Assessments of CrIS Spectral Calibration Accuracy and Stability

The Cross-track Infrared Sounder (CrIS) on Suomi National Polar-orbiting Partnership Satellite (S-NPP) is a Fourier transform spectrometer and provides a total of 1305 channels for sounding the atmosphere. Quantifying the CrIS spectral accuracy, which is directly related to radiometric accuracy, is crucial for improving its data assimilation in the numerical weather prediction. There are two basic frequency calibration methods developed for the hyper-spectral satellite instruments including AIRS, IASI, and CrIS. The first method requires an accurate forward model to simulate the top of atmosphere radiance under clear conditions. It then correlates the observed radiance to the simulated radiance by shifting the spectra at a certain range either from the observation or the simulation to find the maximum correlation. This method is called an absolute spectral calibration and typically requires a cloud detection algorithm and accurate radiative transfer (RT) model. The second method uses two uniform observations to determine frequency offsets relative to each other, referred as relative spectral calibration. In this study, both methods are used to access the CrIS spectral accuracy. The absolute spectral calibration uses the CrIS observations and the radiative transfer simulations using the Community Radiative Transfer Model (CRTM) and European Centre for Medium-Range Weather Forecasts (ECMWF) forecast fields. All three CrIS bands and 9 field of views (FOVs) are done separately for clear scenes over oceans at nadir (the 15th and 16th field of regards). The relative spectral calibration is performed from the same dataset as used in the absolute spectral calibration, except for comparing the observations from FOV 5 to those from other FOVs. Spectral calibration results show that CrIS has small and consistent FOV to FOV spectral shift in all three bands. However, the spectral shift is very stable during the satellite mission and better than the instrument requirement. Long-term CrIS SDR spectral stability is also presented in this study.