Geoscience Reference
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instrument were processed for the retrieval of cloud optical parameters. The
pixels with the cloud amount exceeding 0.5 were only considered.
The following sequence of the procedures for every pixel is proposed for
processing POLDER data:
1. At the first step the angular dependent functions are calculated.
2. The next step includes the calculation of the approximate optical thick-
ness for every viewing direction with the simple formula, assuming
theconservativescattering.Theobtainedvaluesshowthedegreeofthe
shadowinginfluence(ortheinfluenceofthecloudtopdeviationfrom
theplane)andgivethepossibilitytoevaluateparameter
r
with (6.39).
Besides, they allow choosing the pairs of viewing directions where the
optical thickness is approximately equal.
3. Thethirdstageconsistsoftheparameter
s
2
retrieval fromthe radiances at
each pair of viewing directions with the equal optical thickness [(6.13)].
If the optical thickness defined at the previous stage without accounting
of the absorption is more than 100, parameter
s
2
is obtained according
to (6.16). Then the averaging over all pairs of the viewing directions is
accomplished, and the relative mean square deviation is estimated.
4. At the fourth stage optical thickness
τ
0
is calculated for every viewing
direction, assuming the true absorption, and the results are averaged.
5. Then, the similar procedure is repeated for every available wavelength.
6. At the sixth stage the results are prepared for mapping (inserting the
missed pixels; inserting the values averaged over the neighbor pixels
to the missed pixels or to the pixels with only one viewing direction;
rejecting the edge pixels). The uncertainties are calculated for every
pixel using the formulas similar to (6.46).
7. Finally, the images of the single scattering co-albedo and optical thick-
ness are figured with the GRADS editor. The space distribution of single
scattering co-albedo (1 −
τ
0
is shown in Fig. 7.10 (Melnikova and Nakajima 2000a,b). The values of
(1 −
ω
0
) is shown in Fig. 7.9, optical thickness
ω
0
) are in the range 0.001-0.010; the optical thickness is about
15-25 and can reach 100 in the Tropics. Black gaps in the images cor-
respond to the pixels with the cloud amount less than 0.5. Four images
are presented in Figs. 7.9 and 7.10, the upper picture join three images
registered during the successive satellite pass with time interval about
one hour (i. e. these images are presenting one cloud field). Figure 7.11
demonstrates the values of (a) - single scattering co-albedo (1 −
ω
0
),
τ
0
and shadow parameter
r
multiplied by 10
2
in three spectral channels versus pixel numbers. The latter turns not to
depend on wavelength, and in contrast the spectral dependence of the
optical thickness decreases with wavelength for all (!) processed pixels.
Please remember that the processing has been accomplished for every
wavelength independently. The size of every pixel is about 60 km.
and (b) - optical thickness
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