1National Institute for Environmental Studies (NIES), Tsukuba, Japan
2Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan
3Meteorological Research Institute (MRI), Tsukuba, Japan
4National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
5Research Institute for Global Change, JAMSTEC, Yokohama, Japan
Abstract. Column-averaged volume mixing ratios of carbon dioxide (XCO2) retrieved from Greenhouse gases Observing SATellite (GOSAT) Short-Wavelength InfraRed (SWIR) observations were compared with aircraft measurements by the Comprehensive Observation Network for TRace gases by AIrLiner (CONTRAIL) project, the National Oceanic and Atmospheric Administration (NOAA), and the National Institute for Environmental Studies (NIES). Before validation, we investigated the impacts of GOSAT SWIR column averaging kernels (CAK) and the shape of a priori profiles on the calculation of XCO2 based on aircraft measurements (aircraft-based XCO2). The differences between aircraft-based XCO2 with and without the application of GOSAT CAK were evaluated to be less than ±0.4 ppm at most, and less than 0.1 ppm on average. Therefore, we concluded that the GOSAT CAK produces only a minor effect on the aircraft-based XCO2 calculation in terms of the overall uncertainty of GOSAT XCO2.
In this study, two approaches were used to validate GOSAT products (Ver. 02.00). First, we performed a comparison of GOSAT data retrieved within ±2-degree or ±5-degree latitude/longitude boxes centered at each aircraft measurement site and aircraft-based data measured on a GOSAT overpass day (i.e. extraction of temporally matched cases). As this method resulted in no matched data for observation sites where no aircraft measurement was made on the GOSAT overpass day, we also attempted to validate GOSAT products by gap-filling the aircraft-based XCO2 time series through curve fitting. Both methods indicated that GOSAT XCO2 agreed well with aircraft-based XCO2, except that the former is negatively biased by 1–2 ppm with a standard deviation of 1–3 ppm.