Intercomparison of radiative transfer codes under non-local thermodynamic equilibrium conditions

Limb infrared spectra calculated by the Optimized and Precise Radiative Transfer Algorithm (KOPRA), GENLN2, Reference Forward Model (RFM), and Simulation Program for Infrared Radiative Transfer (SPIRT) radiative transfer algorithms were intercompared for both local thermodynamic equilibrium (LTE) and non-local thermodynamic equilibrium (non-LTE) conditions, in the latter case for given profiles of vibrational temperatures. The overall agreement is usually better than approximately 0.5%, except for isolated phenomena that have been assessed in more detail. Most spectral radiance discrepancies occur also under LTE conditions and are attributed to different treatment of far wings of lines, integration schemes, and mass-weighted averaging of atmospheric temperature and pressure along slant path segments as well as different spectral sampling during the calculation of monochromatic radiances.