Wednesday, 13 January 2016
The Cross-track Infrared Sounder (CrIS) on the Suomi NPP (SNPP) is a high spectral resolution Fourier Transform Spectrometer (FTS). The instrument was developed primarily to provide high vertical resolution of temperature and water vapor profiles for weather forecasting. The 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. The interferometer axis is nominally centered in the middle of FOV5. All nine detectors detect off-axis rays and thus require corrections for the instrument self-apodization. The spectral calibration accuracy of CrIS 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. CrIS optical alignments and FOV geometry parameters were pre-determined during thermal vacuum gas cell testing and refined to optimal values after launch. To take the metrology laser wavelength variation into account, CrIS provides a neon calibration system which uses spectrally ultra stable neon emission lines to periodically measure the metrology laser wavelength roughly once per orbit. Quantifying the spectral accuracy of CrIS, which is directly related to the radiometric accuracy, is crucial for improving its data assimilation in the numerical weather prediction and climate applications. 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 Line-by-Line Radiative Transfer Model (LBLRTM) and European Centre for Medium-Range Weather Forecasts (ECMWF) forecast fields to systematically evaluate the spectral accuracy of CrIS full resolution SDR, processed at NOAA/STAR since December 2014, 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.
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