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physical features of the dissipation functions which have actually been observed in field
experiments, and techniques which allow us to calibrate the dissipation function indepen-
dently, before substituting it into the model where many source terms are operational simul-
taneously and their contributions are not possible to separate. One of the main constraints
in the wind-wave system is the total wind stress, whose parameterisations are available and
have to be matched by the wind-input integral. This integral allows independent calibration
of the input, and the integrand of the dissipation spectrum has to be a fraction of it, whose
magnitude and its variations are again known independently from field observations. Test-
ing of a new observation-based dissipation function by such constraint-based means and
its implementation in a research spectral model are also described.
In the context of the dissipation function in spectral modelling, non-breaking dissipation
of wave energy has to be clearly outlined and defined too, and studied in conjunction
with the breaking dissipation. Such dissipation is often forgotten about, but not by the
wave forecasters. Not all the wave-energy dissipation comes through the breaking, and
this fact has to be realised and clearly stated. This will help confusions in forecasting, for
example, the swell propagation. Non-breaking loss of wave momentum/energy, passed to
the ocean, facilitates the upper-ocean mixing, and that passed back to the atmosphere is
another potentially significant physical element at the atmosphere-ocean interface. Both
of these may claim importance in the case of large-scale applications including climate
modelling.
Thus, the topic of wave-energy dissipation is incomplete without discussing non-breaking
dissipation and this is done in
Section 7.5
of the topic. Two mechanisms of non-breaking
dissipation, which appear to be of the same order of magnitude, are discussed and com-
pared to measurements of the swell propagation across the oceans. These are wave-induced
non-breaking turbulence and tangential turbulent stresses imposed by the waves against the
air above the interface.
While small, by comparison, in the presence of breaking, the non-breaking loss of energy
spent on the generation of turbulence is only insignificant as far as the wave attenuation
is concerned. There is an emerging argument that the role of non-breaking wave-induced
turbulence in the upper ocean may be more essential than that of the breaking-produced
turbulence. This argument is further discussed in the topic in
Chapter 9
, which is dedicated
to the multiple roles that the breaking plays in the system of air-sea interactions.
The consequences of the wave-breaking physics are many, and these are not necessar-
ily related to the energy dissipation. Non-dissipative impacts of the breaking are multiple
and they deserve special attention and revision in
Chapter 8
. Among others, they include
the breaking-caused spectral peak downshift, the role of wave breaking in maintaining the
level of the spectrum tail, and wind-input enhancement due to wave breaking. The first
of the three features mentioned, the downshift, is most universally attributed to the weak
nonlinear interactions, and this is how it is treated in spectral wave models with very few
exceptions. In the meantime, there is compelling experimental evidence supported to an
extent by a theoretical argument that the breaking does move the spectral peak to lower fre-
quencies/wavenumbers. And under appropriate conditions, the rate of this peak-frequency
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