Geoscience Reference
In-Depth Information
of the Coriolis force in each case imply that the wind
speed around the low pressure must be lower than
the geostrophic value (
subgeostrophic
), whereas in the
case of high pressure it is
supergeostrophic
. In reality,
this effect is obscured by the fact that the pressure
gradient in a high is usually much weaker than in a low.
Moreover, the fact that the earth's rotation is cyclonic
imposes a limit on the speed of anticyclonic flow. The
maximum occurs when the angular velocity is
f
/2 (= V
sin ϕ), at which value the absolute rotation of the air
(viewed from space) is just cyclonic. Beyond this point
anticyclonic flow breaks down ('dynamic instability').
There is no maximum speed in the case of cyclonic
rotation.
The magnitude of the centripetal acceleration is
generally small, but it becomes important where high-
velocity winds are moving in very curved paths (i.e.
around an intense low-pressure vortex). Two cases are
of meteorological significance: first, in intense cyclones
near the equator, where the Coriolis force is negligible;
and, second, in a narrow vortex such as a tornado. Under
these conditions, when the large pressure-gradient force
provides the necessary centripetal acceleration for
balanced flow parallel to the isobars, the motion is called
cyclostrophic
.
The above arguments assume steady conditions
of balanced flow. This simplification is useful, but in
reality two factors prevent a continuous state of balance.
Latitudinal motion changes the Coriolis parameter,
and the movement or changing intensity of a pressure
system leads to acceleration or deceleration of the air,
causing some degree of cross-isobaric flow. Pressure
change itself depends on air displacement through the
breakdown of the balanced state. If air movement were
purely geostrophic there would be no growth or decay
of pressure systems. The acceleration of air at upper
levels from a region of cyclonic isobaric curvature (sub-
geostrophic wind) to one of anticyclonic curvature
(supergeostrophic wind) causes a fall of pressure at
Figure 6.3
(A) The geostrophic wind case of balanced motion
(northern hemisphere) above the friction layer. (B) Surface wind
V represents a balance between the geostrophic wind, V
g
, and
the resultant of the Coriolis force (C) and the friction force (F).
Note that F is not generally directly opposite to the surface wind.
about a local axis of high or low pressure. Here the
curved path of the air (parallel to the isobars) is main-
tained by an inward-acting, or centripetal, acceleration.
Figure 6.4 shows (for the northern hemisphere) that
in a low-pressure system balanced flow is maintained in
a curved path (referred to as the
gradient wind
) by the
Coriolis force being weaker than the pressure force. The
difference between the two gives the net centripetal
acceleration inward. In the high-pressure case, the
inward acceleration exists because the Coriolis force
exceeds the pressure force. Since the pressure gradients
are assumed to be equal, the different contributions
Figure 6.4
The gradient wind case of
balanced motion around a low pres-
sure (A) and a high pressure (B) in the
northern hemisphere.
