Geography Reference
In-Depth Information
Since from (3.9), the speed must be constant in this case, the radius of curvature
is also constant (neglecting the latitudinal dependence of f). Thus the air parcels
follow circular paths in an anticyclonic sense.
3
The period of this oscillation is
=
1
2πR
V
2π
|
2
day
P
=
|
=
(3.13)
f
|
sin φ
|
P
is equivalent to the time that is required for a Foucault pendulum to turn through
an angle of 180
◦
. Hence, it is often referred to as one-half
pendulum day
.
Because both the Coriolis force and the centrifugal force due to the relative
motion are caused by inertia of the fluid, this type of motion is traditionally referred
to as an
inertial oscillation
, and the circle of radius
is called the inertia circle.
It is important to realize that the “inertial flow” governed by (3.12) is not the same
as inertial motion in an absolute reference frame. The flow governed by (3.12)
is just the constant angular momentum oscillation referred to in Section 1.5.4. In
this flow the force of gravity, acting orthogonal to the plane of motion, keeps the
oscillation on a horizontal surface. In true inertial motion, all forces vanish and
the motion maintains a uniform absolute velocity.
In the atmosphere, motions are nearly always generated and maintained by
pressure gradient forces; the conditions of uniform pressure required for pure
inertial flow rarely exist. In the oceans, however, currents are often generated
by transient winds blowing across the surface, rather than by internal pressure
gradients. As a result, significant amounts of energy occur in currents that oscillate
with near inertial periods. An example recorded by a current meter near the island
of Barbados is shown in Fig. 3.3.
|
R
|
3.2.4
Cyclostrophic Flow
If the horizontal scale of a disturbance is small enough, the Coriolis force may
be neglected in (3.10) compared to the pressure gradient force and the centrifugal
force. The force balance normal to the direction of flow is then
V
2
R
=−
∂
∂n
If this equation is solved for V , we obtain the speed of the
cyclostrophic wind
1/2
R
∂
∂n
V
=
−
(3.14)
3
Anticyclonic flow is a clockwise rotation in the Northern Hemisphere and counterclockwise in the
Southern Hemisphere. Cyclonic flow has the opposite sense of rotation in each hemisphere.
