Geography Reference
In-Depth Information
3.2.2
Geostrophic Flow
Flow in a straight line (R
) parallel to height contours is referred to
as geostrophic motion . In geostrophic motion the horizontal components of the
Coriolis force and pressure gradient force are in exact balance so that V
→±∞
=
V g ,
where the geostrophic wind V g is defined by 2
fV g =−
∂/∂n
(3.11)
This balance is indicated schematically in Fig. 3.2. The actual wind can be in
exact geostrophic motion only if the height contours are parallel to latitude circles.
As discussed in Section 2.4.1, the geostrophic wind is generally a good approxi-
mation to the actual wind in extratropical synoptic-scale disturbances. However,
in some of the special cases treated later this is not true.
3.2.3
Inertial Flow
If the geopotential field is uniform on an isobaric surface so that the horizontal
pressure gradient vanishes, (3.10) reduces to a balance between Coriolis force and
centrifugal force:
V 2 /R
+
fV
=
0
(3.12)
Equation (3.12) may be solved for the radius of curvature
R
=−
V/f
Fig. 3.2
Balance of forces for geostrophic equilibrium. The pressure gradient force is designated by
P and the Coriolis force by Co .
2 Note that although the actual speed V must always be positive in the natural coordinates, V g ,
which is proportional to the height gradient normal to the direction of flow, may be negative, as in the
“anomalous” low shown in Fig. 3.5c.
Search WWH ::




Custom Search