A polarized discrete ordinate scattering model for radiative transfer simulations in spherical atmospheres with thermal source

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This work details the development of a new discrete ordinate scattering algorithm, part of the Atmospheric Radiative Transfer Simulator (ARTS), and its applications, such as studying the influence of cirrus clouds on microwave limb sounding. The theoretical foundation is based on electromagnetic scattering theory, defining key quantities and discussing methods for computing single scattering properties of small particles. The derivation of the vector radiative transfer equation, central to the model, is outlined. To accurately represent clouds as scattering media, input on their micro-physical state—characterized by particle phase, size and shape distributions, orientation, ice mass or liquid water content, and temperature—is essential. The model employs the Discrete Ordinate ITerative (DOIT) method to solve the vector radiative transfer equation, addressing challenges posed by the spherical geometry of the atmosphere necessary for limb radiance simulations. Numerical issues, grid optimization, and interpolation methods are explored. The new scattering algorithm was validated against three contemporaneous models, showing good agreement and reinforcing confidence in the new approach. Scattering simulations for limb- and down-looking geometries were conducted across various atmospheric conditions, revealing significant impacts of cloud particle characteristics on brightness temperatures and polarization of microwave radiation, p