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In most weather forcasting models, multiple scattering properties of the terrestrial radiation are treated in an approximate manner. In climate models, the thin clouds play a very important role in the energy redistribution processes. The radiative impact of cirrus clouds has been suggested to constitute an important atmospheric `thermostats' (Ramanathan and Collins, 1992).
An accurate and also simple parameterization of cloud optical properties such as extinction coefficient, single scattering albedo and asymmetry factor is needed in radiative transfer calculations of the terrestrial radiation in climate models if the climate system is sensitive to the single scattering properties in the longwave radiation. Figure 4 and 5 show that the longwave cloud radiative forcing is indeed sensitive to the changes in cloud equivalent radius if the cloud is not too thick. For clouds which are black in terrestrial radiation, the scattering is still very important. Figure 6 shows that clouds with different equivalent radii may have significantly different longwave radiative forcing.
Figure 4: The longwave cloud radiative forcing at
the top of the atmosphere for different
equivalent radii with fixed cloud liquid water path (in 
)
Figure 5: The change in longwave cloud radiative forcing caused by 
increase in equivalent radius for different
equivalent radii with fixed cloud liquid water path(in )
Figure 6: The change in longwave cloud radiative forcing
for black clouds with different equivalent radii.
