Geoscience Reference
In-Depth Information
Shortwave heat
uxes
Longwave and turbulent heat
uxes
Incident
solar
radiation
341
Re
ected
solar
radiation
102
Emitted
terrestrial
radiation
239
Top of the atmosphere
Absorbed in
stratosphere
11
Direct emission
to space from
surface
40
Emission
from
atmosphere
199
Absorbed by
atmosphere
356
Re
ected
by clouds
62
Back-
scattered
17
Absorbed in
troposphere
67
Absorbed by
atmosphere
98
Scattered to
the surface
(di‰use beam)
81
Back-
radiation
from
atmosphere
333
Radiation
from surface
396
Direct
solar beam
103
Latent
heat
ux
80
Sensible
heat
ux
18
Re
ected
from surface
23
Surface of the Earth
Absorbed by surface 161
Net emission from surface 161
Figure 5.4 The estimated global heat balance (W/m
2
). Based on Trenberth, Fasullo,
and Kiehl (2009).
the direct solar beam (~103 W/m
2
) and some in the diffuse beam (~81 W/m
2
)
after multiple Rayleigh and Mie scattering events in the atmosphere. Only a
relatively small amount of the total incoming solar radiation (~11 W/m
2
) is
absorbed by gases (e.g., ozone) in the stratosphere, and about 67 W/m
2
is ab-
sorbed by aerosols, water vapor, and water droplets in the troposphere.
The right half of
Figure 5.4
illustrates the passage of outgoing longwave,
or terrestrial, radiation through the atmosphere. The longwave emission rate
from the surface (~396 W/m
2
) implies a surface temperature of about 289 K
according to Eq. 4.2 with 1. Only about 10%, or ~40 W/m
2
, of this emis-
sion from the surface radiates directly to space, largely through the longwave
atmospheric window (8-13 mm).
The lion's share of the longwave emission from the surface (~356 W/m
2
) is
absorbed within the atmosphere. This is the primary heat source for the at-
mosphere, so the atmosphere is heated from below (from the surface) and not
from above (directly by the incident solar radiation). Molecules, water droplets,
clouds, and dust in the atmosphere reradiate the absorbed longwave energy at
their own temperatures. About 333 W/m
2
is radiated back to the surface where
it is absorbed by the land or ocean. This is the longwave back radiation asso-
ciated with the greenhouse effect (section 4.5). Outgoing longwave radiation
(OLR) includes emission from clouds (~30 W/m
2
) and atmospheric molecules
(~169 W/m
2
). These combine with the 40 W/m
2
emitted directly from the sur-
face to space, balancing the 239 W/m
2
solar absorbed.
Longwave emission from the surface (~396 W/m
2
) is more than twice the
amount of shortwave radiation absorbed (~161 W/m
2
), and the largest source
