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TheinterpretationoftheUVradiationobservationsinthecloudyskyby(Mayer
et al. 1998) also demonstrates the strong extinction: the cloud optical thickness
in the UV region has been retrieved to be equal to several hundreds.
Mie theory calculations yield volume scattering coefficient
α
(and optical
τ 0 )forensembleoftheparticleswithsize > 5
µ
thickness
m independent of
wavelength in the shortwave region, and the magnitude of the volume absorp-
tion coefficient in the cloud has to be in range 10 −5 -10 −8
(single scattering
ω 0 is about 0.99999-1.0).
Here we propose a possible explanation of this contradiction. It links with
the multiple scattering within clouds. Qualitatively the similar assumption
has been proposed in the topic by Kondratyev and Binenko (1984), while
considering the airborne observational data.
The cloud layer is considered to consist of droplets, sometimes with addi-
tion of aerosols within the droplet. The molecular scattering is accounted for
with summarizing the scattering coefficients and as the molecular scattering
coefficientismuchlower(byafactorof10 3 ) than the cloud scattering coef-
ficient, its yield turns out to be negligible. It's known that the mean number
of the scattering events in the cloud with optical thickness
albedo
τ 0 is proportional
0 owing to the multiple scattering (Minin 1981,1988; Yanovitskij 1997);
forreflectingphotonsitisproportionalto
τ
to
τ 0 . Thus, the photon path within
the optically thick cloud significantly increases compared to the photon path
within the clear sky, and the number of collisions with air molecules (more
rigorous with fluctuations of the molecular density) increases as well. The
radiation absorption removes the part of photons and weakens the increasing
effect of the molecular scattering. Since it is necessary to take into account that
the cloud layer does not simply superpose to the molecular atmosphere, but
it increases the molecular scattering. We should mention that the increasing
of the molecular absorption within oxygen absorption band
λ =
µ
m due
to the increasing of the photon path within the cloud has been considered in
various studies (Dianov-Klokov et al. 1973; Marshak et al. 1995; Kurosu et al.
1997; Pfeilsticker et al. 1997; Wagner et al. 1998; Pfeilsticker 1999). The same
reasons are also valid for radiation scattering and absorption by the aerosol
particles between droplets.
It is clear that the multiple scattering theory and the radiative transfer equa-
tion takes into account all processes of scattering and absorption, but it is
right only, if they are accurately put in the model of scattering and absorbing
medium. Usually the averaging values of scattering and absorption coefficients
over the elementary volume are substituted to the transfer equation and then
the solving is accomplished with one of the radiative transfer methods. How-
ever, from the physical point it is incorrect to average the initial parameters
over the elementary volume before solving. The incorrectness is intensified
with the essentially different scales of the elementary volumes for different
particles (molecules, aerosols and droplets), whose sizes distinguish by an or-
der of magnitude and much more (look Sect. 1.2) and the transfer equation
is derived in a phenomenological way for this incorrect elementary volume.
Strictly speaking, the equation of the radiative transfer for the complex multi-
component medium is to be inferred from Maxwell equations accounting all
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