Geoscience Reference
In-Depth Information
in air velocity and diffluence with a decrease. In
the intermediate case, confluence is balanced by
an increase in wind velocity and diffluence by a
decrease in velocity. Hence, convergence (diver-
gence) may give rise to vertical stretching
(shrinking), as illustrated in
Figure 6.6C
. It is
important to note that if all winds were
geostrophic, there could be no convergence or
divergence and hence no weather!
Convergence or divergence may also occur as
a result of frictional effects. Onshore winds
undergo convergence at low levels when the air
slows down on crossing the coastline owing to
the greater friction overland, whereas offshore
winds accelerate and become divergent. Frictional
differences can also set up coastal convergence
(or divergence) if the geostrophic wind is
parallel to the coastline with, for the Northern
Hemisphere, land to the right (or left) of the air
current, viewed downwind.
systems are to persist and there is to be no
continuous density increase or decrease. Air
rises above a low pressure cell and subsides over
high pressure, with compensating divergence
and convergence, respectively, in the upper
troposphere. In the middle troposphere, there
must clearly be some level at which horizontal
divergence or convergence is effectively zero; the
mean 'level of non-divergence' is generally at
about 600mb. Large-scale vertical motion is
extremely slow compared with convective up- and
downdrafts in cumulus clouds, for example.
Typical rates in large depressions and anticyclones
are of the order of ±5-10cm s
-1
, whereas updrafts
in cumulus may exceed 10m s
-1
.
3 Vorticity
Vorticity implies the rotation, or angular velocity,
of small (imaginary) parcels in any fluid. The air
within a low pressure system may be regarded as
comprising an infinite number of small air parcels,
each rotating cyclonically around an axis vertical
to the earth's surface (
Figure 6.8
). Vorticity has
three elements - magnitude (defined as
twice
the
angular velocity,
2 Vertical motion
Horizontal inflow or outflow near the surface has
to be compensated by vertical motion, as
illustrated in
Figure 6.7
, if the low or high pressure
Ω
) (see Note 3), direction (the
CONVERGENCE
DIVERGENCE
Mean level of
non-divergence
(about 600mb)
ASCENT
SUBSIDENCE
CONVERGENCE
DIVERGENCE
SURFACE
LOW
PRESSURE
HIGH
PRESSURE
Figure 6.7
Cross-section of the patterns of vertical motion associated with (mass) divergence and convergence in
the troposphere, illustrating mass continuity.
