Abstract
Atmospheric bores are disturbances whose passage is accompanied by a
pressure rise and a semi-permanent upward displacement of the isentropic
surfaces. A series of waves often trails behind the bore’s leading edge,
and in contrast to density currents, the near-surface temperature
remains relatively unchanged, or even warms, after the bore passes. One
of the most spectacular and well-studied examples of an atmospheric bore
is the “Morning Glory”, which occurs in the Gulf of Carpentaria region
of northeastern Australia. Atmospheric bores also occur frequently in
the nocturnal environment over the Great Plains of the United States,
where they are often initiated by gust fronts and density currents in
thunderstorm outflows. In favorable conditions, these nocturnal bores
can propagate hundreds of kilometers and trigger new convection through
low-level lifting that can grow upscale into large organized convective
systems. The dynamics of a prototypical atmospheric bore are
investigated through a series of two-dimensional numerical simulations
and linear theory. These simulations demonstrate that the bore dynamics
are inherently finite amplitude. Although the environment supports
linear trapped waves, the supported waves propagate in roughly the
opposite direction to that of the bore. Qualitative analysis of the
Scorer parameter can therefore give misleading indications of the
potential for wave trapping, and linear internal gravity wave dynamics
do not govern the behavior of the bore. The presence of a layer of
enhanced static stability below a deep layer of lower stability, as
would be created by a nocturnal inversion, was not necessary for the
development of a bore. The key environmental factor allowing bore
propagation was the presence of a low-level jet directed opposite to the
movement of the bore. Significant turbulence developed in the layer
between the jet maximum and the surface, which reduced the low-level
static stability behind the bore. Given the essential role of jets and
thereby strong environmental wind shear, and given that idealized bores
may persist in environments in which the static stability is constant
with height, shallow-water dynamics do not appear to be quantitatively
applicable to atmospheric bores propagating against low-level jets,
although there are qualitative analogies.