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Rathke, C, Fischer, J (2002). Evaluation of four approximate methods for calculating infrared radiances in cloudy atmospheres. JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER, 75(3), 297-321.

The goals of this study are the estimation of the accuracy of four approximations for the calculation of infrared radiances emerging at the top and bottom of a cloud layer and the identification of those useful for radiative transfer modelling and remote sensing. Therefore, results of two simple methods, the absorption approximation and the effective beam emissivity parameterization, as well as of two more elaborate methods, the multiple-stream absorption approximation with scattering effects and the delta-Eddington two-stream source function technique, are compared with exact results of the discrete ordinate radiative transfer method. The relative errors of the different approaches are evaluated at nine wavelengths located in the mid-infrared (3.99-17.86 mum), near the position of the channels of the MODIS and SEVIRI satellite spectrometers, over a wide range of cloud microphysical properties, observation angles, and cloud and clear-sky temperature contrasts. For the multiple-stream absorption approximation, the errors are presented as a function of the number of streams. The computational efficiencies of the four approximations are also compared. It is found that in several spectral ranges, the best accuracy is attained with the multiple-stream absorption approximation with scattering effects, and that the delta-Eddington two-stream source function technique offers the best overall applicability. Detailed error figures and tables are provided in order to facilitate the identification of particular pros and cons of each method. (C) 2002 Elsevier Science Ltd. All rights reserved.



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