Numerical simulations with a non-hydrostatic anelastic model are carried out to
reproduce hydraulic tank experiments on stratified flow past a two-dimensional
mountain ridge, for a Froude number of 0.6 and a Reynolds number of 200. The
gravity wave thus generated steepens, overturns and breaks. Numerical simulations
and experiments are directly compared showing close agreement. Ground friction is
found to have a major influence. It induces a boundary-layer separation on the lee
slope of the mountain and a low-level trapped lee wave inhibiting the downstream
propagation of the breaking region above. Consequently, the three-dimensional
vortices generated within the unstable two-dimensional overturning wave have a toroidal
shape in agreement with experimental observations. Sensitivity to the shape of the
initial three-dimensional perturbation is studied. In the case of harmonic disturbances,
spectral analysis reveals that during the growth phase of the instability, harmonics
are coherently produced by the nonlinear transverse advection term. During the
later phase of quasi-steady turbulence, the vortices have a morphology that does not
depend on the type of the initial perturbation.