Geoscience Reference
In-Depth Information
assuming a mean surface temperature of 288 K, is
equivalent to 114 units of infra-red (long-wave) radi-
ation. This is possible since most of the outgoing
radiation is reabsorbed by the atmosphere; the
net
loss
of infra-red radiation at the surface is only nineteen
units. These exchanges represent a time-averaged state
for the whole globe. Recall that solar radiation affects
only the sunlit hemisphere, where the incoming radi-
ation exceeds 342 W m
-2
. Conversely, no solar radiation
is received by the night-time hemisphere. Infra-red
exchanges continue, however, due to the accumulated
heat in the ground. Only about twelve units escape
through the atmospheric window directly from the
surface. The atmosphere itself radiates fifty-seven units
to space (forty-eight from the emission by atmospheric
water vapour and CO
2
and nine from cloud emission),
giving a total of sixty-nine units (
L
u
); the atmosphere in
turn radiates ninety-five units back to the surface (
L
d
);
thus
L
u
+
L
d
=
L
n
is negative.
These radiation transfers can be expressed symbol-
ically:
equator in the annual mean (cf. Table 3.1). The mean
annual totals on a horizontal surface at the top of the
atmosphere are approximately 420 W m
-2
at the equator
and 180 W m
-2
at the poles. The distribution of the
planetary albedo (see Figure 3.13B) shows the lowest
values over the low-latitude oceans compared with the
more persistent areas of cloud cover over the continents.
The highest values are over the polar ice-caps. The
resulting planetary short-wave radiation ranges from
340 Wm
-2
at the equator to 80 Wm
-2
at the poles. The
net (outgoing) long-wave radiation (Figure 3.22B)
shows the smallest losses where the temperatures are
lowest and highest losses over the largely clear skies
of the Saharan desert surface and over low-latitude
oceans. The difference between Figure 3.22A and 3.22B
represents the net radiation of the earth-atmosphere
system which achieves balance about latitude 30°. The
consequences of a low-latitude energy surplus and a
high-latitude deficit are examined below.
The diurnal and annual variations of temperature are
related directly to the local energy budget. Under clear
skies, in middle and lower latitudes, the diurnal regime
of radiative exchanges generally shows a midday maxi-
mum of absorbed solar radiation (see Figure 3.23A). A
maximum of infra-red (long-wave) radiation (see Figure
3.1) is also emitted by the heated ground surface at
midday, when it is warmest. The atmosphere re-radiates
infra-red radiation downward, but there is a net loss at
the surface (
L
n
). The difference between the absorbed
solar radiation and
L
n
is the net radiation,
R
n
; this is
generally positive between about an hour after sunrise
and an hour or so before sunset, with a midday maxi-
mum. The delay in the occurrence of the maximum
air temperature until about 14:00 hours local time
(Figure 3.23B) is caused by the gradual heating of the
air by convective transfer from the ground. Minimum
R
n
occurs in the early evening, when the ground is still
warm; there is a slight increase thereafter. The temper-
ature decrease after midday is slowed by heat supplied
from the ground. Minimum air temperature occurs
shortly after sunrise due to the lag in the transfer of heat
from the surface to the air. The annual pattern of the net
radiation budget and temperature regime is closely
analogous to the diurnal one, with a seasonal lag in the
temperature curve relative to the radiation cycle, as
noted above (p. 47).
There are marked latitudinal variations in the diurnal
and annual ranges of temperature. Broadly, the annual
range is a maximum in higher latitudes, with extreme
R
n
= (
Q
+
q
) (1 -
a
) +
L
n
where
R
n
= net radiation, (
Q
+
q
) = global solar
radiation,
a
= albedo and
L
n
= net long-wave radiation.
At the surface,
R
n
= 30 units. This surplus is conveyed
to the atmosphere by the turbulent transfer of sensible
heat, or enthalpy (seven units), and latent heat (twenty-
three units):
R
n
=
LE
+
H
where
H
= sensible heat transfer and
LE
= latent heat
transfer. There is also a flux of heat into the ground (B.5,
this chapter), but for annual averages this is approx-
imately zero.
Figure 3.22 summarizes the total balances at the
surface (± 144 units) and for the atmosphere (± 152
units). The total absorbed solar radiation and emitted
radiation for the entire earth-atmosphere system is
estimated to be ±7 GJ m
-2
yr
-1
(± 69 units). Various
uncertainties are still to be resolved in these estimates.
The surface short-wave and long-wave radiation
budgets have an uncertainty of about 20 W m
-2
, and the
turbulent heat fluxes of about 10 W m
-2
.
Satellite measurements now provide global views
of the energy balance at the top of the atmosphere. The
incident solar radiation is almost symmetrical about the
