Geography Reference
In-Depth Information
Fig. 9.7
Streamlines of flow over a broad isolated ridge showing upstream phase tilt with height. The
pattern is periodic in height and one vertical wavelength is shown. Orographic clouds may
form in the shaded areas where streamlines are displaced upward from equilibrium either
upstream or downstream of the ridge if sufficient moisture is present. (After Durran, 1990.)
Vertically propagating gravity waves generated by flow over broad topography
can produce clouds both upstream and downstream of the topography depending
on variations of the moisture distribution with altitude. In the example shown in
Fig. 9.7 the positions labeled A and B indicate regions where streamlines are dis-
placed upward downstream and upstream of the ridge, respectively. If sufficient
moisture is present, orographic clouds may then form in region A or B as suggested
by the shading in Fig. 9.7.
9.4.2
Lee Waves
If
u and N are allowed to vary in height, then (7.46) must be replaced by
∂
2
w
∂x
2
¯
∂
2
w
∂z
2
l
2
w
=
+
+
0
(9.31)
where the
Scorer parameter
, l, is defined as
N
2
u
2
u
−
1
d
2
u
dz
2
l
2
=
−
and the condition for vertical propagation becomes k
s
<l
2
.
If the mean cross mountain wind speed increases strongly with height, or there
is a low-level stable layer so that N decreases strongly with height, there may
