THE NEAR-SURFACE LAYER OF THE OCEAN
spectra from wind speed or wave height of breaking waves as an
environmental input. The model is also able to explain the main observed
features of ambient noise produced by breaking waves, including the slope
and frequency range of the spectra.
Crum (1995) and Kolaini et al. (1995) suggested that the bubbles could
change their oscillation mode from individual to collective in a transition
from a less violent to more violent type of wave breaking. The question of
which mode, individual or collective, is most important in the process of
noise generation has not yet been answered.
7.4.6 Ambient noise produced by rain
Rainfall generates sounds in the ocean. The underwater sound of rain is
the ensemble of individual raindrop splashes on the water surface. These
splashes represent a loud and distinctive sound, allowing detection,
classification, and measurement of rainfall parameters in the ocean (Shaw et
al., 1978; Nystuen et al, 2000).
The acoustic sensor (hydrophone) collects data from a relatively large
area on the ocean surface, which is proportional to the depth of the
hydrophone. Since the area of averaging for an acoustic sensor can be orders
of magnitude larger than for other types of rain gauges, much higher
temporal resolution of the rainfall measurement can be achieved. The
acoustic measurement of rainfall with a 5 s temporal resolution has revealed
instantaneous rainrates exceeding 1000 mm/hr under conditions of an
extremely strong rain event (>100 mm/hr, according to conventional rain
gages). These extreme rain rates could not be detected with conventional
rain gages typically using the longer temporal resolutions (Nystuen and
Amitai, 2003).
Acoustic analysis of underwater sound applied to identifying and
quantifying geophysical processes consists of two steps (Nystuen et al.,
2000): 1) The source of sound is identified from the spectral characteristics
of the sound; 2) The geophysical quantity of interest is quantified.
Distinctive spectral shapes for different geophysical or anthropogenic
noise sources aid the first step. Figure 7-13 shows examples of the sound
spectra generated by geophysical sources during an experiment in the South
China Sea. The spectral shape of wind-generated sound is attributed to
spectral slope from a peak near 500 Hz to over 25 kHz characterizes the
wind-generated sound. It is remarkable that five minutes after the start of an
extreme rainfall event (rainfall rate 200 mm h
-1 ), the sound level at 40 kHz
dropped 15 dB relative to the start of the downpour. This is presumably
because of sound absorption by the intense bubble layer formed due to heavy
470
breaking wind waves (see Section 7.4.7). A relatively uniform negative
spectra from wind speed or wave height of breaking waves as an
environmental input. The model is also able to explain the main observed
features of ambient noise produced by breaking waves, including the slope
and frequency range of the spectra.
Crum (1995) and Kolaini et al. (1995) suggested that the bubbles could
change their oscillation mode from individual to collective in a transition
from a less violent to more violent type of wave breaking. The question of
which mode, individual or collective, is most important in the process of
noise generation has not yet been answered.
7.4.6 Ambient noise produced by rain
Rainfall generates sounds in the ocean. The underwater sound of rain is
the ensemble of individual raindrop splashes on the water surface. These
splashes represent a loud and distinctive sound, allowing detection,
classification, and measurement of rainfall parameters in the ocean (Shaw et
al., 1978; Nystuen et al, 2000).
The acoustic sensor (hydrophone) collects data from a relatively large
area on the ocean surface, which is proportional to the depth of the
hydrophone. Since the area of averaging for an acoustic sensor can be orders
of magnitude larger than for other types of rain gauges, much higher
temporal resolution of the rainfall measurement can be achieved. The
acoustic measurement of rainfall with a 5 s temporal resolution has revealed
instantaneous rainrates exceeding 1000 mm/hr under conditions of an
extremely strong rain event (>100 mm/hr, according to conventional rain
gages). These extreme rain rates could not be detected with conventional
rain gages typically using the longer temporal resolutions (Nystuen and
Amitai, 2003).
Acoustic analysis of underwater sound applied to identifying and
quantifying geophysical processes consists of two steps (Nystuen et al.,
2000): 1) The source of sound is identified from the spectral characteristics
of the sound; 2) The geophysical quantity of interest is quantified.
Distinctive spectral shapes for different geophysical or anthropogenic
noise sources aid the first step. Figure 7-13 shows examples of the sound
spectra generated by geophysical sources during an experiment in the South
China Sea. The spectral shape of wind-generated sound is attributed to
spectral slope from a peak near 500 Hz to over 25 kHz characterizes the
wind-generated sound. It is remarkable that five minutes after the start of an
extreme rainfall event (rainfall rate 200 mm h
-1 ), the sound level at 40 kHz
dropped 15 dB relative to the start of the downpour. This is presumably
because of sound absorption by the intense bubble layer formed due to heavy
470
breaking wind waves (see Section 7.4.7). A relatively uniform negative