Multiple-scattering microwave radiative transfer for data assimilation applications

A multiple-scattering radiative transfer model for microwave radiance data assimilation in global numerical weather-prediction models is presented. The model is part of the RTTOV software package and includes forward, tangent-linear, adjoint and Jacobian models. The model is based on the Eddington approximation to radiative transfer which produces mean errors of less than 0.5 K at the targeted microwave frequencies between 10 and 200 GHz. The simplified treatment of subgrid-scale cloud cover may produce biased model calculations that show a maximum at 0.5 cloud cover and may reach several degrees K. These errors may be corrected with a simple bias correction. Linearity tests indicate that, given a screening procedure that excludes situations in which the model responds nonlinearly to input perturbations, channels near 50.3, 19.35, 22.235 and 183.31 GHz may be used in global radiance data assimilation.