Contribution of the AIRS shortwave sounding channels to retrieval accuracy

AIRS contains 2376 high spectral resolution channels between 650 cm-1 and 2665 cm-1, including channels in both the 15 micron (near 667 cm-1) and 4.2 micron (near 2400 cm-1) CO2 sounding bands. Use of temperature sounding channels in the 15 micron CO2 band has considerable heritage in infra-red remote sensing. Channels in the 4.2 micron CO2 band have potential advantages for temperature sounding purposes because they are essentially insensitive to absorption by water vapor and ozone, and also have considerably sharper lower tropospheric temperature sounding weighting functions than do the 15 micron temperature sounding channels. Potential drawbacks with regard to use of 4.2 micron channels arise from effects on the observed radiances of solar radiation reflected by the surface and clouds, as well as effects of non-local thermodynamic equilibrium on shortwave observations during the day. These are of no practical consequences, however, when properly accounted for. We show results of experiments performed utilizing different spectral regions of AIRS, conducted with the AIRS Science Team candidate Version 5 algorithm. Experiments were performed using temperature sounding channels within the entire AIRS spectral coverage, within only the spectral region 650 cm-1 to 1614 cm-1; and within only the spectral region 1000 cm-1-2665 cm-1. These show the relative importance of utilizing only 15 micron temperature sounding clouds, only the 4.2 micron temperature sounding channels, and both, with regards to sounding accuracy. The spectral region 2380 cm-1 to 2400 cm-1 is shown to contribute significantly to improve sounding accuracy in the lower troposphere, both day and night.