THE NEAR-SURFACE LAYER OF THE OCEAN
It is remarkable that there are three different regimes for the phase speed
that are common to both distributions (Figure 7-11c). The phase speed
becomes non-dispersive at frequencies much lower than the resonant
frequencies of the bubbles. The phase speed also asymptotes to the value for
bubble-free water at frequencies much greater than the resonant frequencies
of the bubbles. Strong frequency dependence is observed in the intermediate
region where the dispersion varies according to the range of bubble sizes and
the shape of the size distributions.
Figure 7-10 shows the sound-speed field disturbed by a large bubble
cloud from a wave-breaking event. For this particular event, the sound speed
was dramatically reduced at the surface (to approximately 500 m s
-1 ) for 3040 s. At a depth of a few meters, the sound speed was reduced by only about
100 m s
-1 and for a shorter time period. Short-lived events like this one are
primarily responsible for injecting air into the water column and lowering
the mean sound speed.
The average sound-speed reduction is at a maximum near the ocean
surface and monotonically decreases with depth (because the average bubble
concentration does so). The flux of bubbles into the surface waters
apparently increases as the incidence of wave breaking increases, further
reducing the sound speed in the near-surface layer.
In order to describe the depth dependence of the layer of reduced sound
speed, Terrill and Melville (1997) calculated a contour representing the
depth, 0.99c
z , at which the mean sound speed is 99% of the bubble-free value.
They derived a linear relationship between the depth of the reduced soundspeed layer and the significant wave height H s :
0.99
0.26
0.31
c
s
z
H
,
(7.9)
with the correlation coefficient, r
2 = 0.97. Since the significant wave height
is approximately four times the RMS wave amplitude (Komen et al, 1994),
the results suggest that the 99% contour depth has a characteristic length that
scales with the RMS wave amplitude.
Significant wave height and the wind speed have often been used for
obtaining correlations with bubble-driven acoustic phenomena such as
backscatter and ambient noise, though with limited success. The correlation
between
0
0.99C
z
and significant wave height in the experiment of Terrill and
Melville (1997) was high, though under certain conditions it could be
disturbed. Presence of swell can significantly alter the measured significant
wave height only contributing second-order effects to the wave breaking, or
the acoustically active portion of the wave field (Felizardo and Melville,
1995). Some other factors can also affect the wave breaking process. For
example, Terrill and Melville (1997) report the effect of strong surface
466
It is remarkable that there are three different regimes for the phase speed
that are common to both distributions (Figure 7-11c). The phase speed
becomes non-dispersive at frequencies much lower than the resonant
frequencies of the bubbles. The phase speed also asymptotes to the value for
bubble-free water at frequencies much greater than the resonant frequencies
of the bubbles. Strong frequency dependence is observed in the intermediate
region where the dispersion varies according to the range of bubble sizes and
the shape of the size distributions.
Figure 7-10 shows the sound-speed field disturbed by a large bubble
cloud from a wave-breaking event. For this particular event, the sound speed
was dramatically reduced at the surface (to approximately 500 m s
-1 ) for 3040 s. At a depth of a few meters, the sound speed was reduced by only about
100 m s
-1 and for a shorter time period. Short-lived events like this one are
primarily responsible for injecting air into the water column and lowering
the mean sound speed.
The average sound-speed reduction is at a maximum near the ocean
surface and monotonically decreases with depth (because the average bubble
concentration does so). The flux of bubbles into the surface waters
apparently increases as the incidence of wave breaking increases, further
reducing the sound speed in the near-surface layer.
In order to describe the depth dependence of the layer of reduced sound
speed, Terrill and Melville (1997) calculated a contour representing the
depth, 0.99c
z , at which the mean sound speed is 99% of the bubble-free value.
They derived a linear relationship between the depth of the reduced soundspeed layer and the significant wave height H s :
0.99
0.26
0.31
c
s
z
H
,
(7.9)
with the correlation coefficient, r
2 = 0.97. Since the significant wave height
is approximately four times the RMS wave amplitude (Komen et al, 1994),
the results suggest that the 99% contour depth has a characteristic length that
scales with the RMS wave amplitude.
Significant wave height and the wind speed have often been used for
obtaining correlations with bubble-driven acoustic phenomena such as
backscatter and ambient noise, though with limited success. The correlation
between
0
0.99C
z
and significant wave height in the experiment of Terrill and
Melville (1997) was high, though under certain conditions it could be
disturbed. Presence of swell can significantly alter the measured significant
wave height only contributing second-order effects to the wave breaking, or
the acoustically active portion of the wave field (Felizardo and Melville,
1995). Some other factors can also affect the wave breaking process. For
example, Terrill and Melville (1997) report the effect of strong surface
466