Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 117

has shown that the horizontal transport in the case of a stochastic cloud
top structure is revealed as stronger than in the case of the cloud inner
parameter variations. To estimate the absorption in the layer correctly,
the scale of the reflected and transmitted irradiances averaging over the
cloud horizontal extension should be 30 km for case 1 and 6 km for case 2
correspondingly. The case of the stochastic cloud top structure corre-
sponds to real cumulus clouds and the case of the cloud inner parameter
variations corresponds to real stratus clouds. Different combinations of
the absorption and scattering coefficients in the cloud layer and different
scales of the horizontal and vertical heterogeneity have been considered
in the study by Hignett and Taylor (1996) and the authors has revealed
that “the internal inhomogeneity in the cloud microphysics and in the
macrophysical structure in terms of cloud thickness are both important
inthedeterminationofthecloudradiativeproperties”.
4. In addition to other reasons the anomalous absorption in clouds is
suggested to be explained with the water vapor absorption within the
absorption bands in the NIR spectral region, which has not been ac-
counted for before (Evans and Puckrin 1996; Crisp and Zuffada 1997;
Nesmelova et al. 1997; O'Hirok and Gautier 1997; Savijarvi et al. 1997;
Harshvardhan et al. 1998; Ramaswami and Freidenreih 1998). However,
while computing, the detailed and careful accounting of the molecular
absorption in the NIR region has not provided the observed magnitude
of the cloud absorption (Kiel et al. 1995; Ramaswami and Freidenreih
1998). Besides, the results of spectral observations (Titov and Zhuravleva
1995) have demonstrated the strongest effect of the anomalous absorp-
tioninthevisualspectralregion,wherethewatervaporabsorptionistoo
weak. Thus, it is seen that themolecular absorption by water vapor in the
NIR region is not enough for an explanation of anomalous absorption.
5. The microphysical properties of clouds have been implied as a reason
of the excessive absorption in various studies (Ackerman and Cox 1981;
Wiscombe et al. 1984; Hegg 1986; Ackerman and Stephens 1987). Very
large drops of the cloud are considered in the studies by Ackerman and
Stephens (1987) and Wiscombe et al. (1984); it is suggested the presence
of them actually increases the radiation absorption within clouds, but it
is too weak and insufficient to explain the anomalous absorption. The
authors of another study (Hegg 1986) have calculated in detail the optical
and radiative parameters of clouds containing two-layer particles with
absorbing nuclei and a nonabsorbent shell and have not obtained high
enough values of the absorption by clouds either. In all considered mod-
els, the noticeable absorption by clouds succeeds only when assuming
a significant amount of the atmospheric aerosols (Wiscombe 1995; Bott
1997; Vasilyev A and Ivlev 1997).
6. The authors of three studies (Kiel et al. 1995;Hignett and Taylor 1996; Ra-
maswami and Freidenreich 1998) have considered the above-mentioned
reasons in different combinations and they conclude that with certain