Atmospheric transmittance of an absorbing gas. 4. OPTRAN: a computationally fast and accurate transmittance model for absorbing gases with fixed and with variable mixing ratios at variable viewing angles

A fast and accurate method for the generation of atmospheric transmittances, optical path transmittance (OPTRAN), is described. Results from OPTRAN are compared with those produced by other currently used methods. OPTRAN produces transmittances that can be used to generate brightness temperatures that are accurate to better than 0.2 K, well over 10 times as accurate as the current methods. This is significant because it brings the accuracy of transmittance computation to a level at which it will not adversely affect atmospheric retrievals. OPTRAN is the product of an evolution of approaches developed earlier at the National Environmental Satellite, Data, and Information Service. A major feature of OPTRAN that contributes to its accuracy is that transmittance is obtained as a function of the absorber amount rather than the pressure.