Geography Reference
In-Depth Information
H
L
+
-
300 hPa
10
20
30
40
u 300
H
L
+
-
850 hPa
0
10
20
u 850
Fig. 6.15
Isallobaric and advective contributions to the ageostrophic wind for baroclinic waves in
westerlies. Solid ellipses indicate perturbation geopotential patterns at 300 and 850 hPa.
Dashed ellipses show a geopotential tendency pattern with positive and negative tendencies
indicated by + and - signs, respectively. The mean zonal flow distribution in which the waves
are embedded is indicated on the right for each level. Solid arrows show the isallobaric part
of the ageostrophic wind, and open arrows show the advective part. (Adapted from Lim et
al., 1991.)
The forcing of the ageostrophic wind can be divided conveniently into the two
terms on the right in (6.56). The first term is referred to as the isallobaric wind. It
can be shown easily to be proportional to the gradient of the geopotential tendency
(see Problem 13) and is directed down the gradient of the tendency field. This
contribution to V a is shown schematically for a baroclinic wave disturbance by
the black arrows in Fig. 6.15. The second term on the right in (6.54) may be called
 
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