Geography Reference
In-Depth Information
H
L
I
I
′
250
Negative
vorticity
advection
Positive
vorticity
advection
500
Cold
advection
Warm
advection
750
W
E
A
B
C
Fig. 6.16
Secondary circulation associated with a developing baroclinic wave: (top) schematic
500-hPa contour (solid line), 1000-hPa contours (dashed lines), and surface fronts;
(bottom) vertical profile through line II' indicating the vertical motion field.
differential vorticity advection. For this reason we have neglected boundary layer
friction in developing the equations of the quasi-geostrophic system.
It is also of interest to note that the secondary circulation in a developing baro-
clinic system always acts to oppose the horizontal advection fields. Thus, the
divergent motions tend partly to cancel the vorticity advection, and the adiabatic
temperature changes due to vertical motion tend to cancel partly the thermal advec-
tion. This tendency of the secondary flow to cancel partly the advective changes has
important implications for the flow evolution. These are discussed in Chapter 8.
It should now be clear that a secondary divergent circulation is necessary to
satisfy the twin constraints of geostrophic and hydrostatic balance for a baroclinic
system. Without such a circulation, geostrophic advection tends to destroy the ther-
mal wind balance. The secondary circulation is itself forced, however, by slight
departures from geostrophy. Referring again to Fig. 6.16, we see that in the region
of the upper-level trough (column A), cold advection causes the geopotential height
to fall and thus intensifies the horizontal pressure gradient. The wind, therefore,
