots of 

 appendix B. Also, a linear ordinate of spectral density (cm2 sec~l) is used to 

 emphasize the high energy peaks. The abscissa lists both frequency (mcps) and 

 period (sec). The time of sampling, wind velocities, and the variances are shown 

 for reference. 



The first three spectra (030, 031, and 032) display large variances in excess 

 of 1000 cm^ sec-2„ Strong bands of spectral energy occur from 150-350 mcps 

 (12.9- 6.6 sec), with peaks at about 250 mcps (k sec). Since observation 031 was 

 only a one-minute sample, there is large uncertainty in the variance and spectral 

 estimate. 



Even with relatively high ENE winds of 10-11 m sec - -'-, the wave frequencies 

 are concentrated at 250 mcps. Winds of 10-11 m sec~l from the S or SW (where 

 there is an "infinite" fetch) produce waves having spectral peaks between 150-200 

 mcps (lj-,0 - 6.6 sec), as shown by figure V-15 (spectrum 080) . The waves portrayed 

 in spectra 030-032 are probably "fetch limited" because of the land masses directly 

 to the east (see figure IV-3), and are therefore of relatively high frequency. 



Observation 0^8 indicates a drop in wind speed. The spectrum now shows two 

 peaks; the highest at 300 mcps (3.3 sec) and the lesser one at about 200 mcps 

 (5 sec). The variance value has decreased sharply to 686 cm^ sec - ^. The lower 

 frequency peak may be attributed to longer period swell-like waves generated 

 away from the local area. The spectrum of these waves could have been masked by 

 the strong wind waves depicted by 030, 031, and 032. 



128 



