Solar Radiation in the Atmosphere - Short-Wave Solar Radiation in the Earth's Atmosphere - page 40

role unlike the clear atmosphere. Only the horizontally homogeneous atmo-
sphere will be considered below. Applying to the cloudy atmosphere, it means
the considering of the model of the infinitely extended and horizontally homo-
geneous cloud layer. In reality, it is the stratus cloudiness, which corresponds
best of all to this case. Fix on the properties of the stratus clouds, which allow
applying the considered theoretical methodologies to the real cloudiness.
Remember the classification of the stratiform clouds: the stratiform clouds
of the lower level are Stratus (St), Stratus-cumulus (Sc), Nimbus stratus (Ns);
the stratiform clouds of the medium level are Alto-stratus (As), Alto-cumulus
(Ac); the high stratiform clouds are Cirrus-stratus (Cs); and also the Frontal
cloud systems are Ns-As, As-Cs, Ns-As-Cs (Feigelson 1981; Matveev et al.
1986; Marchuk et al. 1986; Mazin and Khrgian 1989). The extended stratus
clouds are important in the feedback chain of the climatic system influencing
essentially the albedo, radiative balance of the “atmosphere plus surface”,
and total circulation of the atmosphere (Marchuk et al. 1986; Marchuk and
Kondratyev 1988). The stratus clouds widening over vast regions impact on
the Earth radiative balance not only in the regional but also in the global scale.
The cloud albedo is significantly higher than the ground or ocean albedo
withoutsnowcover.Basingonthisandassumingthatcloudspreventthe
heating of the surface and lower atmospheric layer in the low and middle
latitudes, a negative yield to the Earth radiative balance is usually concluded.
The clouds in the high latitudes don't increase the light reflection because the
snow albedo is also high and, in this case, the clouds play a prevailing role in
atmospheric heating.
However, it has been elucidated in the last few decades that the situation
is more complicated: the clouds themselves absorb a certain part of incom-
ing radiation providing the atmospheric heating in all latitudes. Thus, the
problem of the interaction between the clouds and radiation comes to the
foreground in the stratus clouds study. The climate simulation requires in-
putting the adequate optical models of the clouds and so it is necessary to
obtain the real cloud optical parameters (volume scattering and absorption
coefficients). The including of atmospheric aerosols in the processes of the
interaction between short-wave radiation (SWR) and clouds affect equivocally
the forming of the heat regime of the atmosphere and surface. The direct and
indirect aerosol heating effects are indicated in the literature (Hobbs 1993;
Charlson and Heitzenberg 1995; Twohy et al. 1995). The radiation absorption
bythecarbonaceousandsilicateaerosolscausesthedirecteffect.Theindirect
effect is attributed to the hydrophilic atmospheric aerosols necessary to the
water vapor condensation and to the generating of the cloud droplets. Hence,
thehighconcentrationoftheseaerosolsincreasesthedropletsnumberand
optical thickness of the cloud that, in turn, intensifies the reflection of solar
radiation and reduces the radiation absorption in the atmosphere and on the
surface.Fromtheresultsoftheairborneradiativeobservationsofthelastfew
decades, it has been revealed that direct and indirect effects of the aerosols
influence differently the increasing or extinction of the solar radiation absorp-
tion in clouds of different origin in different geographical regions (Hobbs 1993;
Harries 1996).