Geography Reference
In-Depth Information
3
S 3
UP
2
S 2
DOWN
1
S 1
S 1
S 2
S 3
latitude
Fig. 12.5
Parcel motions for an adiabatic planetary wave in a westerly zonal flow. (a) Solid lines
labeled S 1 ,S 2 ,S 3 , are parcel trajectories, heavy dashed lines are latitude circles, and light
dashed lines are contours of vertical velocity field. (b) Projection of parcel oscillations on
the meridional plane.
along the same isentropic surface with trajectories whose meridional projections
are closed ellipses as shown in Fig. 12.5b. In such a situation there is no net vertical
parcel motion, and thus no net vertical transport.
Only if there is net diabatic heating or cooling can there be mean transport
across the isentropes. In the winter stratosphere, where stationary planetary wave
amplitudes tend to be large and there is diabatic heating at low latitudes and diabatic
cooling at high latitudes, the meridional projection of actual parcel motions is
a combination of the elliptical orbits of Fig. 12.5b and mean vertical drift as
indicated in Fig. 12.6, which clearly shows that a vertically stratified long-lived
trace constituent is transported upward at low latitudes and downward at high
latitudes due to vertical transport of fluid parcels associated with diabatic heating
and cooling. Thus, in order to represent net transport effects efficiently, a zonal
averaging process should yield an average vertical circulation that mimics the net
cross isentropic parcel motion.
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