Abstract
Abstract. Column-averaged dry-air mole fractions of methane (XCH4), retrieved from Greenhouse gases Observing SATellite (GOSAT) short-wavelength infrared (SWIR) spectra, were validated by using aircraft measurement data from the National Oceanic and Atmospheric Administration (NOAA), the US Department of Energy (DOE), the National Institute for Environmental Studies (NIES), the HIAPER Pole-to-Pole Observations (HIPPO) program, and the GOSAT validation aircraft observation campaign over Japan. In the calculation of XCH4 from aircraft measurements (aircraft-based XCH4), other satellite data were used for the CH4 profiles above the tropopause. We proposed a data-screening scheme for aircraft-based XCH4 for reliable validation of GOSAT XCH4. Further, we examined the impact of GOSAT SWIR column averaging kernels (CAK) on the aircraft-based XCH4 calculation and found that the difference between aircraft-based XCH4 with and without the application of the GOSAT CAK was less than ±9 ppb at maximum, with an average difference of −0.5 ppb. We compared GOSAT XCH4 Ver. 02.00 data retrieved within ±2° or ±5° latitude–longitude boxes centered at each aircraft measurement site with aircraft-based XCH4 measured on a GOSAT overpass day. In general, GOSAT XCH4 was in good agreement with aircraft-based XCH4. However, over land, the GOSAT data showed a positive bias of 1.5 ppb (2.0 ppb) with a standard deviation of 14.9 ppb (16.0 ppb) within the ±2° (±5°) boxes, and over ocean, the average bias was 4.1 ppb (6.5 ppb) with a standard deviation of 9.4 ppb (8.8 ppb) within the ±2° (±5°) boxes. In addition, we obtained similar results when we used an aircraft-based XCH4 time series obtained by curve fitting with temporal interpolation for comparison with GOSAT data.
Highlights
It is well known that atmospheric methane (CH4) is an important greenhouse gas (GHG) that plays a crucial role in global climate change and atmospheric chemistry
The aircraft-based XCH4 at a certain time of day was calculated by using the short-wavelength infrared (SWIR) column averaging kernels (CAK) of the gases Observing SATellite (GOSAT) data nearest to the aircraft site from among all GOSAT data obtained within ±10◦ latitude–longitude boxes centered at the observation site on the same day
In the western part of Chesterfield Islands on 15 November 2009, the CH4 profile derived from aircraft measurements, observed by a HIAPER Pole-to-Pole Observations (HIPPO) mission, and the corrected Halogen Occultation Experiment (HALOE) data were almost coincident with the GOSAT a priori data (Fig. 6b)
Summary
It is well known that atmospheric methane (CH4) is an important greenhouse gas (GHG) that plays a crucial role in global climate change and atmospheric chemistry. Inoue et al.: Validation of GOSAT XCH4 with aircraft measurements concentrations have been measured from various in situ platforms, including ground-based stations, tall towers, ships, aircraft, and balloons (e.g., Cavanagh et al, 1969; Fraser et al, 1981; Steele et al, 1987; Blake and Rowland, 1988; Aoki et al, 1992; Dlugokencky et al, 1994; Matsueda and Inoue, 1996; Tohjima et al, 1997, 2002; Andrews et al, 2001; Sasakawa et al, 2010; Terao et al, 2011; Wada et al, 2011) These measurements have provided extensive information on the spatial and temporal variations of atmospheric CH4, the distribution of the CH4 sources and sinks is still poorly understood because of the sparseness of available in situ observations and their limited altitudinal range. Near-infrared (NIR) spectra obtained by the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) onboard Envisat, launched in March 2002, provide column-averaged dry-air mole fractions of CH4 (XCH4), which have been compared to ground-based Fourier transform spectrometer (FTS) measurements and model results (e.g., Dils et al, 2006; Schneising et al, 2009, 2012)
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