Geoscience Reference
In-Depth Information
available data. Also shown are the radiative flux values at the top of the atmosphere
and their attenuation on their way down. It can be seen that globally the net radiation
at the Earth's surface is
R
n
=
104Wm
−
2
, which is also roughly equal to the sum of
the sensible and latent turbulent heat fluxes
H
+
L
e
E
. The evaporative flux is shown
85Wm
−
2
; because 1 W m
−
2
produces an evaporation rate of roughly
1.07 mm of water per month, this is equivalent with an annual evaporation of 1.09 m,
which agrees with the values listed in Table 1.1. If it is assumed that there is no global
warming (or cooling), the sum of the incoming and outgoing radiative fluxes at the top
of the atmosphere must also be zero. The ratio of the outgoing and incoming short-
wave radiation shows that the average albedo of the Earth-atmosphere system for extra-
terrestrial radiation is of the order of 0.3. Globally, the atmosphere is being warmed by
the short-wave radiation at a rate of 342
to be
L
e
E
=
96Wm
−
2
; however, the rate of
net long-wave radiation input into the atmosphere is 385
−
+
=
169
25
200Wm
−
2
,
which results in a cooling. Thus the net cooling rate of the global atmosphere due to
radiation is
−
−
=−
345
240
104Wm
−
2
, and this is balanced by the energy input into the
atmosphere by the surface turbulent heat fluxes
H
−
96
+
200
=
L
e
E
.
The energy fluxes at the surface and at the top of the atmosphere, as shown in
Figure 2.28, were derived with the constraint that they would exhibit a perfectly balanced
steady state. In fact, the separately measured fluxes do not exhibit a perfect balance, but
the discrepancy is only of the order of a few W m
−
2
; the issue continues to be the subject
of intense investigations.
+
REFERENCES
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Water Resour. Res.
,
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,
719-730.
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,
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,
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, 742-744.
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