Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 63

breaking is affected by the breaking of larger waves or modulation due to the larger waves.
In such a case, the cumulative effect at high frequencies will be such that the induced
breaking of the short waves will dominate the breaking caused for inherent reasons. There-
fore, breaking rates at small enough scales should be fully determined by the behaviour
of the longer waves in the system, and will be poorly correlated with the background
wind ( Babanin & Young , 2005 ; Babanin et al. , 2007c , see also Section 7.3.4 ). Another
important conclusion of Katsaros & Atakturk ( 1992 ) is that, if the breaking statistics is
investigated in terms of the wave-generating wind, it will depend on two parameters:
the wind speed and the wind forcing u

∗ /

c p (here, c p is the phase speed of waves at the

spectral peak f p ).
Other examples of visual observations and manual tagging of breaking events include
investigations by Weissman et al. ( 1984 ) and Stolte ( 1992 , 1994 ) where the tags were used
to develop ad hoc criteria in order to then computerise the subsequent data processing.
These and other contact measurements of wave breaking, based on assumed trial-and-error
criteria, will be reviewed in Section 3.3 .
Thus, the traditional visual observations gave a significant boost to understanding, descrip-
tion and parameterisations of breaking rates in different wind-wave fields. This is, how-
ever, a very laborious kind of study involving an observer who marks and then counts
breaking events by visually monitoring wave probes, where the surface elevations are
recorded, and waiting for whitecapping to occur. While the visual observations do not
suffer from uncertainties due to empirical thresholds set in 'automated' wave-breaking
detection techniques described later in this chapter, they are nevertheless subject to human
error and are too manually intensive and time consuming to be broadly employed in modern
wave research.

3.3 Contact measurements
Contact measurements of wave breaking imply instrumentation placed at the air-sea inter-
face where the breaking actually happens. What should they measure however? There are
vast volumes of records by wave probes sensing surface elevations, velocities and acceler-
ations that have accumulated over decades; most of them undoubtedly had breaking waves
passing over and recorded, but they are impossible to identify without some criteria which
would distinguish breaking from non-breaking waves.
A significant number of clever and elaborate attempts and approaches have been tried
in this regard. Since the most obvious feature of breaking is discontinuity of just about
every geometrical, physical and even chemical property at the ocean surface, many studies
aimed to develop an ad hoc criterion by means of trial and error to detect such a disconti-
nuity. Others have relied on known physical limiters, i.e. limiting Stokes steepness (2.47) ,
limiting acceleration (2.60) or limiting orbital velocity (2.49) .
Contact measurements of wave breaking is an area of research extensively explored and
developed over a period of some 30 years, and it is impossible to mention even briefly
all relevant studies. Here, we will suggest a review of a small selection of examples in an