Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 100

of dashed lines. All values of the albedo increasewhen the atmospheric pressure
decreases (with the altitude) especially if the surface is dark. It confirms the
aboveconclusionaboutthepredominanceofscatteringoverabsorptionwithin
the short-wavelength range excluding the absorption bands in the NIR region
above the sand surface.
Figure 3.12 apparently indicates spectral transformation of the albedo in
molecular absorption bands with the increasing of atmospheric thickness
especially in the example of the water surface (Vasilyev A et al. 1997a, 1997b,
1997c). The figure also demonstrates that themagnitudes of the snow albedo of
the Ladoga Lake surface are not very high compared with other observations
(Chapurskiy 1986) that could be explained with the destruction and pollution
of ice in spring (April). Carrying out the observations in winter is complicated
owingtothelowSun.Thestandarddeviationofthealbedoiscalculatedwiththe
covariance matrix of the couple of corresponded irradiances. The calculation
methodology will be described in Chap. 4. The average uncertainty of the
albedo is about 5%.
3.3.1
Results of Airborne Observations Under Overcast Conditions
The experiments on the overcast sky were carried out in the field by companies
and conducted as components of CAENEX, GAAREX, GARP and GATE scien-
tific programs. The results of these programs are considered in several topics
(Kondratyev 1972; Kondratyev and Ter-Markaryants 1976; Kondratyev et al.
1977; Kondratyev and Binenko 1981, 1984) and in several studies (Kondratyev
et al. 1976; Vasilyev A et al. 1994; Kondratyev et al. 1996, 1997a, 1997b). The
observations were carried out with K-2 and K-3 instruments (Mikhailov and
Voitov 1969) and each experiment in the cloudy atmosphere was accompanied
with themeasurement for the same region under the clear sky conditions at the
same height levels and at the close time. Only optically thick stratus clouds of
large extension were studied during the overcast-sky experiments. The experi-
mental results in different latitudinal zones in different time during 1971-1985
were analyzed using the uniform observational data sets. The geographical
latitudes of the observations were changing from 15 ◦ N (the East part of the
Atlantic Ocean close to the African coast) to 75 ◦ N (above the Cara Sea). All
aircraft observations were accomplished above the homogeneous surfaces (sea
and snow surface, deserts). Under these conditions, it was possible to exclude
such factors as a horizontal heterogeneity of clouds and surface, broken cloudi-
ness, radiation escape through the cloud sides. To estimate the cloud radiative
forcing the data of the pyranometric (total SWR) and spectral observations
were used simultaneously.
The surface albedo was calculated as a ratio of the upwelling to downwelling
irradiances at the lowest level under the cloud layer. The information about the
cloudy experiments, which will be further interpreted in Chap. 7, is presented
inTable 3.2. The thickness of the cloud layer, the cosine of the solar zenith angle,
the latitudes, the surface type and albedo, the total values of the radiative flux
divergence over the spectral region in cases of the cloud and clear atmosphere