Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 56

with caution for applications in extreme weather conditions and even at reasonably strong
winds (see also Section 9.1.2 ).
This brings us to further cautious considerations regarding the validity of the interpre-
tations of whitecap coverage as a characteristic for wave energy dissipation. Bortkovskii
& Kuznetsov ( 1977 ) and Bondur & Sharkov ( 1982 ) and Bortkovskii ( 1987a ) introduced a
classification of whitecap systems that we will describe by using a direct quote from a later
topic of one of the authors ( Sharkov , 2007 ):

“ specific form of foam systems in optical images allows us to confidently identify at least two

classes (types) of foam formations: (1) 'wave crest foam' (i.e. 'whitecaps'), the so-called 'short-
living form' (i.e. 'dynamic foam') of foam activity with a lifetime of units of seconds; and (2) spotty
structures (or 'foam streaks'), 'static foam' (or 'residual foam') with a lifetime of about 10 seconds
to several minutes ... At wind velocities higher than 15m / s there arises a special class of stable
foam systems: the thread-like systems caused by capture of air bubbles by Langmuir vortices (i.e.
Langmuir circulation)”

...

(e.g. Langmuir , 1938 ; Craik & Leibovich , 1976 ; Smith , 1992 , 1998 ; McWilliams et al. ,
1997 ; Melville et al. , 1998 ; Phillips , 2003 , 2005 ; Thorpe , 2004 ; Sullivan & McWilliams ,
2010 ).
Within this classification, Bortkovskii & Kuznetsov ( 1977 ) produced parameterisations
both for the total whitecap coverage W :

0166 U 2 . 25
10

W = 0 . (3.14)

and for the ratio of active whitecapping W A and passive whitecapping W B :

0

. 255 U 20 − 1 . 99 if U 20 > 9m / s,

W B
W A =

(3.15)

0 if U 20 ≤ 9m / s .

Here, W W B , and U 20 is the wind speed at 20m height. Bortkovskii & Kuznetsov

( 1977 ) also provided an interesting observation that
W B
W A ∼

= W A +

u

∗ ,

(3.16)

but concluded that the exact dependence is much more complicated than this.
Bortkovskii ( 1987a ) further extended this kind of observation and proposed a variety
of statistics for active and passive stages of whitecapping. In particular, he connected the
duration t A of the active stage with wind speed:

t A = 0 . 296 U 10 + 0 . 593 (3.17)

and also suggested a connection between wind speed and velocity of propagation of the
breaking fronts. In view of the present understanding of breaking processes, these depen-
dences on the wind are most likely indirect, that is they reflect changes that occurred in
the wave system, such as downshifting of the wave energy under persistent action of the
wind. This way, for higher winds the waves will be longer and faster moving, and even