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The contact measurements are apparently more difficult, and often impossible in the
field, given the sporadic, powerful and destructive nature of the breaking events to be
measured. Many elaborative remote-sensing methods have been developed to help this
problem. The infrared techniques based on disruption of the surface skin layer, for exam-
ple, have been shown not only to detect the breaking occurrences, but also to indicate the
breaking severity.
Most promising in this regard, in our view, are the passive acoustic methods. In partic-
ular, the bubble-detection method allows us to identify the wave which is breaking and
producing bubbles, and therefore to obtain the distribution of breaking occurrences and of
rates of such occurrences along the spectral frequency and wavenumber scales. The size of
the produced bubbles can also be estimated which has a connection with the wave-energy
loss, and thus information on the breaking severity is also available within this method.
So, the technique has a demonstrated potential to measure all the elements of the spec-
tral distribution of the wave-energy dissipation, and to do that by using a hydrophone. The
hydrophones are simple and cheap sensors, and can be deployed below the surface and thus
avoid the violent power of the wave breaking as such. Their energy consumption is small
and they can be operated on batteries for long-term observations. Information on the time
of bubble appearance and on its size is basically just two numbers, and therefore memory
requirements for such devices are very modest too.
Analytical methods, also described in
Chapter 3
, allow us to point out the presence of
breaking events in wave records where the breaking was not registered by any experimental
means. Since there are large quantities of wave records available, such methods can be
very helpful in enhancing their value in the case where some information on breaking
occurrence and breaking statistics is necessary, even if approximate, but was not recorded
directly.
Also in the absence of direct counting and measuring the wave breaking, a set of statisti-
cal approaches outlined interprets the probability functions in order to infer information on
the dissipation due to wave breaking. If compared with other theoretical methods intended
for this purpose, this method has the least limiting underlying assumptions. Basically, all
that is needed are the valid maximal and minimal experimentally confirmed values of wave
steepness of individual waves achieved in the course of breaking events.
Chapter 4
has been partially re-written compared to the review (Babanin, 2009). The first
section of this chapter remained. It is dedicated to numerical simulations of the evolution
of nonlinear waves leading to the breaking onset, and of the imminent-breaking stage itself,
by means of analytical approaches. The fully nonlinear Chalikov-Sheinin model was used,
which was also coupled with a model for the atmospheric boundary layer, and allowed
investigations of the wind influences on breaking onset. Many new and interesting findings
of this section were further verified, investigated and researched into the stages beyond the
breaking onset in experimental
Chapters 5
and
6
.
The former second section of
Chapter 4
on the kinematics of the wave-breaking onset has
now been removed: this was a scientific criticism of one of the analytical approaches and
was important to point out in the journal review, but does not have an independent value for
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