Methane profiles from GOSAT thermal infrared spectra

Abstract

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 1310 cm −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 9 km altitude but little sensitivity to methane in the lower troposphere. For verification, we compare the GOSAT methane abundance at measurement sites of the Total Carbon Column Observing Network (TCCON) to methane 5 profiles provided by the Monitoring Atmospheric Composition and Climate (MACC) model fields scaled to the total column observations at the 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 10 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 are to 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 measurements and is consistent with the cloud filter based on the GOSAT-SWIR measurements, despite the fact that the TIR-filter is less stringent.

Methane Mitigation: Methods to Reduce Emissions, on the Path to the Paris Agreement

“…Many earlier studies used the European MIPAS and SCIAMACHY instruments on ENVISAT (Bergamaschi et al, ; Frankenberg et al, ; Houweling et al, ), which also retrieved CO 2 in the 1.65 μm band, and other IR‐active gases, with a coarse pixel resolution. More recently, the Japanese GOSAT (Hu et al, ; Kuze et al, ) has provided global mapping of methane, albeit with averaging kernels in the midtroposphere and limited sensitivity to methane in the lower troposphere (e.g., Lange & Landgraf, ). Although SCIAMACHY (2002–2012) and GOSAT (launched 2009) were capable of identifying regionally important high column measurement over hotspots, the spatial and boundary layer resolution was poor compared to in situ and aircraft mapping, and complicated by biases and poor accuracy of satellite measurement.…”

An Analysis of Interhemispheric Transport Pathways Based on Three‐Dimensional Methane Data by GOSAT Observations and Model Simulations

“…Since CH 4 is a well‐mixed long‐lived tracer with a lifetime of ∼9.3 years, as per our model simulation, this adopts a climatological approach to derive the meridional transport variability signals from rather smooth concentration gradients. As the TIR spectral signatures depend on thermal contrast between the atmosphere and surface (De Lange & Landgraf, 2018; Saitoh et al., 2016), we analyze CH 4 levels between 350 and 150 hPa, where the sensitivity of TIR CH 4 observations is stronger in comparison with other levels. A priori information on the shape of the vertical profile and the averaging kernel (AK) functions are critical for the interpretation of satellite retrievals of a trace gas.…”