Assessment of Hyper-spectral Infrared Sensors CrIS and IASI Spectral Accuracy Using Community Radiative Transfer Model

Both the Infrared Atmospheric Sounder Interferometer (IASI) on the METOP A/B platforms and the Cross-track Infrared Sounder (CrIS) on the Suomi NPP (SNPP) are high spectral resolution Fourier Transform Spectrometer (FTS). These instruments were developed primarily to provide high vertical resolution of temperature and water vapor profiles for weather forecasting. Their high accuracy, high stability, and extensive information about trace gases, cloud properties, and surface properties make these observations great potential for climate applications. One CrIS scan comprises 30 fields of regard (FORs) Earth Scenes. Each FOR includes nine fields of view (FOVs) which are arranged in a 3 × 3 array of detectors. IASI has same number of FORs as CrIS in each scan, but each FOR includes four FOVs instead. For CrIS, the interferometer axis is nominally centered in the middle of FOV5. All nine detectors detect off-axis rays and thus require software corrections for the instrument self-apodization. For IASI, all four detectors are off-axis rays and require the corrections of the self-apodization effect. The spectral calibration accuracy of CrIS and IASI can be compromised by the inaccurate geometry of the focal plane detectors that determines the exact alignment of the detectors to the interferometer boresight axis and inaccurate knowledge of the metrology laser wavelength that determines the interferogram optical path difference (OPD) sampling positions. Quantifying the spectral accuracy of CrIS/IASI, which is directly related to the radiometric accuracy, is crucial for improving its data assimilation in the numerical weather prediction. Two basic spectral calibration methods are used to assess the CrIS Sensor Data Records (SDR) spectral accuracy and stability: 1). Relative spectral calibration, which uses two uniform observations to determine frequency offsets relative to each other; 2). Absolute spectral calibration, which requires an accurate forward model to simulate the top of atmosphere radiance under clear conditions and correlates the simulation with the observed radiance to find the maximum correlation. In this study, we use Community Radiative Transfer Model (CRTM) and European Centre for Medium-Range Weather Forecasts (ECMWF) forecast fields to systematically evaluate the spectral accuracy of CrIS and IASI at different spectral ranges for all three bands. Based on these results, the best spectral ranges can be chosen to evaluate the spectral accuracy and stability for CrIS, IASI and future FTS infrared instruments. Long-term CrIS spectral accuracy and spectral stability will also be presented in this study.