A Rapid Transmittance Procedure for Use in Numerical Models

When satellite radiance are used to determine the atmospheric state, a method for rapidly calculating the satellite measurements expected for a given atmospheric state is required. Two methods have gained widespread acceptance. In one version, transmittances are calculated at even increments of pressure. This has been implemented in a number of forms under a variety of names (Tranh, RTTOVS, PLOD, and PFAST). This approach works well for the fixed gases, but is not as accurate for water vapor. For water vapor, an alternative approach that calculates transmittances at fixed intervals of absorber amounts has been implemented as OPTRAN. OPTRAN has also been applied to fixed gases and is used operationally at the National Centers for Climate and Environmental Prediction / Environmental Modeling Center (NCEP/EMC). In this application, several optimizations have been made that are not inherently limited to the OPTRAN version. At the current time, OPTRAN is the most accurate for water vapor and has the fastest execution time, while the constant pressure approach is the most accurate for the dry gases. Ozone is more variable than the dry gases but not as variable as water vapor and the choice for the best method is not as clear. In other words, either can be used. When OPTRAN was used for operational processing, some optimizations were developed that are not necessarily limited to OPTRAN. In this paper, these optimizations are applied to the constant pressure approach to provide an algorithm with the best combination of speed and accuracy for the dry gases and the current OPTRAN approach for water vapor. Results in terms of accuracy and speed will be reported.