Miles (1957) showed that the waves on the sea surface must be
matched by waves on the bottom surface of the atmosphère
The speed
of air and water must be equal at the water surface. Under most meteorological conditions, the air speed increases from near 0 to 60 - 90 percent
of the free air value within 66 feet (20 meters) of the water surface.
Within a shear zone of this type, energy is extracted from the me an flow
of the wind and transferred to the waves. The magnitude of this transfer
at any frequency is proportional to the wave energy already présent at
that frequency. Growth is normally most rapid at high frequencies. The
energy transfer is also a complex function of the wind piofile, the
turbulence of the air stream, and the vector différence between wind and
wave velocities.
The théories of Miles and Phillips predict that waves grow most
rapidly when the component of the wind speed in the direction of wave
propagation is equal to the speed of wave propagation.
The wave génération process discussed by Phillips is very sensitive
to the structure of the turbulence; This is affected significantly by
any existing waves, and the température gradient in the air near the
water surface. The turbulence structure in an offshore wind is also
affected by land surface roughness near the shore.
The wave génération process discussed by Miles is very sensitive to
the vertical profile of the wind. This is determined largely by turbulence
in the wind stream, the température profile in the air, and by the roughness of the sea surface.
Shorter waves grow most rapidly. Those waves which propagate obliquely
to the wind are favored, for they are better matched to the component of
the wind velocity in the direction of wave propagation than those moving
parallel to the wind. Thus, the first wave pattern to appear for short
fetches and durations consists of two wave trains forming a rhombic
pattern with one diagonal along the direction of the mean wind.
There is a limit to the steepness to which a wave can grow without
breaking. Shorter waves reach their limiting growth rather quickly;
longer waves, which grow more slowly but can obtain greater heights,
then become more prominent. Thus, the apparent direction of propagation
of the two wave trains tends to coalesce with increasing fetch and duration.
The length of the région in which a rhombic pattern is apparent may extend
from a few meters to a few kilometers depending on the width of the basin,
the wind speed, and previously existing waves.
Wave growth is significantly affected by any preexisting waves. The
empirical data analyzed by Inoue (1966, 1967) indicated that the magnitude
of the effect of seas already présent is about eight times the value given
m the original Miles (1957) theory. Neglecting this effect in early wave
pre iction théories has led to large errors in computing the duration
require
or a fully arisen sea. There are many situations in which the
W3j6j and t^ie waves growing most rapidly are not being propagated
in the wmd direction.
r
3-16
matched by waves on the bottom surface of the atmosphère
The speed
of air and water must be equal at the water surface. Under most meteorological conditions, the air speed increases from near 0 to 60 - 90 percent
of the free air value within 66 feet (20 meters) of the water surface.
Within a shear zone of this type, energy is extracted from the me an flow
of the wind and transferred to the waves. The magnitude of this transfer
at any frequency is proportional to the wave energy already présent at
that frequency. Growth is normally most rapid at high frequencies. The
energy transfer is also a complex function of the wind piofile, the
turbulence of the air stream, and the vector différence between wind and
wave velocities.
The théories of Miles and Phillips predict that waves grow most
rapidly when the component of the wind speed in the direction of wave
propagation is equal to the speed of wave propagation.
The wave génération process discussed by Phillips is very sensitive
to the structure of the turbulence; This is affected significantly by
any existing waves, and the température gradient in the air near the
water surface. The turbulence structure in an offshore wind is also
affected by land surface roughness near the shore.
The wave génération process discussed by Miles is very sensitive to
the vertical profile of the wind. This is determined largely by turbulence
in the wind stream, the température profile in the air, and by the roughness of the sea surface.
Shorter waves grow most rapidly. Those waves which propagate obliquely
to the wind are favored, for they are better matched to the component of
the wind velocity in the direction of wave propagation than those moving
parallel to the wind. Thus, the first wave pattern to appear for short
fetches and durations consists of two wave trains forming a rhombic
pattern with one diagonal along the direction of the mean wind.
There is a limit to the steepness to which a wave can grow without
breaking. Shorter waves reach their limiting growth rather quickly;
longer waves, which grow more slowly but can obtain greater heights,
then become more prominent. Thus, the apparent direction of propagation
of the two wave trains tends to coalesce with increasing fetch and duration.
The length of the région in which a rhombic pattern is apparent may extend
from a few meters to a few kilometers depending on the width of the basin,
the wind speed, and previously existing waves.
Wave growth is significantly affected by any preexisting waves. The
empirical data analyzed by Inoue (1966, 1967) indicated that the magnitude
of the effect of seas already présent is about eight times the value given
m the original Miles (1957) theory. Neglecting this effect in early wave
pre iction théories has led to large errors in computing the duration
require
or a fully arisen sea. There are many situations in which the
W3j6j and t^ie waves growing most rapidly are not being propagated
in the wmd direction.
r
3-16