Atmospheric motion: principles - Atmosphere, Weather and Climate - page 142

B DIVERGENCE, VERTICAL MOTION
AND VORTICITY

crossing the coastline owing to the greater friction over-
land, whereas offshore winds accelerate and become
divergent. Frictional differences may also set up coastal
convergence (or divergence) if the geostrophic wind is
parallel to the coastline with, for the northern hemi-
sphere, land to the right (or left) of the air current viewed
downwind.

These three terms are the key to a proper understanding
of wind and pressure systems on a synoptic and global
scale. Mass uplift or descent of air occurs primarily in
response to dynamic factors related to horizontal airflow
and is affected only secondarily by airmass stability.
Hence the significance of these factors for weather
processes.

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 systems are to persist
and there is to be no continuous density increase or
decrease. Air rises above a low-pressure cell and sub-
sides 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 600 mb. 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 to 10 cm s -1 , whereas updrafts in
cumulus may exceed 10 m s -1 .

1 Divergence

Different types of horizontal flow are shown in Figure
6.6A. The first panel shows that air may accelerate
(decelerate), leading to velocity divergence (conver-
gence). When streamlines (lines of instantaneous air
motion) spread out or squeeze together, this is termed
diffluence or confluence , respectively. If the streamline
pattern is strengthened by that of the isotachs (lines
of equal wind speed), as shown in the third panels of
Figure 6.6A, then there may be mass divergence or
convergence at a point (Figure 6.6B). In this case, the
compressibility of the air causes the density to decrease
or increase, respectively. Usually, however, confluence
is associated with an increase 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, conver-
gence (divergence) 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

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 about an axis vertical to the earth's surface
(Figure 6.8). Vorticity has three elements - magnitude
(defined as twice the angular velocity, Ω ) (see Note 3),

Figure 6.7 Cross-section of the
patterns of vertical motion associated
with (mass) divergence and conver-
gence in the troposphere, illustrating
mass continuity.