Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 240

hydrostatic pressure gradient (Section 3.5.3) that increases
vertically from virtually zero close to sea level to a maxi-
mum toward the top of the troposphere at latitudes
50-60

roughness, the flow approximating to that of the
geostrophic wind set up due to synoptic pressure or tem-
perature gradients. Such a wind drives flow in the atmos-
pheric boundary layer (ABL), but because of the effects of
surface friction the effectiveness of the Coriolis force in
turning the geostrophic wind markedly decreases; for a
northern hemisphere wind the direction of the frictional
wind backs anticlockwise toward the surface, while in the
southern hemisphere the frictional wind veers clockwise. In
both cases the winds tend to progressively diverge from
their geostrophic course parallel to isobars through the
ABL. This is known as the Ekman spiral effect (see discus-
sion in Section 6.4). The effect of the frictional wind on a
low-pressure system is to cause inward spiralling of wind
into the center; this convergence causes compensatory cen-
tral upwelling of air and may cause cloudy conditions as
moist air is cooled and is vice versa for high-pressure systems
when divergence causes central downwelling and clear skies.

N. The overall concept of this thermal-wind rela-
tionship is illustrated by comparing the slopes of average
isobars low down and high up in the troposphere. The
increasing slope of the equal pressure surfaces and there-
fore the increasing strength of the resulting geostrophic
wind (Fig. 6.6) is evident. The thermal-wind relationship
thus refers to a rate of change of wind velocity with height;
that is, it is a measure of geostrophic wind shear . The mag-
nitude of the vertical wind shear is directly proportional to
the horizontal temperature gradient. The high-level wind
strength and its gradient can be mapped as a velocity field
from pressure layer height contour maps. As a geostrophic
phenomenon (Fig. 6.6) the wind travels parallel to the
contours of geopotential height (Fig. 6.4) and is faster
when contours are closer and vice versa. The maximum
magnitude of wind shear defines the fast cores of the polar
front jet streams that dominate the west-to-east zonal cir-
culation of the planetary wind regime. The jets are
strongest in the winter when temperature contrasts
between equator and pole are greatest (Fig. 6.7).

6.1.4 Energy transformation and the global
atmospheric circulation (GAC)

In order to understand the principles of the wider GAC, it
is now necessary to consider the role of thermal energy
transfer: how energy is transported by a unit mass of moist
air. In order to maintain constant total energy, E , in the
face of continuous loss of longwave radiation to space (the
atmospheric radiation deficit earlier discussed), it is neces-
sary to add sensible heat from surface land and ocean and
from the release of latent heat during rainfall. The general
circulations of the atmosphere thus involve a poleward
transport of heat energy to maintain the observed long-
term temperature distributions, which are approximately
constant. The simplest such arrangement possible would
be a general convective upwelling of warm moist air from
the equatorial regions and its transfer toward the poles,
cooling by radiative heat loss as it does so, where it even-
tually sinks, liberating rain, snow, and latent heat. Such a
simple cellular circulation, termed Hadley circulation , has
the right principles (Fig. 6.8) but inevitably the Earth's
atmosphere is more complicated, chiefly because of the
effects of the Earth's rotation, and also because of pressure
effects due to latitudinal variation in thickness of the tro-
posphere. Atmospheric air masses continuously move
around and at the same time energy is continuously being
transformed from one form to another. Neglecting the
very small kinetic energy of air masses, the following forms
of energy are involved (Fig. 6.9):
1 Latent heat energy , E L , arises in moist air from reversible
phase changes of state between liquid, water, and gaseous

6.1.3 Frictional wind

That part of the wind blowing well above the land or sea
may be considered to flow independently of any surface

Low

Geost rophic wind

High

Geostrophic wind

LOW

Constant pressure
gradient force, d p /d s

Coriolis acceleration

Resultant wind

Fig. 6.6 A starting geostrophic wind begins to flow from high to
low pressure along the constant pressure gradient, d p /d s , but it is
progressively and increasingly turned due to the Coriolis accelera-
tion (shown here for the southern hemisphere).