from the FRF. Well-dispersed, low-frequency wave trains with very narrow 

 directional spreads are associated with these events and are characteristic of 

 waves from distant sources. Events E and R may be natural deviates from the 

 common pattern of the remaining 25 events. 



Storm waves driven by wind systems identified in the FRF annual reports 

 (Miller et al. 1988, Leffler et al. 1989, 1990, 1991. 1992, 1993) as Canadian 

 high-pressure systems, frontal passages, or low-pressure cells forming west to 

 south of the FRF have frequency-direction spectra with many common fea- 

 tures. Most of these features are illustrated in Event N (pages A47 to A50), 

 which exemplifies the set of events. The progression of these features as the 

 storm evolves is as follows: 



a. There is usually a prestorm wave field consisting of low energy, long 

 period swell with moderate spread arriving from shore-normal to slight- 

 ly south of shore-normal. For Event N, this is the first case illustrated 

 on the top row of page A48. 



b. During the primary growth stage, at the initial onset of winds, high- 

 frequency, locally generated waves appear. Peak period usually drops 

 from the pre-storm condition. Peak direction shifts nearly to align with 

 wind, but not exactly (bottom-mounted pressure gauges can detect only 

 intermediate to shallow water waves, so some refraction towards shore- 

 normal will take place even in short fetches). Peak directional spreads 

 are of order 10 deg to 20 deg, the smallest of any time in the storm. 

 For Event N, this is the second case illustrated on the top row of page 

 A48. 



c. In the secondary growth stage (remainder of the top row and first two 

 cases of the second row of plots on page A48 for Event N), peak period 

 increases, and the frequency spectrum tends to conform to one of the 

 conventional nearshore models, such as TMA (Bouws et al. 1985) or 

 FRF (Miller and Vincent 1990). It is the directional distribution within 

 S(f,6) that is most curious. As energy at the peak period increases, so 

 does the characteristic spread, perhaps indicating continued local growth 

 plus additional energy radiating in from growth further offshore. More- 

 over, an additional lobe of energy appears at the peak frequency, 

 aligned more nearly with shore-normal or, for Event N, well south of 

 shore-normal. Furthermore, a secondary peak of high-frequency energy 

 (identified as a ridge in three-dimensional plots of S(f,d) or a second 

 tail off the main lobe of energy in contour plots) appears, making a 

 strongly bimodal high-frequency tail to the spectrum. The second 

 energy ridge is aligned nearer to shore-normal, and ranges from 20 deg 

 to 40 deg offset from the initial growth stage spectrum. 



d. In the advanced growth stage (last two cases on the second row and first 

 two cases on the bottom row of plots on page A48 for Event N), the 

 spectral peak continues to move to lower frequencies and becomes 

 centered at very nearly shore-normal. The spread is rather large, being 

 of order 40 deg to 50 deg, and the secondary lobe of low-frequency 



Chapter 4 Discussion 



17 



