The new σ-IASI code for all sky radiative transfer calculations in the spectral range 10 to 2760 cm-1: σ-IASI/F2N

The paper describes a recently developed forward model (sigma-IASI/F2N) to produce spectral radiances from the far to near-infrared spectrum (100 to 2760 cm-1). The model is a pseudo-monochromatic radiative transfer tool that exploits lookup tables to compute the optical depths of atmospheric gas and clouds. Multiple scattering effects are accurately included in both the Far IR and Thermal IR by using a scaling method for cloud and aerosol radiative properties parameterized in terms of their effective radius, which allows them to be handled adopting the same formalism used in the clear sky. In this paper we apply a novel approach to a classical scaling method in the thermal IR relying on our improved parametrization of backscattering parameter over that used by Chou (Martinazzo et al., 2021), while in the Far IR a corrective term is introduced. The code is written in Fortran and runs on Unix-based (Linux and macOS) or MS Windows operating systems. sigma-IASI/F2N can be used to develop custom versions of fast-forward modules for satellite instruments working in the infrared spectral range, such as the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) and the Polar Radiant Energy in the Far-InfraRed Experiment (PREFIRE) missions. We discuss the sigma-IASI/F2N performance in simulating a set of ECMWF analyses at the global scale. For this purpose, we compare observations from the Infrared Atmospheric Sounding Interferometer (IASI) flying on MetOp B, and C. Results show that sigma-IASI/F2N can easily ingest ECMWF analyses data and accurately reproduce cloud patterns. We also show that the difference between sigma-IASI/F2N simulations and corresponding IASI observations is below 1 K in the 8-12 um window region, which is mainly affected by the water vapor continuum absorption and weak lines, while for night-time clear sky, the differences are below 0.3 K. again within the same window region.