Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 59

according to (3.18) . This is exactly the trend observed by Monahan ( 1971 ) and attributed
to atmospheric stability without first ruling out the surface-temperature dependence.
Further uncertainties of (3.5) -like parameterisations of coverage W in terms of wind
speed U , which may prove essential in some circumstances, have to be highlighted. Again,
early researchers were already aware of these. Monahan ( 1971 ) mentions variations of
whitecap coverage as a function of wave fetch and wind duration. These days, this is trans-
lated as dependence of W on the second parameter, U c , which characterises inverse wave

age (stage of wave development or wind forcing) (e.g. Zhao & Toba , 2001 ; Stramska &
Petelski , 2003 ; Guan et al. , 2007 ). Indeed, this should be expected based on the appar-
ent physical argument. As discussed above, the whitecap coverage depends on breaking
probability and breaking strength. Both of these depend on wave age (see e.g. Katsaros &
Atakturk , 1992 ; Banner et al. , 2000 , for the breaking probability). In a laboratory exper-
iment Babanin et al. ( 2009a , 2010a ) demonstrated that, for stronger forcing U

/

c ,the
breaking probability is higher, whereas the breaking severity is weaker. Thus, the trends
are opposite, and therefore their combined effect on whitecap coverage may not be clearly
pronounced, but obviously variations of magnitude of W , at the same wind speed U ,but
for different development stages U

/

c , can be expected.
Another essential uncertainty, also pointed out by the early researchers, is due to the
presence of biological surfactants. Monahan ( 1971 ) mentions

/

“variations in sea water surface tension caused by the occasional presence of organic films ( Abe
et al. , 1963 ; Garrett , 1967 )”.

Such 'occasional presence' may have a significant impact on regional variations of
whitecap-coverage dependence, because of different biological activity in an ocean region.
Stramska & Petelski ( 2003 ), for example, conducted observations in a polar area, where the
amount of dissolved organic material is different to the waters of traditional observations,
and concluded that some of the historical relationships for W -versus- U would overestimate
their observations by a factor as large as 8.
To conclude this brief overview of the early observations of whitecaps, we have to say
that, while studies of this property of wave breaking have now spanned more than six
decades, many uncertainties still remain in parameterisations of whitecap coverage. Given
that the importance of this oceanic characteristic grows as the means of remote sensing of
the ocean develop, it is important to realise these uncertainties and address them. Some of
them are linked with gaps in our understanding of the wave-breaking process, and some are
not. Reliable experimental means for whitecapping detection, monitoring and quantifying
are certainly available (see Section 3.6 ).

3.2 Traditional means (visual observations)

When capacities for measuring and storing surface elevation data became available, further
research of wave breaking concentrated on the hydrodynamic characteristics of breaking,
rather than on the connections with the wind as in Section 3.1 . As far as the wave-breaking