Non-dissipative effects of breaking on the wave field - Breaking and Dissipation of Ocean Surface Waves - page 334

are essentially enhanced due to the presence of breaking. In this section, we will mostly
follow the paper by Babanin et al. ( 2007b ).
The topic focuses on the long-standing problem of the aerodynamic consequences of
wave breaking on wind-wave coupling and was investigated in the more general context
of the Lake George field experiment ( Young et al. , 2005 , see also Section 3.5 ). In this
experiment, direct measurements of the influence of wave breaking on the wave-induced
pressure in the air flow over water waves, and hence on the energy flux to the waves, were
conducted. As described in Section 3.5 , the forcing covered the range U 10 / 3-7. The

propagation speeds of the dominant waves were limited by the water depth and the waves
were correspondingly steep ( Young & Babanin , 2006b ). These measurements allowed an
assessment of the magnitude of any breaking-induced enhancement operative for these
field conditions and provided a basis for parameterising the effect.
Overall, appreciable levels of wave breaking occurred for the strong wind-forcing con-
ditions prevailing during the observational period. Associated with these breaking wave
events, a significant phase shift in the local wave-coherent surface pressure was observed.
This produced an enhanced wave-coherent energy flux from the wind to the waves, with a
mean value of two times the corresponding energy flux to the non-breaking waves. Thus,
it was proposed that the breaking-induced enhancement of the wind input to the waves
can be parameterised by the sum of the non-breaking input and the contribution due to the
breaking probability.
This aerodynamic effect had been investigated before on the basis of laboratory meas-
urements (e.g. Banner & Melville , 1976 ; Reul , 1998 ; Giovanangeli et al. , 1999 ; Banner ,
1990 ) and numerical simulations ( Maat & Makin , 1992 ; Kudryavtsev & Makin , 2001 ;
Makin & Kudryavtsev , 2002 ), but detection and quantifying the wind-input enhancement
in field conditions was first reported following the Lake George observations ( Young &
Babanin , 2001 ; Babanin & Young , 2006 ; Babanin et al. , 2007b ). It was expected that local
air-flowseparation accompanieswave breaking, and causes a phase shift of thewave-induced
pressure, and that this significant modification to the near-surface aerodynamics can result
in enhanced wave-coherent momentum and energy fluxes from the wind to the waves.
Instrumentation and measurement techniques for such a fine phenomenon is quite a
complicated issue which was described in great detail in a special paper by Donelan et al.
( 2005 ), and will not be repeated here. In short, in order to measure microscale oscilla-
tions of induced pressure above surface waves, a high precision wave-follower system was
developed at the University of Miami, Florida. The principal sensing hardware included
Elliott pressure probes, hot film anemometers and Pitot tubes. The wave-follower record-
ings were supplemented by a complete set of relevant measurements in the atmospheric
boundary layer, on the surface and in the water body. The precision of the feedback wave-
following mechanism did not impose any restrictions on the measurement accuracy in the
range of wave heights and frequencies relevant to the problem. Thorough calibrations of
the pressure transducers and moving Elliott probes were conducted. It was shown that the
response of the air column in the connecting tubes provides a frequency-dependent phase
shift, which was then accounted for to recover the low-level induced pressure signal.

c p =