Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 363

available for wind speeds in excess of 28m s. In this regard, the new laboratory study

by Haus et al. ( 2010 ) should be mentioned, which extends the measurement range of this
property up to 38m

/

s wind speeds, scaled as environmental winds.
In this section, we obviously do not intend to review the full topic, that is the physics of
spray, its measurements, theories and applications. Rather, we will just outline it and direct
the reader to some references where he/she can find more details and further references
from this research field.
Spray must be mentioned in a topic dedicated to the effects of wave breaking, of course,
as most of its existence is either due to wave breaking or is part of the wave breaking
as such. Like almost anything related to the breaking, when details are looked into, a
phenomenon splits into further subgroups and subphenomena, where different physical
arguments and different approaches apply, and with different relative importance. In this
regard, spray produced by breaking waves can be broadly classified into three groups (e.g.
Andreas , 1998 ; O'Dowd & de Leeuw , 2007 ; Rastigejev et al. , 2011 ; Soloviev & Lukas ,
2010 ); but note the fourth mechanism of droplet generation as a result of modulational
instability of capillary waves (Shats et al. , 2010).
The first group (smallest size) and the last group (largest size) of droplets are rou-
tinely produced in benign wave conditions by the breakers. The tiny submicrometre- to
several-μm-diameter droplets result from bursting of bubbles in regular whitecaps. These
are sometimes called film droplets, but they can be further subdivided into two groups,
one of which (the submicrometer sub-group) is indeed due to instability of the bubble-
produced films, and the other (the supermicrometer sub-group) is caused by the jet formed
in the centre of the bursting bubble.
At the other end of the scale are a few orders of magnitude larger droplets from plunging
jets and spilling splashes. Neither of the two groups play an essential part in the momentum
exchange and surface stresses. The first kind of droplets are too small, and the second kind
are too few to affect the dynamics of the lower boundary layer and the interface. The film
spray, however, is very important in producing aerosols (see e.g. Monahan et al. , 1986 ),
with all the meteorological, applied and other consequences attached.
Their absolute-mass production may be increasing in high-wind conditions, but then their
relative amount gives way to the second type of spray, the spume ( Koga , 1981 ). These are
particles of water resulting from direct tearing of wave crests by the wind. Their size is from
tens to hundreds of μm, and in strong wind they fill the air within the lower WBL, fromwave
troughs up to a vertical distance above the wave crests of the order of wave height.
Quantitative account of this phenomenon, however, is not well defined. While there
seems to be a general agreement that this effect is small in moderate winds up to some
20m

/

s (e.g. Monahan , 1966 ; Wu , 1973 ), estimates for high-wind conditions differ. Pielk &
Lee ( 1991 ), for example, found consequences of the spray significant in winds of the order
of 40m

/

s, whereas Fairall et al. ( 1994 ) concluded that it is negligible (i.e. of the order
of 10%) up to 50m

/

s winds. In a more recent model, Andreas ( 2004 ) suggests, on the
contrary, that the same 10% of the spray's contribution to the total stress is achieved for a
30m

/

/ s wind, and at 60m / s it supports all of the stress.