Geoscience Reference
In-Depth Information
(A)
Low pressure
(B)
High pressure
LOW
HIGH
CORIOLIS
FORCE
PRESSURE
GRADIENT
DIRECTION OF
GRADIENT WIND
DIRECTION OF
GRADIENT WIND
DIRECTION OF
CENTRIPETAL
ACCELERATION
DIRECTION OF
CENTRIPETAL
ACCELERATION
PRESSURE
GRADIENT
CORIOLIS
FORCE
Figure 6.4
The gradient wind case of balanced motion around low pressure (A) and high pressure (B) in
the Northern Hemisphere.
exceeds the pressure force. Since the pressure
gradients are assumed to be equal, the different
contributions of the Coriolis force in each case
imply that the wind speed around the low pressure
must be lower than the geostrophic value (
sub-
geostrophic
), 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 anti-
cyclonic flow. The maximum occurs when the
angular velocity is
f
/2 (= V sin
useful, but in reality two factors prevent a contin-
uous 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 (subgeostrophic
wind) to one of anticyclonic curvature (super-
geostrophic wind) causes a fall of pressure at lower
levels in the atmosphere to compensate for the
removal of air aloft. The significance of this fact
will be discussed in Chapter 9G. The interaction
of horizontal and vertical air motions is outlined
in B.2, (this chapter).
In cases where the curvature of the flow is tight,
as near the eye of a tropical cyclone (see Chapter
11B.2), the centripetal acceleration may balance
the pressure gradient force; the resulting wind is
termed cyclostrophic.
), 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 condi-
tions of balanced flow. This simplification is
φ
5 Frictional forces and the
planetary boundary layer
The last force that has an important effect on air
movement is that due to friction from the earth's
surface. Towards the surface (i.e., below about
500m for flat terrain), friction due to form drag
