Characterization and validation of methane products from the Atmospheric Infrared Sounder (AIRS)

This paper presents the characterization and validation of retrievals of atmospheric methane (CH4) vertical profiles by the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua platform. AIRS channels near 7.6 μm are used for CH4 retrieval, and they are most sensitive to the middle to upper troposphere, i.e., about 200–300 hPa in the tropics and 400–500 hPa in the polar region. The atmospheric temperature‐humidity profiles, surface skin temperature, and emissivity required to derive CH4 are obtained from retrievals using separate AIRS channels and the Advanced Microwave Sounding Unit (AMSU). Comparison of AIRS retrieved profiles with some in situ aircraft CH4 profiles implied that the forward model used in the AIRS retrieval system V4.0 required a 2% increase in methane absorption coefficients for strong absorption channels, and this bias adjustment was implemented in the AIRS retrieval system V5.0. As a new operational product in V5.0, AIRS CH4 were validated using in situ aircraft observations at 22 sites of the NOAA Earth System Research Laboratory, Global Monitoring Division (NOAA/ESRL/GMD), ranging from the Arctic to the tropical South Pacific Ocean, but their altitudes are usually above 300 hPa. The results show the bias of the retrieved CH4 profiles for this version is −1.4∼0.1% and its RMS difference is about 0.5–1.6%, depending on altitude. These validation comparisons provide critical assessment of the retrieval algorithm and will continue using more in situ observations together with future improvement to the retrieval algorithm. AIRS CH4 products include not only the CH4 profile but also the information content. As examples, the products of AIRS CH4 in August 2004 and the difference of CH4 in May and September 2004 are shown. From these results a few features are evident: (1) a large AIRS CH4 plume southwest of the Tibetan plateau that may be associated with deep convection during the Asian summer monsoon; (2) high mixing ratios of AIRS CH4 in southeastern Asia and in the high northern hemisphere in the summer; and (3) the increase of AIRS CH4 from May to September in the high northern hemisphere that is likely linked with wetland emission but needs more study. Further analysis of these data and its comparison with model data will be addressed in a separate paper.