Atmospheric infrared fast transmittance models: a comparison of two approaches

The next generation of atmospheric temperature and humidity sounders will have thousands of radiometrically accurate spectral channels throughout the infrared. The retrieval of atmospheric parameters from these radiances will stress both the accuracy and efficiency of forward model radiative transfer algorithms. We are developing a forward model for the Atmospheric Infrared Sounder (AIRS) which will fly on the EOS PM platform. The work presented here is based on algorithms developed over a number of years by McMillin, Fleming, and others for low resolution infrared sounders (HIRS) and microwave sounders. We have developed tow 'high resolution' AIRS forward model algorithms for water vapor, one based on atmospheric layers with fixed pressures and variable water amounts, and other based on layers of fixed absorber amount but with variable pressures. These algorithms are compared for speed, accuracy, ease of development, and other factors that must be considered in developing a complex operational retrieval system.