East Asia and Western Pacific METEOROLOGY AND CLIMATE

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— -~ 1, thus the above relation becomes
IV
Sf ^
a = - » T . (41)
Then the vertical velocity at the top fo the boundary layer in 2
coordinate reads
< 42)
III order to appeciate the orographic contribution, the influence of
geostrophic momentum can be neglected temporarily. In this case
CZ* *^f
D = f, C f = Yj, t z -- -7y y^ . (43)
And the vertical velocity is expresed as follows
1 = -av 4 .Vlnp -* - (K • VAVlnp-) + - L - (44)
T
4 2 2 ^
These three tens on the right Jiand side of eq. (44) denote three
different physical processes* First, the orographic effect, ~ay*vlnp".
If the surface is flat then vp^ =0, and this ter» vanishes. Second,
frictional effect, £ . This means that in a tow pressure
2 3
system, £ > Q, and this term makes positive contribution. The otherwise
is true in a high pressure system. Since the flow is always deflect
to the low pressure side when the infuence of friction is taken into
consideration. Thus it results in convergence and vertical motion at
the top of the boundary layer in a region of low pressure as the
continuity principle requires. Similar argumemts apply to high pressure
systems. This was discussed by Charney and Eliassen in 1949. The third,
. _
is the joint effect of orography and friction, (K * VAvlnp ).
2 *
If the surface is flat or the motion is fric-tionless, then .lftp^= 0
or K= 0 respectively. In this case, the third term vanishes. It is
non-zero only if both effects coexist. The physical explanation of
this term is that under the influence of friction, even if the
geostrophic wind is parallel to the configuration of orography, the
ageostrophic component will force the air upward or downward. These
three factors can be visualised in the following schematic
illustrations.