Conclusions. What else do we need to know about wave breaking? - Breaking and Dissipation of Ocean Surface Waves - page 420

overcome by coupling the two-phase models with fully nonlinear potential models. The
latter cannot simulate the breaking in progress, but do exceptionally well in modelling the
long-term evolution to the breaking and are much less demanding in terms of computing
resources.
From the point of view of experimental research on the dissipation, one of the major
problems is the breaking severity. Combined with the breaking-probability dependences,
the breaking-severity parameterisations would provide experimental estimates of the dis-
sipation and its behaviour. Chapter 6 , dedicated to the breaking strength, however, offers
more questions for future studies on the breaking severity than answers that are available.
It is relevant to reiterate them in the Conclusions.
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? How do the 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 the buoyancy forces while entraining the bubbles into the water,
and what fraction is passed on to the water turbulence? These and other questions should
be answered and parameterised for the practical applications, and this topic represents a
very interesting physics too.
Certainly, the severity is more difficult to measure and approach, compared to the fre-
quency of breaking occurrence, particularly by contact in situ means, but it is not impos-
sible, and a variety of remote-sensing techniques capable of quantifying the breaking
strength are available, like those acoustic, infrared and others described in Chapter 3 .
Therefore, the problem of answering the many questions raised should be a matter of
dedicated effort rather than of principal difficulty.
One difficulty, however, has to be specifically mentioned and highlighted: the directional
dependence of the breaking strength. Since, as discussed in the topic, there are indications
that the directional distribution of breaking probability is more or less uniform, and there
is evidence that the dissipation is not, this implies that breaking strength can be a function
of an angle between the propagation direction of wave groups, where the breaking occurs,
and the main direction of the waves/wind. What it is exactly and how this can be explained
in the context of the physics of the air-sea interactions, is an interesting question to keep
in mind. Dependences and parameterisations for the breaking severity may turn out to be
more complex than it seems at first sight.
Directional behaviours related to the breaking, not only to the breaking strength, but in
general, are among the most significant outstanding problems of wave-breaking studies.
Understandably, a lot of the research was conducted in two dimensions, in theoretical,
numerical and laboratory approaches, but as we can see the directional properties can be
most essential.
In this regard, the word 'directional' artificially unifies quite different and even unrelated
physics. For example, the directional distributions of the breaking probability, breaking