Recent improvements to the fast transmittance model, OPTRAN, for NOAA data assimilation

In this paper we present recent improvements to the fast and accurate transmittance-calculation procedure, Optical Path TRANsmittance (OPTRAN), which is used for satellite data assimilation in NOAA. One of the most important improvements is the introduction of a correction term to account for the polychromatic effect of the channel transmittance. This replaces the effective transmittance widely used in many current fast models. Use of the correction term solves some numerical problems that are associated with the use of effective transmittances, and greatly reduces the line-by-line computational burden, while allowing for the efficient inclusion of multiple gases. Other improvements include additional predictors and changes to the absorber space, as well as some procedures to reduce the number of coefficients for its implementation for data assimilation. Significant improvements in the accuracy of the OPTRAN for High-resolution Infrared Radiation Sounders (HIRS) and Atmospheric Infrared Sounder (AIRS) are demonstrated.