Laboratory experiments were conducted to study the interaction
between two
downward propagating internal wave rays with identical properties but opposite
horizontal phase velocities. The intersection of the rays produced a velocity
field with
stagnation points, and these points propagated vertically upwards within
the
intersection region. Nonlinear non-resonant interactions between the two
rays
produced evanescent modes, with frequencies greater than the ambient buoyancy
frequency, trapped within the intersection region. These evanescent modes
provided a
mechanism whereby energy could accumulate locally and, even though the
vertical
wavelength of the primary resultant wave remained the same, the local isopycnal
displacements increased in time. Eventually, the isopycnals were forced
to
overturn in the region just above the stagnation points by the variation
with
depth in the local horizontal strain rate.
The gravitationally unstable overturning ultimately broke down releasing
its
available potential energy and generating turbulence within the intersection
region.
The results showed that the release of available potential energy was disrupted
by
the wave motions and even the dissipative scales were directly affected
by the
ambient stratification and the background wave motion. The distribution
of the
centred displacement scales was highly skewed towards the Kolmogorov scale
and the
turbulent Reynolds number Ret was low. Thus,
the net
buoyancy flux was very small
and almost all turbulent kinetic energy was dissipated over the parameter
range
investigated. The results also showed that for such dissipative events
the
square of the strain Froude number
(ε/νN20)
and the turbulent Reynolds number Ret can
be less than
one.