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
équation embodying the Miles-Phillips theory to predict wave growth. A
comparison of significant wave heights from shipboard observations ànd by
hindcasting at two separate locations near the weather ships is shown in
Figure 3-7. The location of Océan 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 Océan as shown in Figure 3-8. The
agreement between observed and computed values seems to justify a higl>
level of confidence in the basic prédiction model. Observed meteorological
data were interpolated in time and space to provide the required data,
thus these prédictions were hindcasts.
Bunting and Moskowitz (1970) and Bunting (1970) hâve compared forecast 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 prédiction was the détermination of the surface wind field over
the océan.
(Pore and Richardson, 1967), and (Bunting, 1970). It is partly
because of the difficulty in obtaining a satisfactory spécification of the
wind field over the sea that simpler wave prédiction 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 génération ceases,
or the waves may propagate out of the génération 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 prédiction 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 génération. In many of these areas, wave characteristics
3-17
Inoue (1967) prepared hindcasts for Weather Station J (located near
53°N, 18°W), for the period 15-28 December, 1959, using a differential
équation embodying the Miles-Phillips theory to predict wave growth. A
comparison of significant wave heights from shipboard observations ànd by
hindcasting at two separate locations near the weather ships is shown in
Figure 3-7. The location of Océan 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 Océan as shown in Figure 3-8. The
agreement between observed and computed values seems to justify a higl>
level of confidence in the basic prédiction model. Observed meteorological
data were interpolated in time and space to provide the required data,
thus these prédictions were hindcasts.
Bunting and Moskowitz (1970) and Bunting (1970) hâve compared forecast 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 prédiction was the détermination of the surface wind field over
the océan.
(Pore and Richardson, 1967), and (Bunting, 1970). It is partly
because of the difficulty in obtaining a satisfactory spécification of the
wind field over the sea that simpler wave prédiction 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 génération ceases,
or the waves may propagate out of the génération 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 prédiction 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 génération. In many of these areas, wave characteristics
3-17