Geography Reference
In-Depth Information
Fig. 9.8
Streamlines for trapped lee waves generated by flow over topography with vertical variation
of the Scorer parameter. Shading shows locations where lee wave clouds may form. (After
Durran, 1990.)
be a layer near the surface in which vertically propagating waves are permitted,
which is topped by a layer in which the disturbance decays in the vertical. In that
case vertically propagating waves in the lower layer are reflected when they reach
the upper layer. Under some circumstances the waves may be repeatedly reflected
from the upper layer and the surface downstream of the mountain, leading to a
series of “trapped” lee waves as shown in Fig. 9.8.
Vertical variations in the Scorer parameter can also modify the amplitude of
waves that are sufficiently long to propagate vertically through the entire tro-
posphere. Amplitude enhancement leading to wave breaking and turbulent mix-
ing can occur if there is a
critical level
where the mean flow goes to zero
(l
→∞
).
9.4.3
Downslope Windstorms
Strong downslope surface winds are occasionally observed along the lee slopes
of mountain ranges. Although partial reflection of vertically propagating linear
gravity waves may produce enhanced surface winds under some conditions, it
appears that nonlinear processes are essential to account for observed windstorms
associated with stable flow over topography.
