Wave-breaking severity - Breaking and Dissipation of Ocean Surface Waves - page 231

generated wave trains of a certain height were forced by the wind. In Figure 6.7 , energy
loss increases when the wind forcing grows, but here stronger winds also correspond to
higher waves.
In the field, this feature is responsible for a change of the ambient noise level
(e.g. Manasseh et al. , 2006 ). As a result, this level depends on many factors., i.e. wind
speed, wave height, breaking rate of dominant waves etc., and it is hardly possible to
set a universal acoustic threshold to distinguish individual breaking events in the field by
acoustic means (e.g. Babanin et al. , 2007b ).
These details expand and reinforce the conclusion already made in Section 6.1 that,
although correlations between the wind and the breaking strength can be well-traced in a
particular circumstance, generally speaking wind influences are indirect. These influences
must be coming through the wave dynamics which is imparted by the wind on temporal
scales greater than the wave-breaking event duration. Such dynamics can differ between
the two-dimensional modulated wave trains and fully spectral directional oceanic environ-
ments, even if both are subject to the same wind forcing. A similar conclusion has also
been made with respect to the breaking probability ( Section 5.3.1 and Figure 5.19 ). It
should also be noted that both breaking probability and severity can themselves affect the
wind input and therefore the wind ( Section 8.3 ).
To conclude this breaking-severity chapter, we will summarise what is unknown rather
than what is definite knowledge, because the former certainly outweighs the latter. What
does the severity depend on? Will it be constant, even if on average, in different wave
fields with the same background steepness, for instance? Does the breaking strength change
across the spectrum, i.e. is the relative energy loss different for the dominant waves and
the shorter waves away from the peak? For the latter waves, does their severity depend on
whether they break due to inherent reasons or their breaking is induced by larger waves?
Is the severity dependent on the wind (see Stolte , 1992 ; Babanin et al. , 2010a ,inthis
regard)? How do wave directionality and shortcrestedness impact the breaking strength, if
they do? Also, for example, how much of the energy loss is spent on work against buoyancy
forces while entraining the bubbles into the water, and what fraction is passed on to water
turbulence? These and other questions should be answered and parameterised for practical
application, and this topic represents very interesting physics too.