A Fast Radiative Transfer Model for Satellite Radiance Data Assimilation

The Optical Path Transmittance (OPTRAN) is a fast transmittance model in which regression is performed on surfaces of constant absorber amount, as opposed to fixed pressure levels. This frees pressure to become a predictor, implicitly includes satellite zenith angle, and permits arbitrary input atmospheric state profiles. The algorithm is uniform over all wavelengths, and thus has the same formulation for the infrared and microwave. Coefficients have been generated for all of the HIRS and AVHRR instruments on TIROS-N through NOAA-15, GOES 8 and 9 imager and sounder, METEOSAT 3 through 7, GMS-5, the MSU, SSM/T and SSM/T-2, AMSU-A and AMSU-B. The code has been developed as forward model, tangent linear, adjoint and jacobian for use in numerical weather prediction, and is being used as the operational radiance assimilation portion of the Global Data Assimilation System at the National Centers for Environmental Prediction/ Environmental Modeling Center. This paper will present sensitivity studies via the jacobian, and outline proposed improvements to the methodology.