to give an upper limit to ^he magnitude thaii- any particular spectral component could 

 attain. This does not imply that this upper limit is ever reached or, if reached, 

 maintained. 



(iv) Attention is now returned to the features present in the near zone of the 

 f-x diagrams. The most obvious of these is the occurrence of a "peak" (high wave 

 energy) and then a "valley" (low wave energy) in the downwind diagrams in the 

 region f = .12 - .15 cps and x = 35 - 45 nautical miles. These features do not repeat 

 in the upwind diagram, but considerable certainty can be attached to the belief that 

 the features are indeed reoL This raises the problem of trying to explain their cause. 

 Since the frequencies involved indicate that the corresponding waves are in relatively 

 deep water, there is little possibility of accounting for the result through shoaling or 

 some other bottom effect. The only other explanation that seems at all reasonable was 

 the presence of a small area of relatively stronger wind located some 35 nautical miles 

 from the coast. Since the plane was travelling much faster than this area, the result 

 would be shown on the f~x diagrams as an area of high wave energy. Frequencies 

 higher than those in the peak area would already be at or near equilibrium and hence 

 shov/n no significant change. Frequencies lower than the peak respond more slowly to 

 changes in wind speed and once again show little evidence of such an area. If one 

 takes this point of view, the "valley" is now only a relative feature due entirely to the 

 occurrence of the peak area. A close inspection of the upwind and downwind diagrams 

 will show that tfie energy content of the valley area (in f-x space) is practically the 

 same for both. While the occurrence of an area of stronger wind accounts qualitatively 

 for the observations, and at the same time does not strain one's imagination, such an 

 explanation must be considered as speculation. There are no definite facts to back it 

 up„ This indicates how vital it is to know more about the fine scale features of the 

 wind field. At the same time it points up the folly of attempting anything but a 

 macroscopic analysis of the present, limited data. 



One other near zone feature of the f-x diagrams deserves comment. This 

 is the presence of a shallow, upward sloping valley in the region (.09,45). The fact 

 that this feature appears on both sets of diagrams and at essentially the same location 

 indicates that it is significant. Detailed topographic charts show that the flight path 

 of the aircraft crossed over a north-south extending ridge on the sea floor at a distance 

 of 42 nautical miles from the coast. The ridge was just shoal enough to make waves 

 with frequencies less than 0.10 cps feel bottom. Although this undersea ridge could 

 account for the above mentioned f-x feature, the magnitude of the effect is small in 

 comparison with the statistical variability inherent in the spectral estimates. The 

 error will be presently neglected, but hopefully not repeated in later work. 



7.2 Wave Growth 



7.2.1 Determination of Wave Growth Parameters 



It is the purpose of this section to obtain quantitative estimates of the wave 

 growth parameters OC and P (Equation 2), and then to compare these estimates with 



38 



