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Abstract 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. Copyright © 2006 Royal Meteorological Society.
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