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permit a rigorous determination of discriminants from future field data, possibly leading to
a universal discriminant.
3.6 Remote sensing (radar, optical and infrared techniques)
The acoustic techniques described in
Section 3.5
are based on non-intrusive measurements
and therefore are themselves remote-sensing methods for studying various wave-breaking
features and properties. They have been singled out into a separate section because of a
number of practical advantages they appear to provide compared to other remote-sensing
means, in this particular area of oceanography.
The current section is dedicated to radar and optical sensing techniques, including those
based on infrared sensing. Of these, microwave radar backscattering has been applied in
wave-breaking studies as often as, and sometimes together with, acoustic methods (see e.g.
Melville
et al.
,
1988
;
Lowen & Melville
,
1991a
).
The radar active backscatter has been extensively used for remote sensing of the ocean
surface with the purpose of detecting, monitoring and studying surface winds, ocean cur-
rents and other oceanic features. A large number of laboratory experiments have shown
that the Bragg scattering mechanism, based on backscatter of microwave radiation by sur-
face water waves with wavelengths of the order of a few cm, is the main process to be
accounted for in this regard, although a variety of other contributing physical mechanisms
are possible (e.g.
Lowen & Melville
,
1991a
). The strength of the reflected signal depends
on the height of the cm-waves, and because of the rapid response of the heights of such
waves to environmental forcing, the backscattered signal is a good indicator of the activity
and strength of the oceanographic phenomena of interest.
It was quite early by comparison with techniques other than visual observations, that it
was noticed that radar backscatter could be used to study wave breaking.
Pigeon
(
1968
),
Long
(
1974
),
Lewis &Olin
(
1980
),
Alpers
et al.
(
1981
),
Kwoh & Lake
(
1984
),
Chaudhry &
Moore
(
1984
),
Keller
et al.
(
1986
) and
Trizna
et al.
(
1991
) all observed strong microwave
return, so-called 'sea spikes' in the reflected signal, and associated it with breaking.
Breaking of very short waves is mostly induced by long waves (see
Section 5.3.2
). This
can be either due to modulation of short waves by underlying longer waves (e.g.
Longuet-
Higgins & Stewart
,
1960
;
Phillips
,
1963
;
Donelan
,
2001
;
Donelan
et al.
,
2010
), in which
case they occur on the front face of the dominant crests, or forced by larger-scale breaking
(
Young & Babanin
,
2006a
), in which case it should happen in the wake of large breaking.
If the short waves break because they overcome a steepness limit like the dominant waves
(i.e.
Brown & Jensen
,
2001
;
Babanin
et al.
,
2007b
,
2009a
,
2010a
), then the Bragg scattering
due to such height-saturating short waves should be locked to the phase of the dominant
waves, in front of the crest or behind the crest, depending on what effect induces small-
scale breaking.
Microwave backscatter indeed makes this distinction. Already in the earlier studies
Alpers
et al.
(
1981
) and
Kwoh & Lake
(
1984
) noticed that the Bragg spikes are linked
to the steep crests of breaking waves.
Kwoh & Lake
(
1984
) further concluded that Bragg
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