Geoscience Reference
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is a special case because the energy of the photons is so large that the molecules
(ozone and oxygen) are broken up in atoms that reorganize into other types of mol-
ecules.
To summarize, the extinction of direct beam radiation can be treated as the sum of
a number of processes. In terms of transmissivities:
(
)
⋅
(
)
τ τ τ τ
=
⋅
⋅
τ τ τ
⋅ ⋅
(2.14)
λ
θ
λ
θ
,Ray
λ
θ
,Mie
λ
θ
,
geo
λ
θ
,
gas
λ
θ
,
O
λ
θ
,
w
where the irst group of transmissivities is related to scattering (Rayleigh and Mie
scattering, and geometric optics theory) and the second group to absorption (ozone,
water vapour and other, uniformly mixed, gases). It depends on the variability of the
concentration of atmospheric constituents whether the extinction due to the respec-
tive processes is variable from time to time, or between locations. Whereas
τ
λθ
,Ray
, and
τ
λθ
,gas
will be relatively constant, other transmissivities may be highly variable due
to variations in aerosols and clouds (
τ
λθ
,Mie
), ozone concentrations (
τ
λθ
,O
) and atmo-
spheric water vapour content (
τ
λθ
,w
).
The direct beam radiation that enters the atmosphere (
I
0
) but does not reach the
surface (
I
0
- I
) has to go somewhere. There are three fates for this energy:
Radiation that is scattered in a direction away from Earth's surface leaves the atmosphere
•
as relected radiation (especially important for cloudy situations).
Radiation that is scattered in a direction towards the surface reaches the surface some-
•
where as diffuse radiation.
Radiation that is absorbed heats up the air where it is absorbed. This in turn will result
•
in extra thermal emission. Note that the air will re-emit the absorbed radiation at a very
different wavelength than at which it was absorbed: absorption takes place at wavelength
in the shortwave region (left-hand-side of
Figure 2.2c
), whereas emission occurs in the
longwave region, corresponding to the temperature of the air and only at wavelengths
where emission lines exist (right-hand side of
Figure 2.2c
).
Impact of the Atmosphere on Radiation Reaching the Surface
The clearest impact of absorption and scattering on the amount of radiation reach-
ing the surface can be seen when cloudiness changes.
Figure 2.6a
shows observa-
tions of global radiation (
K
↓
) for two consecutive, but contrasting days. The input
of solar radiation at the top of the atmosphere was nearly identical (only a one-
day difference) but the presence of clouds on May 22 leads to a reduction of total
incoming radiation of about 75%. One remarkable feature is the high peak around
15 Coordinated Universal Time (UTC) at May 22 which exceeds the value of
K
↓
on
the clear day. This is probably due to a combination of direct insolation (sunlight
peeking through the clouds) and relection of solar radiation on the sides of cumu-
lus clouds.
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