Energy dissipation across the wave spectrum - Breaking and Dissipation of Ocean Surface Waves - page 266

The surface roughness is largely determined by the waves in general and the wave break-
ing in particular (see e.g. Donelan , 1990 ; Komen et al. , 1994 ; Kudryavtsev et al. , 2001 ;
Kudryavtsev & Makin , 2002 ; Babanin et al. , 2007c ; Babanin & Makin , 2008 ), and there-
fore the characteristics of the surface waves are apparently different in such circumstances,
and perhaps much different. The interface is smeared as the water is covered with foam
and the air is full of spray, and the anecdotal description of the ocean surface in hurricane
conditions says that the water is too difficult to breathe, but the air is not dense enough to
swim (see Figure 1.9 ). Obviously, the very notion of short-wave scales loses its physical
meaning attributed to waves with a distinct air-sea interface, but the longer waves would
still persist and be breaking. The behaviour of the whitecapping dissipation at such extreme
wind forcing, however, can and will be altered.
Apparently, there is no sudden change of the wave-surface and boundary-layer
conditions at wind speed (7.28) . There is a gradual build-up of the change when the
moderate-condition features of the air-sea interface are not necessarily cancelled, but the
new physical exchanges certainly appear and grow in significance to the point of the new
physics becoming dominant.
In Sections 5.3.4 and 6.2 , examples of the wave-breaking response to wind speeds as low
as U 10 > s (5.77) were demonstrated, when the growth of the wind did not cause a

further growth of the waves, but rather brought about an increase of the breaking rates and
dissipation. Thus, at least one effect of the strong winds on the breaking can be outlined:
when the wind-energy input rates at some spectral scales become too strong for the waves
of those scales to 'digest' them (that is to redistribute this energy to different scales and
directions through nonlinear interactions), the waves respond by more frequent breaking
( Figure 5.27 ) which dissipates the extra-input energy ( Figure 6.6 ), without a noticeable
change to the wave spectrum at those scales ( Figure 5.42 ).
This conclusion of Sections 5.3.4 and 6.2 , however, was not unambiguous because the
results shown in the above-mentioned figures correspond not just to different wind speeds,
but to different magnitudes of the dominant waves also. The higher wind speeds are associ-
ated with larger waves, and these larger waves, too, in addition to the wind, could contribute
to enhanced breaking rates of shorter waves, without having the spectrum altered at those
shorter scales.
On the other hand, the conclusion agreed with the independent observation-based
deep-water parameterisation (5.75) of Babanin & Soloviev ( 1998a ), also discussed in
Section 5.3.4 . This parameterisation was obtained in a broad range of spectra and winds,
and implies that for younger waves (stronger wind forcing) the level of the wave-spectrum
tail remains relatively constant. It certainly does not respond in any consistent way to an
increase in wind forcing, that is the shorter waves do not on average grow higher if the
wind is stronger, which can only mean that they break more frequently.
The results of Sections 5.3.4 and 6.2 on wind influence on wave breaking and dissipation
were obtained for quite strong winds, up to mean wind speeds U 10 in excess of 20m

14m /

s,
but such magnitudes are still very distant from extreme conditions like those hurricanes
identified by (7.28) .

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