Geoscience Reference
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either directly (
Q
= 28) or as diffuse radiation (
q
=21) transmitted via clouds or by downward
scattering.
The pattern of outgoing terrestrial radiation is
quite different (see
Figure 3.22
). The black-body
radiation, assuming a mean surface temperature
of 288K, is equivalent to 114 units of infrared
(longwave) radiation. This is possible because
most of the outgoing radiation is reabsorbed by
the atmosphere; the
net
loss of infrared radiation
at the surface is only 19 units. These exchanges
represent a time-averaged state for the whole
globe. Recall that solar radiation affects only the
sunlit hemisphere, where the incoming radiation
exceeds 342W m
-2
. Conversely, no solar radiation
is received by the night-time hemisphere. Infrared
exchanges continue, however, due to the accumu-
lated heat in the ground. Only about 12 units
escape through the atmospheric window directly
from the surface. The atmosphere itself radiates
57 units to space (48 from the emission by
atmospheric water vapor and CO
2
and 9 from
cloud emission), giving a total of 69 units (
L
u
); the
atmosphere in turn radiates 95 units back to the
surface (
Ld
); thus,
Lu
+
Ld
=
Ln
is negative.
These radiation transfers can be expressed
symbolically:
R
n
= (
Q
+
q
) (1 -
atmosphere system is estimated to be ±7GJ m
-2
yr
-1
(± 69 units). Various uncertainties have still
to be resolved in these estimates. The surface
short-wave and long-wave radiation budgets have
an uncertainty of about 20W m
-2
, and the
turbulent heat fluxes of about 10W 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 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 420W m
-2
at the equator and
180W 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
shortwave radiation ranges from 340W m
-2
at
the equator to 80W m
-2
at the poles. The net
(outgoing) longwave radiation (
Figure 3.22B
)
shows the smallest losses where the temperatures
are lowest and largest losses over the mainly clear
skies of the Saharan desert surface and over low-
latitude oceans. The difference between
Figure
3.22A
and
3.23B
represents the net radiation of the
earth-atmosphere system which achieves balance
about latitude 30
) +
L
n
where
R
n
= net radiation, (
Q
+
q
) = global solar
radiation,
α
. The consequences of a low-
latitude energy surplus and a high-latitude deficit
are examined below.
The diurnal and annual variations of temper-
ature are directly related to the local energy
budget. Under clear skies, in middle and lower
latitudes, the diurnal regime of radiative
exchanges generally shows a midday maximum of
absorbed solar radiation (see
Figure 3.23A
). A
maximum of infrared (longwave) radiation
(see
Figure 3.1
) is also emitted by the heated
ground surface at midday, when it is warmest.
The atmosphere re-radiates infrared 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
°
= albedo and
L
n
= net longwave
radiation. At the surface,
R
n
= 30 units. This
surplus is conveyed to the atmosphere by the
turbulent transfer of sensible heat, or enthalpy
(7 units), and latent heat (23 units),
α
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 approximately 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-
