frequency and less spread is at higher frequencies. This pattern is very dif- 

 ferent from the commonly assumed unidirectional distribution of energy. It 

 initiates questions as to whether or not this is a common occurrence and, if 

 so, how it may depend on other sea state descriptors. These and other 

 questions are addressed below. 



Early Storm Example 



126. Figure 6 shows a frequency- direction spectrum in the early stages 

 of a storm. Two distinct groupings of energy are apparent in the figure. One 

 group, located on the negative -angle side of the graph, has little high- 

 frequency energy and is likely the remnants of the prestorm sea state. The 

 other group is characterized by sharp peaks at the higher frequencies. These 

 are centered at directions in the range 40 to 50 deg. The contour plot. 

 Figure 6b, indicates that the high-energy part of this group is spread over 

 about 10 to 20 deg in direction. This is narrower than the low-energy swell 

 shown in Figure 5 but is still very different from unidirectional. 



127. The integrated frequency spectrum in Figure 6a shows an almost 

 uniform distribution of energy with frequency for frequencies less than about 

 0.22 Hz. This is the contribution from the low-energy waves coming from the 

 negative -angle directions. The peak in the frequency spectrum is near the 

 high-frequency end of the analysis band. This structure is consistent 

 qualitatively with wind wave growth models in which wind forcing acts first to 

 build high-frequency waves and, in time, results in migration of the spectral 

 peak frequency to lower values . The total energy in this case is relatively 

 low, as indicated by the H^^ value of about 0.6 m, again suggesting a young 

 sea. 



128. The integrated direction spectrum in Figure 6a shows the low 

 background energy spread from about -60 to 40 deg (an arc of about 100 deg) . 

 The young part of the sea state has a peak direction of about 50 deg and is 

 concentrated in the direction range from about 35 to 60 deg (an arc of about 

 25 deg) . This suggests that directional spread may be small in the early 

 stages of wind generation, another question for later analysis. 



129. An interesting property of Figure 6a is that there appears to be 

 two clear groupings of energy, each of which resides in a distinct region of 

 the frequency-direction domain. Such a condition is not always expected. A 



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