ATMOSPHERIC TRANSMITTANCE OF AN ABSORBING GAS .4. OPTRAN - A COMPUTATIONALLY FAST AND ACCURATE TRANSMITTANCE MODEL FOR ABSORBING GASES WITHFIXED AND WITH VARIABLE MIXING RATIOS AT VARIABLE VIEWING ANGLES

A fast and accurate method for the generation of atmospheric transmitt ances, optical path transmittance (OPTRAN), is described. Results from OPTRAN are compared with those produced by other currently used metho ds. OPTRAN produces transmittances that can be used to generate bright ness 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 whi ch it will not adversely affect atmospheric retrievals. OPTRAN is the product of an evolution of approaches developed earlier at the Nationa l Environmental Satellite, Data, and Information Service. A major feat ure of OPTRAN that contributes to its accuracy is that transmittance i s obtained as a function of the absorber amount rather than the pressu re.