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Published January 2016 | Published
Journal Article Open

Information-rich spectral channels for simulated retrievals of partial column-averaged methane


Space‐based remote sensing of the column‐averaged methane dry air mole fraction (XCH_4) has greatly increased our understanding of the spatiotemporal patterns in the global methane cycle. The potential to retrieve multiple pieces of vertical profile information would further improve the quantification of CH_4 across space‐time scales. We conduct information analysis for channel selection and evaluate the prospects of retrieving multiple pieces of information as well as total column CH_4 from both ground‐based and space‐based near‐infrared remote sensing spectra. We analyze the degrees of freedom of signal (DOF) in the CH_4 absorption bands near 2.3 μm and 1.6 μm and select ∼1% of the channels that contain >95% of the information about the CH_4 profile. The DOF is around 4 for fine ground‐based spectra (resolution = 0.01 cm^(−1)) and 3 for coarse space‐based spectra (resolution = 0.20 cm^(−1)) based on channel selection and a signal‐to‐noise ratio (SNR) of 300. The DOF varies from 2.2 to 3.2 when SNR is between 100 and 300, and spectral resolution is 0.20 cm^(−1). Simulated retrieval tests in clear‐sky conditions using the selected channels reveal that the retrieved partial column‐averaged CH_4 values are not sensitive to the a priori profiles and can reflect local enhancements of CH_4 in different partial air columns. Both the total and partial column‐averaged retrieval errors in all tests are within 1% of the true state. These simulated tests highlight the possibility to retrieve up to three to four pieces of information about the vertical distribution of CH_4 in reality.

Additional Information

© 2015 The Authors. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. Received 15 JUN 2015; Accepted 16 DEC 2015; Accepted article online 18 DEC 2015; Published online 16 JAN 2016. The authors would like to thank Jack Margolis, John Worden, and Christian Frankenberg for their valuable comments. The authors also gratefully acknowledge detailed and helpful comments from three anonymous reviewers. This research is supported by the Orbiting Carbon Observatory 2 (OCO‐2) project, a NASA Earth System Science Pathfinder (ESSP) mission, and Project JPL.1382974 to California Institute of Technology.

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