Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 121

Fig. 3.21. The annual zonal cloud amount: (1) averaged over the latitude; (2) above the sea
surface and (3) above the ground surface in 1971-1990 according to Matveev et al. (1986)

The common features of the considered relationship are clear because of
the evident relation between the solar zenith angle and geographical latitude
(keeping in mind that the radiative experiments are accomplished around
midday). However, the original reason is not clear: whether it is the solar
height or different cloud optical properties in different latitudinal zones.
It is obvious that for elucidation of the cloud absorption a sufficient amount
of clouds is necessary. It is of special interest that the comparison of the
latitudinal dependence of the cloud amount (Fig. 3.21) from the study by
Matveev et al. (1986) and the dependence of parameter f s characterizing the
cloud radiative forcing as per Fig. 3.20b are seen to coincide qualitatively.
The airborne radiative experiments accomplished in the range of CAENEX,
GAAREX, GARP and GATE programs have apparently demonstrated a signif-
icant absorption of SWR by clouds. In the remainder of this subsection the
following thesis are given:
The excessive absorption of SWR is defined just by the optical properties of
cloudiness and is not caused by the observational or processing uncertainties
as some investigators have presented.

1. The relationship between the scattering and absorbing properties of
stratus clouds and the geographical latitude, solar zenith angle, and type
of the atmospheric aerosols within clouds is experimentally proved.
2. The increase in radiation absorption is stronger in thick cloud layers in
a dusty atmosphere containing carbon or sand aerosols.
The effect of the excessive absorption is observed over the shortwave spectral
regionasawholebutitisespeciallyhighfortheshorterwavelengths( λ µ ).

The existence of the anomalous absorption fundamentally changes the current
understanding of the energetic budget of the atmosphere. In this connection,
it is of great importance to account for the atmospheric heating caused by the
cloudabsorptionofSWRforclimateforecastsimulations.

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