Extended inter-comparison of collocated MetOp-A AVHRR-IASI brightness temperature data and its implication for AVHRR calibration

Abstract

The collocated measurements in 3.74μm, 11μm, and 12μm channels from Advanced Very High Resolution Radiometer (AVHRR) and corresponding simulated AVHRR measurements using hyper-spectral Infrared Atmospheric Sounding Interferometer (IASI) observations are inter-compared. Both of the instruments are placed on MetOp-A satellite launched in October 2006. Because IASI observations did not have complete spectral coverage over AVHRR 3.74μm channel, Line-By-Line Radiative Transfer Model (LBLRTM) simulated IASI spectra were generated to enable complete IASI coverage for this channel. It is shown that the large AVHRR minus IASI negative bias in 3.74μm channel can be explained more or less completely by the part of the AVHRR spectral band not seen by IASI which is an indication of relatively large absorption in that particular portion of the AVHRR spectral band. The near similarity between slopes of bias dependency on scene radiance from the model and those derived from observations with respect to 3.74μm channel indicate that it could be mostly the CO2 absorption in the higher wave-numbers experienced by AVHRR and not experienced by IASI causing the discrepancy between these two observations. Thus the study confirms that AVHRR short wave infrared channel (3.74 μm) is performing very well with no indication, of spectral uncertainties, or of significant radiometric uncertainties. On the other hand, the results suggest that AVHRR 3.74 um channel experiences significant CO₂ absorption which may disqualify it from being recognized as a "window channel." With respect to long wave infrared channels at 11 μm and 12 μm the study reveals that the bias between the two measurements undergo seasonal variations, however, with small magnitudes.