Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 352

to breaking is a very large topic, and it will be briefly reviewed in Section 9.1.2 .The
last subsection, 9.1.3 , will indicate changes to the roles of breaking in extreme wind-
forcing situations.

9.1.1 Sea-drag dependence on wave breaking
Coupling between the atmospheric boundary layer and the ocean surface is often param-
eterised in terms of the drag coefficient C D (3.8) ,see Section 3.1 . The very definition of
C D relies on the idea of a constant-flux layer outlined above. This concept has proved
very consistent in general fluid mechanics when constant-speed flows over solid walls are
considered. In the case of ocean waves, evolving simultaneously at multiple time scales
from very long and continuous (slow growth due to wind input and nonlinear interac-
tions) through to very short and intermittent (wave breaking), with their very complex
physics and multiple mechanism for imparting feedback on the atmospheric flow, devi-
ations from the assumed simple friction forcing can be expected, particularly as the winds
are ever changing and gusty too, with a continuous spectrum of temporal and spatial
inhomogeneities which may disrupt the very concept of the layer with a constant flux of
momentum. Besides, at low wind speeds, the height of this layer can be less than 10m, and
(3.8) would not be valid. When applicable, however, knowledge of C D enables a simple
determination of the wind stress or the flux of momentum from the wind to the waves

τ
(3.7) ,if U 10 is specified. In many models of the air-sea interaction, in fact, and particularly
in large-scale models such as climate models, the sea-drag coefficient is the only property
which defines this momentum exchange between the atmosphere and the ocean. Here, we
will follow Babanin & Makin ( 2008 ) to introduce this property in a broader context of the
air-sea-interaction properties.
Accurate evaluation of C D has proven to be a major challenge experimentally, since it
requires precise field measurements of fine turbulent fluctuations in the atmospheric bound-
ary layer close to the wavy surface. The available field data have resulted in a number of
quite different parameterisations. Guan & Xie ( 2004 ), for example, counted as many as
25 of them produced over a period of 50 years. Routinely, C D is parameterised as a func-
tion of mean wind speed U 10 , but the scatter of experimental data around such parametric
dependences is very significant and has not improved noticeably over some 40 years, in
spite of all the advances in the instrumentation, measurement techniques and data-logging
systems. This scatter imposes a serious limitation on forecasts and predictions that make
use of the sea-surface-drag parameterisations.
Also, since measuring in extreme wind and wave conditions is logistically particularly
difficult, the majority of the data has been obtained during light to moderate winds. This,
in addition to the scatter, further limits the applicability of the available parameterisations
because their extrapolation into extreme conditions is questionable. Different physics is
expected to drive the air-sea interaction in very strong wind-forcing conditions as has
been shown in an escalating series of recent studies (see Section 9.1.3 ). This limitation is
particularly important for modelling extreme events, such as tropical cyclones, and also