Geoscience Reference
In-Depth Information
Although we have considered only two of the forces
acting upon the air parcel, the geostrophic wind is
nevertheless a useful approximation. Strictly, it operates
only when the isobars are straight - a rare event. Normally
isobars are curved and winds are subject to another force
termed
centripetal acceleration
which acts towards the
centre of rotation. When this rotational component is
included, the resultant wind is called the
gradient wind
,
which is closer to observed flow in the upper atmosphere
(
Figure 6.9b
)
.
Convergence and divergence can also be found as a result
atmospheric flows (see
Figure 6.17
).
If convergence or
divergence is maintained for any time, a transfer of mass
of air will result and the original pressure gradient will be
changed. Convergence will produce an accumulation of
air, increase surface pressure and so decrease the pressure
gradient and hence the convergence which produced
the original air flow. The system will stop. To maintain
surface convergence (or divergence), vertical movement
is required. In general, if air is converging at the surface,
it must rise, while if it is diverging it is usually associated
with subsiding air. Because of these vertical movements
resulting from horizontal flows, surface convergence often
produces cloud sheets and precipitation, whilst surface
divergence is associated with clear skies and dry weather.
In the middle troposphere there is a level at about 600 hPa
at which the horizontal convergence and divergence
are effectively zero
(
Figure 6.11b
)
. This link between hori-
zontal and vertical flows in the atmosphere through
convergence and divergence is extremely important in
determining weather events, as we shall see in
Chapter 7.
Friction
Inspection of a surface weather map will show that, at
ground level, the wind does not blow parallel to the
isobars. It blows across the isobars towards the area of
lower pressure. The more observant may notice that this
angle between the wind flow and the isobars is greater over
land areas than over oceans. This may give a clue to the
reasons for the change. Land surfaces are rougher than
seas; they tend to slow the wind down through friction
more effectively. Friction acts as a force pulling against the
direction of flow. We can now rearrange our 'balance of
forces' to include friction. To achieve balance, the flow will
be across the isobars because the Coriolis pull to the right
(or left in the southern hemisphere) decreases as the air
velocity falls (
Figure 6.10
).
From these forces we can now
explain equilibrium horizontal flows of air. They are
initiated by pressure differences, then modified by the
effects of Earth's rotation and friction.
Where flows occur across the isobars in the direction
of lower pressure, there will be a transfer of air towards
the low-pressure centre, leading to
convergence
or a net
accumulation of air. Where flow is away from a high-
pressure centre, there will be a
divergence
of air away from
the surface anticyclone, leading to a net outflow of air.
With these principles in mind we can try to build up
a picture of the global pattern of circulation in Earth's
atmosphere. We can start by considering a highly simpli-
fied model of the atmospheric system: a uniform, non-
rotating, smooth Earth.
As we have seen, the basic force causing atmospheric
motion is the pressure gradient; this gradient arises from
the unequal heating of the atmosphere by solar radiation.
At the equator - the 'firebox' of the circulation, as it has
Northern hemisphere
Southern hemisphere
992 hPa
992 hPa
p.g.f.
p.g.f.
Actual wind
Actual wind
90°
90°
996 hPa
996 hPa
90°
Friction
90°
Friction
1000 hPa
1000 hPa
Coriolis
force
Coriolis
force
Figure 6.10
The effect of friction on the geostrophic wind. The Coriolis force is always at right-angles to the actual wind. It is
smaller than the pressure gradient force because friction has reduced the speed of the wind.
