3.32 VERIFICATION OF WAVE HINDCASTING 



Inoue (1967) prepared hindcasts for Weather Station J (located near 

 53°N, 18°W) , for the period 15-28 December, 1959, using a differential 

 equation embodying the Miles-Phillips theory to predict wave growth. A 

 comparison of significant wave heights from shipboard observations and by 

 hindcasting at two separate locations near the weather ships is shown in 

 Figure 3-7. The location of Ocean Weather Ship J, the mesh points used in 

 the numerical calculations, and four other locations discussed below are 

 are shown in Figure 3-8. The calculations required meteorological data 

 from 519 grid points over the Atlantic Ocean as shown in Figure 3-8. The 

 agreement between observed and computed values seems to justify a high 

 level of confidence in the basic prediction model. Observed meteorological 

 data were interpolated in time and space to provide the required data, 

 thus these predictions were hindcasts. 



Bunting and Moskowitz (1970) and Bunting (1970) have compared fore- 

 cast wave heights with observations, using the same model with comparable 

 results. 



By 1970, it was generally believed that the major remaining difficulty 

 in wind wave prediction was the determination of the surface wind field over 

 the ocean. (Pore and Richardson, 1967), and (Bunting, 1970). It is partly 

 because of the difficulty in obtaining a satisfactory specification of the 

 wind field over the sea that simpler wave prediction systems are still 

 being used operationally. (Pore and Richardson, 1969), (Shields and Burdwell, 

 1970), and (Francis, 1971.) 



3.33 DECAY OF A WAVE FIELD 



Wind-energy can be transferred directly to the waves only when the 

 component of the surface wind in the direction of wave travel exceeds the 

 speed of wave propagation. Winds may decrease in intensity, pass over 

 land, or change in direction to such an extent that wave generation ceases, 

 or the waves may propagate out of the generation area. When any of these 

 events occurs, the wave field begins to decay. Wave energy travels at a 

 speed which increases with the wave period. Thus the energy packet leaving 

 the generating area spreads out over a larger area with increasing time. 

 The apparent period at the energy front increases and the wave height 

 decreases. If the winds subside before the sea is fully arisen, the 

 longer waves may begin to decay while the shorter waves are still growing. 

 This possibility is recognized in advanced wave prediction techniques. 

 The hindcast spectra, computed by the Inoue (1967) model and published by 

 Guthrie (1971) show many examples of this for low swell, as do the aerial 

 photographs and spectra given by Harris (1971). (See Figures 3-2 and 3-6.) 

 This swell is frequently overlooked in visual observations and even in the 

 subjective analysis of pen and ink records from coastal wave gages. 



Most coastal areas of the United States are so situated that most of 

 the waves reaching them are generated in water so deep that depth has no 

 effect on wave generation. In many of these areas, wave characteristics 



3-17 



