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-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.

Evaluation of single-footprint AIRS CH<sub>4</sub> Profile Retrieval Uncertainties Using Aircraft Profile Measurements

“…An example averaging kernel matrix is shown in Figure 2 and shows that AIRS based estimates of methane are most sensitive to methane in the free-troposphere and lower-stratosphere as demonstrated previously for AIRS and other TIR based estimates of tropospheric methane (e.g. Xiong et al, 2016;de Lange and Landgraf, 2018).…”

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, 2013;Frankenberg et al, 2006;Houweling et al, 2014), 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, 2018;Kuze et al, 2009) 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, 2018).…”

Evaluation of single-footprint AIRS CH<sub>4</sub> profile retrieval uncertainties using aircraft profile measurements

“…The AIRS instrument is a nadir-viewing, scanning infrared spectrometer (Aumann et al, 2003;Pagano et al, 2003;Irion et al, 2018;DeSouza-Machado et al, 2018) that is onboard the NASA Aqua satellite and was launched in 2002. AIRS measures the thermal radiance between approximately 3-12 µm, with a resolving power of approximately 1200.…”