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∆τ
0
|τ
0
of the average values of the optical
thickness obtained from the reflected and transmitted irradiance assuming the
conservative scattering versus to the single scattering albedo is demonstrated
in Fig. 6.2. It is clear that the ground albedo strongly increases the uncertainty.
The interpretation of the irradiance observations within the conservative
cloud layer is available using the formula readily derived from(2.46) and (2.49):
- the upper sublayer adjoins the cloud top
The dependence of relative error
µ
0
)−2(
F
1
+
F
1
)
3
F
(
4
K
0
(
τ
1
=
−
q
,
(1 −
g
)
(6.31)
τ
1
)
-thesublayerwithinthecloud
4(
F
i
−1
−
F
i
)
3
F
(
τ
i
−
τ
i
−1
)
=
(1 −
g
)(
,
(6.32)
τ
i
)
- the sublayer adjoins the cloud bottom
−
q
+
,
2(
F
N
−1
+
F
N
−1
)
3
F
(
4
A
3(1 −
A
)
τ
N
−
τ
N
−1
)
=
(1 −
g
)(
(6.33)
τ
N
−1
)
τ
N
=
τ
0
.
where
N
is the number of sublayers and
6.2.2
Estimation of Phase Function Parameter
g
All the above-presented expressions retrieve the
scaled
optical thickness, so
phase function parameter
g
is needed to obtain the optical thickness. The infer-
ring of phase function parameter
g
(asymmetry factor) of ice clouds has been
made in the 90th by measuring the radiative fluxes, calculating the radiative
transfer models, and selecting parameter
g
for the best coincidence with the
observations. However, the methodology of
selecting parameters
is ambiguous
as has been shown inChap. 4 and needs careful error analysis. Probably, it is the
reason for inconsistent results. Besides, parameter
g
dramatically influences
the calculation of reflection function
µ
0
), thus it has to be obtained
from measurements for the adequate interpretation of the satellite radiation
observations.
The attempts to obtain parameter
g
from observations has been made in
two studies (Gerber et al. 2000; Garrett et al. 2001) using the nephelometer
measurements, and the values of parameter
g
is revealed to be equal to 0.85
for stratiform liquid clouds, to 0.81 for convective clouds, and to 0.73 for
nonconvective ice clouds. It is seen that the variation of the asymmetry factor
is significant and it is desirable to retrieve parameter
g
and the other optical
parameters together during one experiment.
Here we propose a way of estimating phase function parameter
g
for the
optically thick cloud from radiative observations as other optical parameters.
ρ
∞
(
µ
,
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