Geoscience Reference
In-Depth Information
sections dedicated to laboratory techniques and to their counterparts applicable in field
experiments where, as far as wave breaking is concerned, requirements for
in situ
devices
are very different from the laboratory. Remote sensing approaches are outlined next. Of
these, the acoustic methods are singled out into a separate section because, in our view,
they are the most advanced and most promising with regard to breaking studies. Finally,
analytical techniques are reviewed briefly. These include a section on detecting breaking
events in existing surface-elevation time series and a section on analyses of wave-breaking
statistics based on some initial assumptions. Both approaches appeal to outliers, either in
wave records or in probability distributions, which would not have taken place if the waves
were not breaking.
It should be mentioned that the majority of the wave-breaking measurement and anal-
ysis methods are intended for studies of breaking rates or probability, whereas breaking
severity measurement methods are underdeveloped if not marginal. In this regard, almost
all experimental techniques mentioned above and described in more detail below show
promise, but have never been sufficiently elaborated for the purpose of estimating severity.
The most direct way of estimating breaking-energy loss is of course measuring the wave
train immediately before and immediately after a breaking event (see
Section 2.7
), but even
in the laboratory this is problematic. Among the other methods, acoustic (
Section 3.5
) and
infrared (
Section 3.6
) techniques have a proven record of being able to qualitatively dis-
tinguish breaking events of different strengths, but even these are still to be advanced into
a quantitative calibration stage. In the meantime, as mentioned in
Section 2.7
, knowledge
and understanding of breaking severity remain quite poor, but are as important as those
of breaking probability. Therefore, it is worth emphasising at this stage that the lack of
progress in the breaking-strength topic is not only due to the need for relevant dedicated
studies, but to a great extent is hampered by the absence of robust and reliable experimental
methodologies.
3.1 Early observations of wave breaking, and measurements
of whitecap coverage of ocean surface
In this section, whitecap-coverage observations and parameterisations are described and
analysed, dating from historical to modern. In this context, definitions of the characteristic
features and properties of the atmospheric boundary layer are also discussed.
Early documented scientific observations of wave breaking date back to the late 1940s
(
Munk
,
1947
) and were followed by a number of research attempts which intensified in
the 1960s (i.e.
Blanchard
,
1963
;
Gathman & Trent
,
1968
;
Cardone
,
1969
;
Monahan
,
1969
,
1971
;
Monahan & Zietlow
,
1969
). They usually dealt with some identification of the white-
cap coverage of the ocean surface, usually by means of photography and without actual
collocated measurements of the waves. Wave measurement techniques were still under
development in those days, and data logging and computer facilities for recording, storage
and processing long time series of surface elevations were not available. Therefore, the
whitecap measurements were interpreted and parameterised in terms of wave-generating
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