Geography Reference
In-Depth Information
Fig. 4.10
As in Fig. 4.9, but for easterly flow.
curvature and the column will be deflected poleward. When the parcel returns to its
original latitude, it will still have a poleward velocity component and will continue
poleward gradually, acquiring anticyclonic curvature until its direction is again
reversed. The parcel will then move downstream, conserving potential vorticity by
following a wave-like trajectory in the horizontal plane. Therefore, steady westerly
flow over a large-scale ridge will result in a cyclonic flow pattern immediately to
the east of the barrier (the lee side trough) followed by an alternating series of
ridges and troughs downstream.
The situation for easterly flow impinging on a mountain barrier is quite different.
As indicated schematically in Fig. 4.10b, upstream stretching leads to a cyclonic
turning of the flow, as in the westerly case. For easterly flow this cyclonic turning
creates an equatorward component of motion. As the column moves westward
and equatorward over the barrier, its depth contracts and its absolute vorticity
must then decrease so that potential vorticity can be conserved. This reduction in
absolute vorticity arises both from development of anticyclonic relative vorticity
and from a decrease in f due to the equatorward motion. The anticyclonic relative
vorticity gradually turns the column so that when it reaches the top of the barrier
it is headed westward. As it continues westward down the barrier, conserving
potential vorticity, the process is simply reversed with the result that some distance
downstream from the mountain barrier the air column again is moving westward
at its original latitude. Thus, the dependence of the Coriolis parameter on latitude
creates a dramatic difference between westerly and easterly flow over large-scale
topographic barriers. In the case of a westerly wind, the barrier generates a wave-
like disturbance in the streamlines that extends far downstream. However, in the
