A fast and accurate model for microwave radiance calculations

A fast and accurate radiative transfer model was developed primarily for microwave remote sensing and radiance data assimilation applications. Radiances at any direction were expressed by formal solution while the scattering source term was determined by a 4-stream discrete ordinate solution using Henyey-Greenstein scattering phase function. The accuracy of this model is analyzed by comparing it to a polarized 32-stream discrete ordinate model with Mie scattering phase function, and an unpolarized 32-stream discrete ordinate model with Henyey-Greenstein scattering phase function. It shows that this model is fast and accurate enough for practical use in microwave remote sending and data assimilation, the error being less than 3 K comparing to the accurate polarized 32-stream model. It is also compared with a 2-stream Eddington model. The results show that it is more accurate than the Eddington model, especially for ice cloud conditions.