m No. 3k2 



a strong negative correlation at a frequency associated -witli the -waves 

 thenselves . This type of simple (BSH) mechanism may well be primarily 

 responsible for the doimward flux of i^find- imparted momentum through the 

 "water column. 



OPEN OCEAN MEASUEEMENTS 



A series of preliminary "open ocean" observations were made with the 

 slightly modified wave meter termed OMDUM II at the Bussards Bay Entrance 

 Light Station^ which is situated in 20 ,m of -water off the southern coast of 

 Jfessachusetts (figure 6). The meter, system was supported in a semi-rigid 

 geometi-y by an array of supporting guys above it and a 50 kgm vertical damping 

 weight suspended below it. 



The measurements were made on 11 Ifey 1964 during a period of steady -wind 

 conditions^ -^zith a mean wind speed of about 6-8 m sec~l. The spectra res-ults 

 of velocity da-ta taken at a depth of 1 meter and h meters below the -wave 

 trough level are sho-wn in figures 7 and 8. 



The auto-covariance spectra of the U* and W -values at both depths indi- 

 cate peaks at about 3-^ seconds, which agrees -with the visual estimates of 

 the wave periods . There is a strong atten-uation of the U* component spectra 

 relative to the W* at periods greater th^an about I.3 seconds. This effect, 

 apparently caused by a s-wlnging motion in the horizontal direction, is the 

 reaction of the meter system to the back-and-forth U* velocity component. 

 Thus, the reaction of the meter system tended to attenioate the amplitude of 

 the U* fluctuation component due to the gross wave motions. This effect has 

 been largely eliminated by modifying the -wire support system and using a 

 hea-vler daaiping weight. At the time of this -writing a new, smaller meter 

 system has been constructed and tested which has about half the cross 

 sectional area as the original de-vice. Raw da-ta from this smaller meter are 

 strongly indicative that, due to the reduction of drag, the reaction to the 

 oscillatory horizontal m-otions has been effectively removed. 



Returning again to figures 7 and 8, note the effective reddening of the 

 spectral peaks of both pairs of spectra -with depth, i.e., the high frequencies 

 are damped more effectively with depth. Measurements were made at (just 

 under "the trough level), 1, 2, S? ^p and 5 meters. This shift to more 

 dominant low frequencies with depth is characteristic of the data. 



Other -fchan observing the gross attenuation of the freque-ncies above, say, 

 1.8 cps, the reader should be cautioned of the possible limitations of the 

 meter* s detectability of t-urbulent or eddy structures which approach the scale 

 dimensions of the meter itself. Although the impellers have a fast response 

 character, the -variations in velocity at the higher frequency band-widths will 

 likely be associated -with smaller eddy scales approximating the dimensions of 

 the meter, i.e., 20 cm« 



