Version 6 Performance and Test Report large as 0.2 (20%) both day and night. For example over the Taklamakan desert in China there are large daytime differences, even although the corresponding LST differences are small. For nighttime data, differences are slightly larger with the largest variations distributed over more diverse areas, e.g. boreal forests of northern Eurasia, over parts of Canada, and China. The surface emissivity retrieval is set to have minimum limits of 0.65 (shortwave) and 0.92 (longwave). Emissivity retrievals that fall below these limits are generally considered unphysical values at AIRS spatial resolution (50km). However, on occasion emissivity values below these limits will be reported for Q0 (best) and Q1 (good) quality flags when there is a valid MODIS first guess emissivity that is unusually low, most likely due to cloud contamination. The logic for the minimum emissivity limit is as follows: For a valid MODIS first guess, the min allowable emissivity is min(MODIS - ∆ε, 0.65) for the shortwave, and min(MODIS - ∆ε, 0.92) for the longwave. The ∆ε, or 'maxchange' values are set to 0.25 (shortwave) and 0.1 (longwave), and set the limits to which the retrieval can move off the MODIS first guess.
Figure 38. AIRS mean emissivity spectra from 2003-2006 over the Grand Erg Oriental validation site in Algeria for V5 (left) using the NOAA surface regression first guess, and V6 (right) using the UW-Madison MODIS baseline-fit emissivity database. Improvement in emissivity spectral shape and absolute magnitude versus ASTER emissivity at this site is clearly seen.
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