Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 365

These are measurements and estimates of the micrometer-scale spray. Knowledge of this
spray is essential in meteorology where such spray can be responsible for condensation,
in practical applications which rely on transparency of the atmosphere. This spray occurs
even in more or less benign wind-wave conditions.
From the point of view of the dynamics of the boundary layer, the spume is more impor-
tant, which occurs at higher winds, and the measurements become much more complicated
in such circumstances (e.g. Anguelova et al. , 1999 ). The obvious retreat for measuring
spume and WBL at extreme winds would be a laboratory wind-wave tank capable of pro-
ducing hurricane-like winds. Such tanks are available and the dedicated experiments have
been conducted.
Donelan et al. ( 2004 ), Fairall et al. ( 2009 ) and Mueller & Veron ( 2009 ) carried out
tests in such tanks, with the centre-line wind speed in the flume varying in the range 0 to
30m s in the second. Obviously,

the full boundary layer is not reproduced in such tanks, with the layer of air above the
waves being of the order of 1m under the tank's lid, but if the wind speed is doubled as
an approximate estimate of the corresponding U 10 value, which produces hurricane-like
winds in both cases, definitely in the range well above U 10

/ s in the first experiment, and being 16 . 7m / s and 17 . 8m /

= 20m / s, spume starts to be

actively generated.
Given the challenge of the problem of direct physical modelling of extreme winds, both
experiments are quite outstanding, but their direct applicability to real hurricanes is in
question. Donelan et al. ( 2004 ), for example, did obtain the saturation of the surface drag
at wind speeds greater than 30m s, as was observed in the field ( Powell et al. , 2003 ; Jarosz

et al. , 2007 , see discussions in Section 9.1.3 ). Kudryavtsev & Makin ( 2007 ), however, (see
also Kukulka et al. , 2007 ) argued that in the laboratory this saturation was due to reasons
other than in the ocean. According to them, at the very short fetches the spray does not
play a major role in the dynamics of WBL, and the observed drag reduction was due to
air-flow separation over continuously breaking crests. This is not the enhancement effect
described in Section 8.3 , but an opposite effect as far as the momentum/energy fluxes are
concerned - because of the small-scale waves, which contribute most to the drag, being
under the separated bubble.
For the spume, similarly to the conclusion of O'Dowd & de Leeuw ( 2007 ) for the film
droplets mentioned above, Fairall et al. ( 2009 ) suggest that u ∗ is not the only scaling
parameter to describe the spray-production strength. According to them, particulars of the
wave breaking can be important, such as whitecap fraction, breaking severity, length of the
breaking crests. Speaking of the ocean conditions, we can add the sea-water temperature
as another factor here (e.g. Exton et al. , 1986 ; Blanchard , 1989 ; Bortkovskii , 1997 ). These
all may be different and bring about different contributions to the spray generation, but
undistinguishable if scaled by the surface stress or u

/

, particularly as the experiment was
only conducted for two wind speeds. As the authors say:

∗

“we do not claim that the spray function we measure in the laboratory is the same as over the
ocean. Rather, we assert that the wind stress interactions with breaking waves in the laboratory are
reasonably similar to oceanic waves.”