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z
h
0
S
h
0
3
2
1
Figure 7.24
The highly idealized density profile used by Dewan and Picard (1998). [After
Dewan and Picard (1998). Reproduced with permission of the American Geophysical
Union.]
can and do occur. The channels in these cases are hypothesized to be layers of
stable air surrounded immediately above and below by less stable layers. Layers
of temperature inversion could provide for such a situation. In explaining the
Aloha front, they believe there must have existed a horizontal waveguiding chan-
nel between the two layers (say, around 90-94 km) in which a bore propagated.
As the bore theoretically passed overhead, it produced symmetrical oscillations
about the center of the channel. The upper layer (5577) was pushed higher, mak-
ing it cooler, less dense, and presumably less bright. The lower layer (OH) was
pushed symmetrically lower, making it warmer, more dense, and brighter.
Dewan and Picard use the very simplified density profile depicted in Fig. 7.24,
which they claim is the simplest guiding structure that can support an internal
bore. The
ρ
1
and
ρ
3
layers are assumed to be semi-infinite in extent. The
ρ
3
layer is of finite thickness 2
h
0
. They argue that any oscillation on the
ρ
3
−
ρ
2
surface will be mirrored in the
ρ
3
−
ρ
1
surface about the plane of symmetry
S
,
where there is no vertical motion. Thus,
S
acts as if it were the rigid bottom of an
open channel bore. Dewan and Picard state that the equations governing open
channel bores will hold for this simple model if the acceleration due to gravity,
g
, is replaced by the buoyant acceleration
g
defined as
g
=
ρ
2
−
ρ
3
ρ
2
g
Figure 7.24 is equivalent to a Brunt-Väisälä profile with a step.
7.9.3 Nonlinear Simulation of Mesospheric Bores
Seyler's (2005) two-dimensional nonlinear model for the mesosphere depends on
the following parameters:
N
0
,
N
1
,
h
,
L
x
,
L
z
, and
A
.
N
1
<
N
0
are Brunt-Väisälä
frequencies associated with inversion layers guiding the resulting waves;
h
is the
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