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

Abstract

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.

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