Efficient parameterization of the infrared effective beam emissivity of semitransparent atmospheric layers

Abstract

A novel parameterization is provided that relates the effective beam emissivity of a semitransparent atmospheric layer (emissivity between 0.05 and 0.95) to its optical depth, single-scattering albedo, and asymmetry factor and to the zenith viewing angle and estimates of the incoming radiation. It is derived from discrete ordinate radiative transfer calculations performed for optical properties representative for liquid water and ice clouds as well as aerosols. The formulation is kept simple and general and can be applied to any infrared window wavelength between 3 and 40 microns. It enables the efficient consideration of scattering of infrared radiation in fast radiative transfer models and remote-sensing algorithms. The greatest improvement over other straightforward approximations occurs when the single-scattering albedo is larger than 0.50. The parameterization also permits a better interpretation of the ratio of the effective optical depths at two wavelenghts, which is highly correlated with cloud and aerosol microphysics. The possibilities are illustrated by a sample analysis of an advanced very high resolution radiometer image containing infrared signatures of contrails and stratocumulus clouds.