A Model for Polarized Microwave Radiative Transfer in the CRTM

This JCSDA-funded project seeks to develop a new model for computing the Stokes parameters (i.e., I, Q, U, and V) within the Community Radiative Transfer Model (CRTM) for more effective use of current and future satellite microwave sensors in operational data assimilation. At higher microwave frequencies, observations have shown that current sensors can detect significant polarization signatures generated by large horizontally-oriented non-spherical ice particles in precipitating systems – a phenomenon commonly observed from the tropics to the mid-latitudes. Also, sensors like WindSat, which measures all four Stokes parameters for observing the ocean surface wind vector, have yet to be exploited in radiance data assimilation. Future instruments that measure submillimeter-wave (325-664 GHz) polarized radiances, such as the Ice Cloud Imager, will have sensitivity not only to ice water content but also ice particle shape. A polarized radiative transfer (RT) model will be necessary in helping to extract the full information content from these observations for use in data assimilation.

Our main effort will be to enhance the non-polarized Successive Order of Interaction (SOI) solver that is already available in the CRTM by expanding it to a vector RT model. The new model will be verified through a comparison to previously published results from a polarized doubling-adding model. Results for a high-resolution numerical weather prediction model simulation based on the latest single-scattering properties for non-spherical ice particles will also be used to demonstrate the characteristics of the polarization signatures over a range of frequencies. Future work will involve using this new RT model in all-sky data assimilation experiments to understand its impact on global forecast model skill.