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homogeneous thick cloudiness during these observations, and the obtained
data seemed suitable for applying the analytical method.
The second experiment was accomplished using the spectral instrument,
whichhad spectral resolution 0.002
µ
µ
m,under
the overcast condition in St. Petersburg's suburb on 12th April 1996 (Melnikova
et al. 1997).
In all these cases, the data were obtained for five viewing angles and for
five azimuth angles. One set of the measurements took about 10 minutes. The
measurements were conducted at noontime, when the solar zenith angle hardly
varied during 10minutes. The transmitted radiance at different azimuth angles
with the same viewing angle varied in the range of the measurement error and
it was averaged during the data processing.
During the Arctic experiment, the observations of the downwelling and up-
welling irradiance were also accomplished and ground albedo A was obtained
(Radianov et al. 1981). Different types of snow cover were studied (fresh snow,
wet snow and so on), and in all cases, a slight spectral dependence of ground
albedo A was observed. On 13th August 1979 the ground surface was covered
with wet snow and ground albedo A was about 0.6. On 8th October 1979 there
was fresh snow and ground albedo A was about 0.9. Thus, the data of 13th Au-
gust 1979 are more favorable for the present interpretation because the lower
value of ground albedo A decreases the errors of the optical cloud parameters
retrieval. During the observation of 12th April 1996 there was heavy snow-
ing, so the ground albedo was assumed equal to 0.9 (for fresh snow) as per
(Radionov et al. 1981).
Besides, the observation of direct solar radiation was carried out in clear
sky during the Arctic experiment of 1979. It gave the opportunity of calibrating
the instrument in units of solar incident flux
mand spectral range 0.35-0.75
π
S at the top of the atmosphere
that is necessary for the retrieval of optical thickness
τ 0 according to (6.17).
The experiment of 12th April 1996 was accomplished likewise excluding the
measurement of direct solar radiation in the clear sky, hence the instrument
was not calibrated and optical thickness
τ
could not been obtained.
Each pair of the transmitted radiance is processed according to (6.20), and
then values s 2 and
τ are averaged. The final spectral values of single scattering
τ 0 are calculated accounting the spec-
tral dependence of parameter g according to the topic by Stephens (1979). The
results are presented in Fig. 7.7 and in Table A.15 of Appendix A [the Arctic
cases (a) and St. Petersburg's suburb case (b); Melnikova and Domnin 1997;
Melnikova et al. 1998, 2000]. Out of the molecular absorption bands spectral
values (1 −
ω 0 ) and optical thickness
co-albedo (1 −
ω 0 ) are about 0.002-0.004, that corresponds to the values obtained
earlier from the airborne experiments (Sect. 7.1). The optical thickness pre-
sented in Fig. 7.8 is typical for stratus clouds and demonstrates the apparent
spectral dependence similar to the one, presented in Sect. 7.1.
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