TM No. 377 



Referring to table IV- 3 , the mean horizontal down-wave flow component u 

 varied from -5.5 to +3.2 cm sec -1 between the time of serial 023 and 02^. Low 

 tide occurred at about 1030 hours. According to the relationship between tide 

 height -rj and current direction 9 shown in figure IV-6, at the time of the 

 wave measurements (from IIU5 to 1229), the current should have been changing 

 from 180° to about 280° . Since the u meter was aimed at l80° , the predicted 

 current change agrees with the observations. 



Interestingly enough, the values of OZ. and Ou^ a " e more nearly equivalent 

 than in any other BBELS observations at 0.5 meter depth (see table V-l, showing 

 the ratios ^Tt*/ ^TH*'). The variances for 2.0 meters are about equal. 



The auto-spectra for u and w from serial 023 (0.5 m i) are shown in appendix B. 

 The curves exhibit two peaks, one centered at 100 mcps (10 sec) and a lesser one 

 at 6OO-65O mcps (1.7 sec). Although the ^^ curve falls slightly below the (^^ 

 curve at all frequencies, the spectra are similar in shape. A third minor peak 

 is displayed by ^^ at about 1200 mcps (O.85 sec). 



The auto- spectra for the second 0.5 meter observation (serial 02^) are also 

 shown in appendix B. Two dominant peaks are again displayed. The highest energy 

 peak again occurs, for both <Jto anci - *&*** j a "t 100 mcps (10 sec). Clear secondary 

 peaks are exhibited at about 500 mcps (2 sec); however, the ^y^ peak falls 

 farther below the ^^ value than in the 023 observation. 



The auto-spectra for 2 meter depth (serial 025) are shown in appendix B. 

 Again the dominant peak occurs at 100 mcps (10 sec), where the curves for each 

 spectral component are essentially identical. Below 200 mcps (above 5 sec), 



^yl slightly exceeds the value of c£> w . At about U00-500 mcps, a slight step 

 or plateau is depicted by the ^^j curve and, to a lesser extent, by Q^ 

 This inflection appears to be a vestige of the secondary peaks of the spectra 

 from the 0.5 meter measurements (serial 023 and 02U). 



Inspection of the auto-spectra of the three observations indicates that by 

 far the greatest energy content lies in the low frequency swells appearing in 

 the spectra in the region from 100 mcps (10 sec) to 250 mcps (U sec). The 

 secondary peaks in the auto-spectra occurring at 500-700 mcps (2.0-1.7 sec) are 

 probably manifestations of the small wind waves radiating from the SE super- 

 imposed upon the long swells from the south. 



In view of the wind and sea build-up during the observations, it is of 

 interest to directly compare the spectra of serial 023 (0.5 m i) with 02^- 

 (0.5 m II). Figure V-31A. displays the superposed auto-spectra of the vertical 

 velocity components. Most striking is the similarity of the curves except in 

 the frequency band from 350 mcps (2.8 sec) to 900 mcps (l.l sec). In this region, 

 there is a contrasted and overall "reddening" and energy increase displayed by 

 the 0.5 m II spectrum relative to that of the 0.5 m I. The peak of 0.5 m I at 

 650 mcps (1.7 sec) attains ^+.6 cm2 sec~l; whereas the corresponding 0.5 m II peak, 

 which has shifted to 500 mcps (2.0 sec), attains 23 cm2 sec"! _ a five-fold 

 increase in spectral density. 



126 



