Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 360

sea drag, then, depending on the frequency of breaking occurrences and their strength, this
aspect of the breaking can also prove important for the C D dependences and their scatter.
The first feature would lead to enhancement of the drag, the last one should result in its
reduction, and the whitecap coverage can potentially go either way, and these ambiguous
roles of the breaking only add to the complexity and the importance of the topic.
Thus, it has to be recognised that a new approach to the problem of sea drag is needed.
C D does increase, on average, once the wind goes up, but it is not a simple function of
mean wind speed and, therefore, attempts to parameterise it in terms of U 10 or sea state
U 10 /

c p are bound to have a great scatter, no matter how extensive and how precise are the
measurements of wind and waves.
Babanin & Makin ( 2008 ) suggest a complex approach to tackle the problem. In short, it
is based on recognition of the complex nature of air-sea interaction at small scales, where
multiple mechanisms affect and alter the sea drag simultaneously, sometimes in opposite
directions. Wave breaking is one of such mechanisms, and the breaking alone can cause
both increase and decrease of the drag depending on the relative importance of its multiple
consequences.
Seventeen features are listed above which can influence the drag, and this list is not
exhaustive. Babanin & Makin ( 2008 ) demonstrated that most significant exaggeration of
the drag occurs due to wind trends and, particularly, due to wind gustiness. The breaking
in Figure 9.3 also has a very high correlation with C D , as has the directional spread of the
wave spectra (Ting et al. , 2010). We believe that this complexity is an underlying reason
for the observed large scatter of C D data. Adopting this perspective, we should eventually
be able to significantly reduce the scatter of C D parameterisations. The contributing mech-
anisms, including the breaking, need to be singled out, studied separately, evaluated and
then reunited in a joint parameterisation for the sea drag. An analytical approach should be
applied to the mechanisms, wherever it is possible. This will enhance our understanding
of their physics to create a complete picture of the complex phenomenon and to produce a
general parameterisation.

9.1.2 Generation of spray

Sea spray is a large topic in its own right, and even at the present stage can easily constitute
a subject for a separate topic. And the present state is still characterised by the lack of
experimental observations and knowledge, and by a selection of theories and numerical
simulations, largely based on semi-empirical approaches, which, at the very least, are not
necessarily compatible, and at the other end are even contradictory.
In the meantime, out of all topics in the field of air-sea interactions, this was one of
the very first attended to both by experimentalists and theoreticians. Here, we will quote
Andreas ( 2004 ) on the most significant role of sea spray in the dynamics of extreme wind-
wave conditions and on the origins of its research:

“When sea spray droplets are thrown into the air, they accelerate almost immediately to the local
wind speed. This process extracts momentum from the near-surface wind and therefore slows it.