| Breaking wind waves play an important role for the majority of upper-ocean processes.
Detailed description of physics related to breaking waves is necessary for the correct
description of heat, mass, air exchange, wind wave dissipation, sea spray generation etc.
Breaking waves and whitecap foam are also important source of information for the ocean
remote sensing methods.
”Lambda distribution” (Î(c)) concept introduced by Phillips (1985) provides a way to
link wind wave spectra and various important measures of the air-sea interface
processes related to wave breaking. Radar backscattering due to wave breaking,
turbulent mixing in the uppermost sea layer, production of sea drops in the near water
atmospheric layer and other phenomena were recently studied on the basis of this
approach.
However, results of field measurements of visible breaking crests appeared to be different
from the form predicted by Phillips (1985). Most of the experimental Î(c) has clear
maximum (phase velocities c - 1.5 - 2 m) and decay with phase velocity at the short
scale spectral bandwidth. The descending c-6 shape only observed at the large
scales.
Current work represents an attempt to describe experimental whitecap distributions in
terms of Phillips’s (1985) equilibrium spectrum approach. Significant part of small scale
breaking events do not produce visible foam patch and can not be detected with traditional
video observation methods. At this work modified Î(c) function proposed which
describes only visible breaking events and can be directly compared with available field
observations.
All theoretical calculations were compared with empirical Î(c) distributions obtained
during experimental campaigns on the Black Sea research platform of Marine Hydrophysical
Institute (National Academy of Sciences of Ukraine). Simultaneous whitecap video
observations and wave 2D-spectra measurements were obtained under variety of wave and
wind conditions.
Breaking crests distributions and total breaking crests length values calculated from the
spectrum of wind waves are in a good quantitative agreement with experimental data. Phase
velocity of breaking waves of the maximum of Î(c) distributions is proportional to friction
velocity. This analytical result fully confirmed by present experimental data and supported by
previously published experimental works. |