Two-year comparison of radiances from the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI)

Abstract

The radiometric intercomparison of instruments is a key element in developing climate-quality data records. In this study we compare data from the first two years of the Infrared Atmospheric Sounding Interferometer (IASI) with the matching data from the Atmospheric Infrared Sounder (AIRS). We compare observed spectra in cloud-free areas of the tropical oceans at night to spectra calculated using data from the European Centre for Medium-Range Weather Forecasts (ECMWF). We use five frequencies-three window channels, one mid-tropospheric sounding channel, and one lower stratospheric sounding channel. The use of ECMWF data as a transfer standard permits comparisons of many more points distributed more widely over the globe than is possible with the traditional simultaneous nadir overpass (SNO) technique. The analysis shows that AIRS and IASI daily mean brightness temperatures track each other within 100 mK, in spite of the fact that the instruments are in different orbits. AIRS was launched into polar orbit on the EOS Aqua spacecraft on May 4, 2002. It is a grating spectrometer with 2378 channels in the range 3.7 to 15.4 microns. IASI was launched into polar orbit in October 2006 on the METOP-A spacecraft. IASI is a Fourier transform spectrometer covering 3.7 to 15.5 microns in three bands with a total of 8461 channels.