Radiometric and Spectral Inter-comparison of IASI: IASI-A / IASI-B, IASI / AIRS, IASI / CrIS

IASI-A has been in operation on board MetOp-A since June 2007 and is now considered as a reference sensor for the radiometric calibration at high spectral resolution. IASI-B on board MetOp-B was launched on September 17th, 2012, and finished its commissioning phase in April 2013. CNES is in charge of the performance monitoring of the two IASI instruments. One of the associated tasks is to compare the calibration of IASI-A with IASI-B, and to compare each one with other similar sensors (AIRS and CrIS). Our goal is to check the IASI data quality with an external reference and to establish the high radiometric accuracy required for climate data records. After more than one year of IASI-B data, we are now in long-term routine monitoring. Our inter-comparisons are now valid on every season and observation condition. We first present the radiometric inter-calibration between IASI-A and IASI-B. An original method has been developed based on the observations of common regions with a 50 minutes temporal gap and in different viewing conditions due to the orbital configuration. The input dataset filtering has been tuned to minimize the geophysical biases. The sounding pixels are spatially averaged on a 300x300km² area. Radiometry is compared at full spectral resolution. We also present the radiometric inter-calibration of IASI (A and B) with Aqua/AIRS and NPP/CrIS. Our method is based on simultaneous nadir overpasses, occurring at high altitudes. The difference in spectral sampling and resolution is handled through either the use of spectrally broad pseudo-channels or the reconvolution of IASI spectra. Results for with more than one year of data for IASI-A / AIRS, IASI-B / AIRS, IASI-A / CRIS, IASI-B / CRIS and IASI-A / IASI-B show small biases of the order of 0 to 0.2K. The five sounders show very stable inter-calibration. Several diagnosis tools are presented to understand the small residuals of inter-calibration. An indirect inter-comparison IASI-B / IASI-A is also presented using double differences with AIRS and CRIS. We also present a long-term monitoring of the spectral cross-calibration, based on the cross-correlation of spectra on several spectrally broad pseudo-channels.