Characterization and evaluation of AIRS-based estimates of the deuterium content of water vapor

Single-pixel tropospheric retrievals of HDO and H2 O concentrations are retrieved from Atmospheric Infrared Sounder (AIRS) radiances using the optimal estimation algorithm developed for the Aura Tropospheric Emission Spectrometer (TES) project. We evaluate the error characteristics and vertical sensitivity of AIRS measurements corresponding to 5 d of TES data (or five global surveys) during the Northern Hemisphere summers between 2006 and 2010 (∼ 600 co-located comparisons per day). We find that the retrieval characteristics of the AIRS deuterium content measurements have similar vertical resolution in the middle troposphere as TES but with slightly less sensitivity in the lowermost troposphere, with a typical degrees of freedom (DOFS) in the tropics of 1.5. The calculated measurement uncertainty is ∼ 30 ‰ (parts per thousand relative to the deuterium composition of ocean water) for a tropospheric average between 750 and 350 hPa, the altitude region where AIRS is most sensitive, compared to ∼ 15 ‰ for the TES data. Comparison with the TES data also indicates that the uncertainty of a single target AIRS HDO / H2 O measurement is ∼ 30 ‰. Comparison of AIRS and TES data between 30◦ S and 50◦ N indicates that the AIRS data are biased low by ∼ −2.6 ‰ with a latitudinal variation of ∼ 7.8 ‰. This latitudinal variation is consistent with the accuracy of TES data compared to in situ measurements, suggesting that both AIRS and TES have similar accuracy.