Assessment of CrIS Radiometric and Spectral Accuracy using Community Radiative Transfer Model

The Cross-track infrared Sounder (CrIS) on Suomi National Polar-orbiting Partnership Satellite (NPP), is a very important Fourier transform spectrometer, and provides soundings of the atmosphere with 1305 channels. The sounding information will be used to enhance weather forecast and help improve understanding of climate change. Quantifying the CrIS radiometric and spectral accuracy and bias with other hyper-spectral infrared sensors such as Atmospheric Infrared Sounder (AIRS) on Aqua and Infrared Atmospheric Sounding Interferometer (IASI) on MetOp are crucial for creating fundamental climate data records and intercalibrating other infrared sensors. In this study, we first detect the clear scenes over ocean for CrIS and IASI at the same day. The IASI inhomogeneities among the four FOVs at nadir (the 14 and 15th field of regard (FOR)) are analyzed. The same analyses are done for CrIS 9 FOVs at nadir. After understanding the radiance uncertainties among FOVs, we then employ a double difference method to assess the CrIS radiance consistency with the IASI. This method uses the differences of CrIS/IASI relative to the radiative transfer (RT) simulations using the Community Radiative Transfer Model (CRTM) and European Centre for Medium-Range Weather Forecasts (ECMWF) forecast fields. The double difference method can remove the forward RT model and NWP model errors, and the intercomparisons are made on common spectral scale by convolving IASI spectral to CrIS spectral. The CrIS spectral accuracy is also tested by scaling the wavenumber with constant factors at high resolution using Fast Fourier transform. The scaled spectrums are then compared to the CRTM simulated spectrums to get the highest correlation, which is an indicator of the true spectrum. All three CrIS bands and 9 FOVs are tested separately. These results can be used to validate the CrIS off-axis self-apodization correction.