Geoscience Reference
In-Depth Information
Fig. 1.2.
Definition of net radiant flux
The value
F
↓
(
z
)iscalledthe
downward flux
(
downwelling irradiance
), the
value
F
↑
(
z
)-an
upward flux
(
upwelling irradiance
), both are also called
semi-
spherical fluxes
expressed in watts per square meter (per micron). The physical
sense of these definitions is evident. The downward flux is radiation energy
passing through the level
z
down to the ground surface and the upward flux
is energy passing up from the ground surface. The downward flux is always
positive (cos
ϑ
ϑ
<
0). In practice (for
exampleduringmeasurements)itisadvisabletoconsiderbothfluxesaspos-
itive ones. We will follow this tradition. Then for the upward flux in (1.6) the
value of cos
>
0), upward is always negative (cos
ϑ
is to be taken in magnitude, and the total flux will be equal to the
difference of the semispherical fluxes
F
(
z
)
=
F
↓
(
z
)−
F
↑
(
z
). This value is often
called a (
spectral
)net
radiant flux
expressed in watts per square meter (per
micron).
Consider two levels in the atmosphere, defined by the altitudes
z
1
and
z
2
(Fig. 1.2). Obtain solar radiation energy
B
(
z
1
,
z
2
) (per unit area, time and
wavelength) absorbed by the atmosphere between these levels. Manifestly, it is
necessary to subtract outcoming energy from the incoming:
=
F
↓
(
z
2
)+
F
↑
(
z
1
)−
F
↓
(
z
1
)−
F
↑
(
z
2
)
=
B
(
z
1
,
z
2
)
F
(
z
2
)−
F
(
z
1
).
(1.7)
The value
B
(
z
1
,
z
2
)iscalleda
radiative flux divergence in the layer between levels
z
1
and
z
2
. It is extremely important value for studying atmospheric energetics
because it determines the warming of the atmosphere, and it is also important
for studying the atmospheric composition because the spectral dependence of
B
(
z
1
,
z
2
) allows us to estimate the type and the content of specific absorbing
materials (atmospheric gases and aerosols) within the layer in question. Hence,
the values of the semispherical fluxes determining the radiative flux divergence
arealsoofgreatestimportanceforthementionedclassofproblems.
To provide the possibility of comparing the radiative flux divergences in
different atmospheric layers we need to normalize the value
B
(
z
1
,
z
2
)tothe
thickness of the layer:
=
|
b
(
z
1
,
z
2
)
B
(
z
1
,
z
2
)
(
z
2
−
z
1
).
(1.8)
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