thesis
Mountain wave breaking in atmospheric flows with directional wind shear
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Abstract
In this thesis, mountain wave breaking triggered by directional wind shear is investigated
using numerical simulations of idealized and semi-idealized orographic flows.
Idealized simulations are used to produce a regime diagram to diagnose conditions
for wave breaking in Richardson number-dimensionless mountain height parameter
space. It is found that, in the presence of directional shear, wave breaking can
occur over lower mountains than in a constant-wind case. Furthermore, the extent of
regions within the simulation domain where Clear-Air Turbulence (CAT) is expected
increases with terrain elevation and background wind shear intensity.
Analysis of the model output, supported by theoretical arguments, suggest the existence
of a link between wave breaking and the relative orientations of the incoming
wind vector and the horizontal velocity perturbation vector. This condition provides
a possible diagnostic for CAT forecast in directional shear flows.
The stability of the flow to wave breaking in the transition from hydrostatic to nonhydrostatic
mountain waves is also investigated. Wave breaking seems to be inhibited
by non-hydrostatic effects for weak wind shear, but enhanced for stronger wind shear.
In the second part of the thesis, a turbulence encounter observed over the Rocky
Mountains (in Colorado, USA) is studied. The role of directional shear in causing
wave breaking is isolated from other possible wave breaking mechanisms through
various sensitivity tests. The existence of an asymptotic wake, as predicted by Shutts
for directional shear flows, is hypothesized to be responsible for a significant downwind
transport of unstable air detected in cross-sections of the flow.
Finally, critical levels induced by directional shear are studied by spectral analysis of
the horizontal velocity wave perturbations. This is done for a fully idealized flow and
for the more realistic flow corresponding to the investigated turbulence encounter.
In these 2D power spectra, a rotation of the most energetic wave modes with the
background wind and their selective absorption can be found. Such behaviour is
consistent with the mechanism leading to wave breaking in directional shear flows