Contributions to climate studies from four years of hyperspectral data from the Atmospheric Infrared Sounder (AIRS)

The Atmospheric Infrared Sounder (AIRS), launched on the EOS Aqua spacecraft on May 4, 2002, has been in routine operations since late August 2002. In this paper we analyze the first four years of AIRS Level 1B data (calibrated radiances) from September 1, 2002 through August 31, 2006 for stability and accuracy of the radiometric and spectral calibration. Both are key to linking the AIRS record to previous and future instruments. The analysis shows that the AIRS absolute radiometric accuracy is better than 200 mK with a stability of better than 10 mK/year. Both upper limits are due to the difficulty of finding ground truth data of sufficient quality. The instantaneous knowledge of the channel frequencies is better than 1 ppmf. Compared to the nominal frequency set adopted in September 2002, AIRS channel frequencies are slowly shifting to higher frequencies at the rate of about 1 ppmf/year. The term "ppmf" means "parts per million in frequency". For all but the most critical climate applications, using the nominal frequency set is sufficient for the radiative transfer code used in atmospheric parameter retrievals.