  We present an investigation of the fundamental physical processes involved in
deep water wave breaking. Our motivation is to identify the underlying reason
causing the deficiency of the eddy viscosity breaking model (EVBM) in
predicting surface elevation for strongly nonlinear waves. Owing to the
limitation of experimental methods in the provision of high-resolution flow
information, we propose a numerical methodology by developing an EVBM enclosed
standalone fully-nonlinear quasi-potential (FNP) flow model and a coupled FNP
plus Navier-Stokes flow model. The numerical models were firstly verified with
a wave train subject to modulational instability, then used to simulate a
series of broad-banded focusing wave trains under non-, moderate- and
strong-breaking conditions. A systematic analysis was carried out to
investigate the discrepancies of numerical solutions produced by the two models
in surface elevation and other important physical properties. It is found that
EVBM predicts accurately the energy dissipated by breaking and the amplitude
spectrum of free waves in terms of magnitude, but fails to capture accurately
breaking induced phase shifting. The shift of phase grows with breaking
intensity and is especially strong for high wavenumber components. This is
identified as a cause of the upshift of wave dispersion relation, which
increases the frequencies of large wavenumber components. Such a variation
drives large-wavenumber components to propagate at nearly the same speed, which
is significantly higher than the linear dispersion levels. This suppresses the
instant dispersive spreading of harmonics after the focal point, prolonging the
lifespan of focused waves and expanding their propagation space.
