Energy dissipation across the wave spectrum - Breaking and Dissipation of Ocean Surface Waves - page 252

While the first and second type instabilities, as far as the breaking is concerned, may
always be locked together because the breaking starts developing if

42, separation of
the superposition cases is more distinct as each of them can lead to a breaking occurrence
independently. These can be subdivided into frequency focusing (e.g. Longuet-Higgins ,
1974 ; Rapp & Melville , 1990 ; Griffin et al. , 1996 ; Meza et al. , 2000 , among many oth-
ers), amplitude focusing ( Donelan , 1978 ; Pierson et al. , 1992 ) and directional focusing
(e.g. Fochesato et al. , 2007 ). Spectral outputs of such breaking mechanisms are still to
be thoroughly examined and documented, but in the amplitude-focusing observations of
Pierson et al. ( 1992 ), including the cases of breaking waves, it was shown that the loss
of energy by dominant waves is accompanied by energy gain both below and above the
spectral peak. That is, for amplitude focusing the spectral outcome of dissipation is yet to
be different.
Another potential group of mechanisms is external forcing. For example, a very strong
wind or a surface current gradient can also lead directly to steepening of the background
waves all the way up to the (2.47) , (2.51) and (2.52) limit. Usually, the role of the wind
in the breaking process is secondary and is limited to gradual growth of the steepness,
to stimulating or negotiating development of instabilities or probability of superpositions
( Sections 4.1.3 , 5.1.3 , 5.3.4 , Babanin et al. , 2010a ). If the wind forcing, however, is very
strong and can provide a wave growth from background to the Stokes steepness within
a few wave periods, or even within one period, then, on the contrary, the other mecha-
nisms become irrelevant. A strong current gradient certainly possesses such capacity too.
Whether the breaking limit will still be the same in such circumstances and what will be
the spectral outcome of such breakings are still to be verified.

> 0 .

7.3.3 Field measurements

Field measurements of the spectral distribution of wave-breaking dissipation are very few.
They are a complicated undertaking for a great number of reasons, ranging from the techni-
cal difficulties of breaking measurements as such, through the difficulties of interpretation
of the physics of breaking, to the mathematical difficulties of presenting properties of ran-
dom and irregular events as a spectral dissipation term, whereas the spectrum by definition
implies a continuous and uniform distribution of some property. Setting the technical and
mathematical difficulties aside, we will draw the attention of the reader to the variety of
physical mechanisms for wave-energy dissipation active simultaneously in a single break-
ing event (see, for example Section 7.2 ) and to the diversity of the spectral outcomes of
wave-breaking dissipation ( Sections 7.3.1 , 7.3.2 ).
The latter particularly presents a considerable challenge for field measurements. Indeed,
if the dissipation output of breaking of a monochromatic or quasi-monochromatic wave
train is spectral, then how can the outcomes of breaking be understood and sorted out in an
environment where a continuous spectrum of breaking waves is the initial condition?
In this section, we will present methods that allow us to identify and isolate break-
ing occurrences by relatively narrow spectral bands where the initially breaking waves