Validation of XCH4 derived from SWIR spectra of GOSAT TANSO-FTS with aircraft measurement data

摘要

Column-averaged dry-air mole fractions of methane (XCH), retrievedfrom Greenhouse gases Observing SATellite (GOSAT) short-wavelength infrared(SWIR) spectra, were validated by using aircraft measurement data from theNational Oceanic and Atmospheric Administration (NOAA), the US Departmentof Energy (DOE), the National Institute for Environmental Studies (NIES),the HIAPER Pole-to-Pole Observations (HIPPO) program, and the GOSATvalidation aircraft observation campaign over Japan. In the calculation ofXCH from aircraft measurements (aircraft-based XCH), othersatellite data were used for the CH profiles above the tropopause. Weproposed a data-screening scheme for aircraft-based XCH for reliablevalidation of GOSAT XCH. Further, we examined the impact of GOSAT SWIRcolumn averaging kernels (CAK) on the aircraft-based XCH calculationand found that the difference between aircraft-based XCH with andwithout the application of the GOSAT CAK was less than ±9 ppb atmaximum, with an average difference of −0.5 ppb. We compared GOSAT XCH Ver. 02.00 data retrieved within ±2° or ±5° latitude–longitude boxes centered ateach aircraft measurement site with aircraft-based XCH measured on aGOSAT overpass day. In general, GOSAT XCH was in good agreement withaircraft-based XCH. However, over land, the GOSAT data showed apositive 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 standarddeviation of 9.4 ppb (8.8 ppb) within the ±2° (±5°) boxes. In addition, we obtained similar results when we usedan aircraft-based XCH time series obtained by curve fitting withtemporal interpolation for comparison with GOSAT data.