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Effect of Greenhouse Gases on Thermal Emissivity by Clouds

Greenhouse gases, most importantly water vapor, increase the emissivity and decrease the albedo of clouds for thermal radiation. The modifications, which can be of order 10% for optically thick clouds, depend on the attenuation coefficient $α^{\{g\}}$ of the greenhouse gases, and also on the attenuation coefficient, $α^{\{c\}}$, the single-scattering albedo $\tilde ω^{\{c\}}$, and the scattering phase function $p^{\{c\}}(μ,μ')$ of the cloud particulates. Cold, high-altitude clouds with low partial pressures of water vapor have smaller emissivities for thermal radiation and larger albedos than otherwise identical but warmer low-altitude clouds with higher partial pressures of water vapor. In $2n$-stream scattering theory, these phenomena can be quantified with the intensity emissivities $\varepsilon_{i}$ of the streams $i=1,2,3,\ldots, 2n$, and with upward or downward flux emissivities, $\varepsilon_{\bf u}$ and $\varepsilon_{\bf d}$. The emissivities are the ratios of the outgoing thermal intensities or fluxes to those of a reference black cloud. Emission from optically-thick, isothermal clouds with scattering, as well as absorption and emission, is limb darkened. Intensity emissivities $\varepsilon_i$ for streams that are nearly normal to the cloud surface are larger than those of more nearly horizontal streams. The limb darkening increases with increasing values of the single scattering albedo $\tilde ω$. For fixed values of $\tildeω$, the onset of limb darkening with increasing zenith angle is more abrupt for phase functions with more forward scattering. Black clouds, which have only absorption and emission but no scattering, have unit (Lambertian) emissivities, $\varepsilon_i = 1$, for all stream directions.