Definitions for wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 41

turbulence generation, bubble penetration depth and gas exchange across the surface, but
as said above the latter is more frequent. For the dynamics of plunging and spilling breakers
in shallow waters see, for example, Janssen ( 1986b ).
The micro-breaking phenomenon deserves to be mentioned separately among other
wave-breaking definitions, not so much because it is a different kind of breaking, but rather
in order to avoid confusion and to state that the dynamics of micro-breaking should be
basically the same as that of regular breaking. Different is the external signature of such
breaking, which is that micro-breaking does not produce whitecapping and therefore does
not exhibit whitecapping-related acoustic, optical and other signatures.
The term 'whitecapping' is often used interchangeably with the term wave breaking,
and even the dissipation function employed in spectral wave modelling is routinely called
whitecapping dissipation. A significant part of the spectral distribution of such dissipa-
tion, however, corresponds to the scales where waves break without producing whitecaps.
These are short gravity waves, short in absolute rather than relative terms, whose breaking
intensity is too weak to warrant air entrainment visible as whitecapping (or too weak to
overcome the surface tension at the wave crest and form a jet ( Tulin & Landrini , 2001 )).
Investigators of such micro-scale breaking point out that this phenomenon is in fact much
more widespread than are the whitecaps (e.g. Jessup et al. , 1997a ). According to Tulin &
Landrini ( 2001 ), these are waves of

λ ≤ 25 cm ,

(2.45)

f ≥ 2 . 5Hz ,

i.e. in most cases this is a major part of the wave spectrum.
Since micro-breaking is not visible, new means have had to be developed in order to
detect and to be able to quantify its breaking rates and severity. Katsaros & Atakturk ( 1992 )
used high-resolution video recording for this purpose, which involved a significant manual
effort to process the data and obtain the statistics. Jessup et al. ( 1997a ) employed infrared
imagery that allowed automatic data processing. The idea is based on the existence of a
'skin' thermal layer at the ocean surface. The depth of this layer is of the order of 0

1mm,
and the top of the layer is a few tenths of a degree Celsius cooler than the bottom. Micro-
breaking disrupts the skin layer and exposes the water with bulk upper-ocean temperature
to the surface where the temperature differences between the micro-breaking wake and the
background surface can be observed in the infrared-light range.
Jessup et al. ( 1997a ) write:

.

“The conceptual model we present, which explains our infrared observations, suggests that thermal
detection of microscale wave breaking may serve as a de facto definition of the phenomenon itself”.

In other words, as much as visually observed whitecapping can be treated as a de facto defi-
nition of breaking in the common sense, regardless of the physics behind the phenomenon,
the infrared signature defines micro-breaking. We agree this is true, with a small addi-
tion: the disruption of the thermal skin layer identifies the micro-breaking provided that