Linear trends and closures of 10- year observations of AIRS stratospheric channels

The AIRS (Atmospheric Infrared Sounder) Level-1b radiances have been shown to be well calibrated (~0.3K or higher) and have little secular drift (~4mK/year) since its operation started in September 2002. We study the linear trends of 10 years (2003-2012) of AIRS global-mean radiances in the CO2 ?2 band that are sensitive to emissions from the stratosphere (stratospheric channels). AIRS lower-stratospheric channels have a cooling trend of no more than 0.23K/decade while its middle-stratospheric channels consistently show a statistically significant cooling trend as large as 0.58K/decade. The 95% confidence interval for the trend is ~±0.20K/decade. Two sets of synthetic AIRS radiances are computed using the PCRTM (Principle Component-based Radiative Transfer Model), one based on a free-running GFDL AM3 model over the same period and one based on ERA-interim reanalysis. The GFDL AM3 simulations overestimate the cooling trends in the middle-upper-stratospheric channels while slightly underestimate in the lower-stratospheric channels. The synthetic radiances based on ERA-interim reanalysis, on the opposite, have statistically significant positive trends at virtually all stratospheric channels. This confirms the challenge to GCM modeling and reanalysis community for a better simulation or assimilation of the stratospheric climate. We show that the linear trends in AIRS radiances can be reproduced to a large extent by the spectral radiative kernel technique and the trends from the AIRS L2 temperature retrievals and from the change of CO2. This suggests a closure between AIRS L1 radiances and L2 retrievals and potential merit of AIRS data in the studies of stratosphere changes.