Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 94

scattering is not the only mechanism responsible for sea spikes at breaking crests. They dis-
tinguished the backscatter from capillary waves generated on the front face of the breaking
wave, from the turbulent wake behind the crest (see also Wetzel , 1990 ) and from the sharp
near-breaking crests themselves (see e.g. earlier work of Lyzenda et al. , 1983 , on the topic
of wedge diffraction).
Melville et al. ( 1988 ), Jessup et al. ( 1990 ), Lowen & Melville ( 1991a ) and Melville
et al. ( 1992 ), in the laboratory and in the field, moved from recognition of the wave-
breaking contribution of radar backscatter to the question of whether these remote-sensing
techniques can be used for studying the breaking statistics and physics, and the answer
was positive. In this regard, combined laboratory studies of microwave backscatter and
the acoustic signature of breaking are most instructive ( Melville et al. , 1988 ; Lowen &
Melville , 1991a ; Melville et al. , 1992 ).
Such laboratory experiments showed that both reflected microwave power and radiated
acoustic energy correlate with wave-energy dissipation (see also Section 3.5 ). As in the
case of acoustic probing, this shows promise for determining the breaking strength, along
with the breaking occurrence/probability. As in the acoustic case, however, this promise
has so far proved difficult to realise in complicated multi-scale oceanic wave fields.
In the laboratory, the main proportion of the backscattered power appeared to precede
the acoustic response to breaking. This interesting detail indicates that radar sensing can
be employed for studying the incipient breaking statistics/probability, because the sound
signature obviously designates the breaking in progress, the stage of active wave collapse
when the whitecapping and bubbles are produced (see Sections 2.1 , 2.2 ). Since the onset
of breaking is difficult to detect because of the absence of distinct visual features, such as
whitecapping, this makes microwave sensing a valuable tool for studying the initial stage
of wave breaking, even if it is only in the laboratory.
Another potential application of microwave radar backscatter uses its ability to meas-
ure the Doppler spectrum of the reflected signal, and therefore orbital velocities including
those at breaking onset. The latter has frequently been used as a breaking criterion ( (2.49) ,
Section 2.9 ) and even provides a basis for deducing a dissipation function based on obser-
vations of probability distributions of the orbital velocity, but it has not been the subject of
a convincing dedicated study or comprehensively measured. In Lowen &Melville ( 1991a ),
the peak of the Doppler spectrum corresponds to a velocity within 10% of (2.49) . Lamont-
Smith et al. ( 2007 ) showed that the spectral density of this peak and its width depend on the
phase velocity of breaking waves, and does not depend on the grazing angle of the radar,
and they further found that the peak magnitude and the radar cross-section for breaking
waves scale with the radar wavelength

λ radar :
∼ λ

1 . 5

radar .

(3.29)

Application of the technique to dedicated field studies of wave breaking followed. Jessup
et al. ( 1990 ) found that the sea-spike contribution to the radar cross-section was

u 3
∗

.This
suggests that the radar return can potentially be used to measure total dissipation of wave
energy (i.e. 3.9 ).

∼