First trends from four years of AIRS data in the middle and upper troposphere

The analysis of the first four years of Atmospheric Infrared Sounder (AIRS) data from clear tropical oceans shows some expected and unexpected trends. The credibility of the AIRS data for climate research is confirmed by two expected trends: The stability of the AIRS radiances, evaluate for atmospheric window channels relative to the Real Time Global SST from NCEP is better then 10 mK/year, and the effect of increase in the CO2 column abundance dominates the trends seen in CO2 sensitive channels. A 1.5 ppmv global increase in CO2 typically results in a 60 mK/year trend. There are a number of small, but significant and unexpected trends in the four years of data: The number clear ocean footprints has decreased by about one percent per year day and night, the thin cirrus signal has increased and the water sensitive channels at the tropopause have become warmer, while those at the 400 mb level are stable. The interpretation of the latter effect is complicated by the interaction between water vapor and temperature profiles. The unexpected trends relate to water vapor and clouds. At this point, with only four years of AIRS data, it is premature to speculate if these trends are evidence of inter-annual variability or if they are global warming related. Understanding of these effects require a longer time series and analysis of other than equatorial zones. With the increase of the estimated 5 year lifetime of AIRS to a currently predicted 12 year lifetime, the extension of the time series will be invaluable. AIRS is the first hyperspectral infrared sounder designed in support of weather forecasting and climate research. AIRS was launched on the EOS Aqua spacecraft in May 2002 into a 704 km altitude polar sun- synchronous orbit, and has is gathering 3 million spectra per day since September 2002.