Geoscience Reference
In-Depth Information
surrounding air, and as a result the spray absorbs some momentum of the existing air flow.
The spume also creates density stratification and damps the near-surface turbulence. In
these and other ways, the suspended droplets may alter the behaviour of the very bottom
of the boundary layer, where the interactions as such occur, in a significant manner. Such
effects can potentially modify the air-sea interactions beyond recognition, if the amount
of suspended water matter is essential, as it is under extreme wind-forcing/wave-breaking
conditions such as bora winds or hurricanes. Measurements of spume in these conditions is
particularly difficult and a new experimental technique able to handle such in situ estimates
is discussed.
The important role of the spray has been the topic of an escalating number of papers
published lately. The main models of the spray influence and spray production are dis-
cussed or outlined in the section. Spray classification is presented, and both its mechanic
and thermodynamic capacities are highlighted. In this regard, models of the spray in the
atmospheric boundary layer can be related to a gross number of applications, ranging from
rocket combustion engines, with fuel droplets suspended in a turbulent gas, through dust
storms, where the two-phase medium consists of the air and solid particles, to tropical
cyclones where the sea droplets can serve as a source of sensible heat.
Section 9.1.3 is dedicated to the change in the dynamic regime of air-sea interactions at
hurricane-like wind forcing. The role of the breaking is most important here as the majority
of models relate this regime change to the breaking directly or indirectly.
Saturation of the sea drag at high wind speeds was foreshadowed by large-scale mod-
ellers of air-sea interaction a while ago, but only over the last decade was it actually
measured. This experimental finding stimulated a boom in theoretical models striving to
reproduce and explain the effect. In Section 9.1.3 the models are briefly reviewed by being
subdivided into two broad classes: those due to the spray effects and due to changes in the
ocean-surface aerodynamics.
The spume models can be conditionally further subdivided into four types. These are
those which rely on the effect of damping the near-surface atmospheric turbulence due
to suspended spume, on the momentum-balance effect of the spume acceleration, on the
rain-like effect of smoothing the short waves by the droplets falling back down on the
wavy surface, and on the thermodynamic effects of wave-induced-pressure reduction by
suspended droplets.
The aerodynamic effects are basically two: full-flow separation and smoothing the short
waves. The latter, apart from the rain-like effect of spume mentioned above, can be achieved
directly by a very strong wind without flow separation. The separation, at the same time,
can be caused in a variety of ways, that is due to centrifugal acceleration of strong wind
cornering the steep crests, due to wave breaking, and due to Kelvin-Helmholtz instability
of capillary waves leading to their microbreaking.
The wave breaking itself in extreme conditions also experiences a number of unusual
behaviours. Direct observations and accounts of such breaking are rare, but multiple mod-
els point to a number of peculiarities of this kind of breaking which distinguish it from
the breaking in benign environmental situations. These include the full flow separation,
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