Retrieval of greenhouse gases from GOSAT and GOSAT-2 using the FOCAL algorithm

Abstract

Abstract. We show new results from an updated version of the Fast atmOspheric traCe gAs retrievaL (FOCAL) retrieval method applied to measurements of the Greenhouse gases Observing SATellite (GOSAT) and its successor GOSAT-2. FOCAL was originally developed for estimating the total column carbon dioxide mixing ratio (XCO2) from spectral measurements made by the Orbiting Carbon Observatory-2 (OCO-2). However, depending on the available spectral windows, FOCAL also successfully retrieves total column amounts for other atmospheric species and their uncertainties within one single retrieval. The main focus of the current paper is on methane (XCH4; full-physics and proxy product), water vapour (XH2O) and the relative ratio of semi-heavy water (HDO) to water vapour (δD). Due to the extended spectral range of GOSAT-2, it is also possible to derive information on carbon monoxide (XCO) and nitrous oxide (XN2O) for which we also show first results. We also present an update on XCO2 from both instruments. For XCO2, the new FOCAL retrieval (v3.0) significantly increases the number of valid data compared with the previous FOCAL retrieval version (v1) by 50 % for GOSAT and about a factor of 2 for GOSAT-2 due to relaxed pre-screening and improved post-processing. All v3.0 FOCAL data products show reasonable spatial distribution and temporal variations. Comparisons with the Total Carbon Column Observing Network (TCCON) result in station-to-station biases which are generally in line with the reported TCCON uncertainties. With this updated version of the GOSAT-2 FOCAL data, we provide a first total column average XN2O product. Global XN2O maps show a gradient from the tropics to higher latitudes on the order of 15 ppb, which can be explained by variations in tropopause height. The new GOSAT-2 XN2O product compares well with TCCON. Its station-to-station variability is lower than 2 ppb, which is about the magnitude of the typical N2O variations close to the surface. However, both GOSAT-2 and TCCON measurements show that the seasonal variations in the total column average XN2O are on the order of 8 ppb peak-to-peak, which can be easily resolved by the GOSAT-2 FOCAL data. Noting that only few XN2O measurements from satellites exist so far, the GOSAT-2 FOCAL product will be a valuable contribution in this context.

Carbon dioxide cover: carbon dioxide column concentration seamlessly distributed globally during 2009–2020

“…Thus, we selected the bias-corrected data of GOSAT_FTS_L3_V2.95. And the accuracy of the comparison between the GOSAT data product and the TCCON site was 0.56 ppm (Noël et al 2021;Watanabe et al 2015;). And the GOSAT orbits at an altitude of approximately 666 km, with 10.5 km of spatial resolution and three-day temporal resolution.…”

Retrieval of greenhouse gases from GOSAT and greenhouse gases and carbon monoxide from GOSAT-2 using the FOCAL algorithm

“…The input data used in this study are essentially the same as for the v1.0 product described in Noël et al (2021) with some updates described in the following. As input spectra, we use calibrated GOSAT and GOSAT-2 L1B radiances for both polarisation directions of the three NIR/SWIR bands at around 0.76, 1.6 and 2.0 µm.…”

“…Recently, Noël et al (2021) presented a first version (v1.0) of an XCO 2 data product from GOSAT (Greenhouse gases Observing SATellite; Kuze et al, 2009Kuze et al, , 2016 and GOSAT-2 (Suto et al, 2021) measurements in the near-infrared (NIR) and shortwave infrared (SWIR) spectral regions derived with the FOCAL (Fast atmOspheric traCe gAs retrievaL) method (Reuter et al, 2017a, b). FOCAL was originally applied to measurements of the Orbiting Carbon Observatory-2 (OCO-2; Eldering et al, 2017; and is based on a full-physics retrieval in which scattering is approximated by a single layer.…”

“…FOCAL was originally applied to measurements of the Orbiting Carbon Observatory-2 (OCO-2; Eldering et al, 2017; and is based on a full-physics retrieval in which scattering is approximated by a single layer. Noël et al (2021) focused on the XCO 2 results, but the application of FOCAL to the GOSAT instruments includes the determination of various other atmospheric quantities. In the current paper, we present results from an updated version (v3.0) of the GOSAT and GOSAT-2 FOCAL retrieval.…”

“…NASA also released an XCO 2 product based on the ACOS v9 retrieval, recently described by Taylor et al (2022). A precursor of the FOCAL XCO 2 product v1.0 from Noël et al (2021) is the BESD v01.04 product, which is also from the Institute of Environmental Physics (IUP), Bremen (Heymann et al, 2015). This is a near-real-time product produced for the Copernicus Atmospheric Monitoring Service (CAMS, https://atmosphere.copernicus.eu/, last access: 30 July 2020).…”