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Volume 11, issue 6 | Copyright
Atmos. Meas. Tech., 11, 3815-3828, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 28 Jun 2018

Research article | 28 Jun 2018

Methane profiles from GOSAT thermal infrared spectra

Arno de Lange and Jochen Landgraf Arno de Lange and Jochen Landgraf
  • Earth Group, SRON Netherlands Institute for Space Research, Utrecht, the Netherlands

Abstract. This paper discusses the retrieval of atmospheric methane profiles from the thermal infrared band of the Japanese Greenhouse Gases Observing Satellite (GOSAT) between 1210 and 1310cm−1, using the RemoTeC analysis software. Approximately one degree of information on the vertical methane distribution is inferred from the measurements, with the main sensitivity at about 9km altitude but little sensitivity to methane in the lower troposphere. For verification, we compare the GOSAT-TIR methane profile retrieval results with profiles from model fields provided by the Monitoring Atmospheric Composition and Climate (MACC) project, scaled to the total column measurements of the Total Carbon Column Observing Network (TCCON) at ground-based measurement sites. Without any radiometric corrections of GOSAT observations, differences between both data sets can be as large as 10%. To mitigate these differences, we developed a correction scheme using a principal component analysis of spectral fit residuals and airborne observations of methane during the HIAPER pole-to-pole observations (HIPPO) campaign II and III. When the correction scheme is applied, the bias in the methane profile can be reduced to less than 2% over the whole altitude range with respect to MACC model methane fields. Furthermore, we show that, with this correction, the retrievals result in smooth methane fields over land and ocean crossings and no differences can be discerned between daytime and nighttime measurements. Finally, a cloud filter is developed for the nighttime and ocean measurements. This filter is rooted in the GOSAT-TIR (thermal infrared) measurements and its performance, in terms of biases, is consistent with the cloud filter based on the GOSAT-SWIR (shortwave infrared) measurements. The TIR filter shows a higher acceptance rate of observations than the SWIR filter, at the cost of a higher uncertainty in the retrieved methane profiles.

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Short summary
Atmospheric methane abundances as function of altitude derived from the thermal infrared band of the Japanese GOSAT satellite show a systematic bias. It has been shown that simple correction schemes can lead to undesired consequences. This paper discusses a more elaborate approach, resulting in methane profiles that fall within 2 % of independent reference measurements and models. This is true for different scenes around the globe and over the full altitude range.
Atmospheric methane abundances as function of altitude derived from the thermal infrared band of...