Fast computation of microwave radiances for data assimilation using the "successive order of scattering" method

Fast and accurate radiative transfer (RT) models are crucial in making use of microwave satellite data feasible under all weather conditions in numerical weather prediction (NWP) data assimilation. A multistream "successive order of scattering" (SOS) RT model has been developed to determine its suitability in NWP for computing microwave radiances in precipitating clouds. Results show that the two-stream SOS model is up to 10 times as fast as and is as accurate as the commonly used delta-Eddington model for weaker scattering [column scattering optical depth (CSOD) < 0.01], but it is less accurate and is slower for higher frequencies (> 30 GHz) in cases of moderately strong to strong scattering (CSOD > 5). If two- and four-stream SOS models are used in combination, however, it was found that 85.5-GHz brightness temperatures computed for V X F global forecast fields were more accurate (< 0.5 K vs 1.5 K for CSOD > 0.1) and were executed 4 times as fast as the delta-Eddington model. The SOS method has been demonstrated as an alternative to other fast RT models for providing accurate and very rapid multiple-scattering calculations at thermal wavelengths for remote sensing studies and demanding applications such as operational NWP data assimilation.