Abstract
Abstract. We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO2 (XCO2) from space, and we illustrate the method by applying it to the v2.8 Atmospheric CO2 Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) measurements over land. The approach exploits the lack of large gradients in XCO2 south of 25° S to identify large-scale offsets and other biases in the ACOS-GOSAT data with several retrieval parameters and errors in instrument calibration. We demonstrate the effectiveness of the method by comparing the ACOS-GOSAT data in the Northern Hemisphere with ground truth provided by the Total Carbon Column Observing Network (TCCON). We use the observed correlation between free-tropospheric potential temperature and XCO2 in the Northern Hemisphere to define a dynamically informed coincidence criterion between the ground-based TCCON measurements and the ACOS-GOSAT measurements. We illustrate that this approach provides larger sample sizes, hence giving a more robust comparison than one that simply uses time, latitude and longitude criteria. Our results show that the agreement with the TCCON data improves after accounting for the systematic errors, but that extrapolation to conditions found outside the region south of 25° S may be problematic (e.g., high airmasses, large surface pressure biases, M-gain, measurements made over ocean). A preliminary evaluation of the improved v2.9 ACOS-GOSAT data is also discussed.
Highlights
The Greenhouse Gases Observing Satellite (GOSAT) was successfully launched on 23 January 2009, with the goal of measuring total column abundances of CO2 and CH4 with unprecedented precision from space (Yokota et al, 2004)
GOSAT is a joint venture of the National Institute for Environmental Studies (NIES), the Japanese Space Agency (JAXA) and the Ministry of the Environment (MOE), and carries the Thermal And Near-infrared Sensor for carbon Observation Fourier Transform Spectrometer (TANSO-FTS, Hamazaki et al, 2005), which measures spectra of sunlight reflected from the Earth
The Atmospheric CO2 Observations from Space (ACOS) retrievals of XCO2 from the GOSAT TANSO-FTS instrument contain global and regional systematic errors
Summary
The Greenhouse Gases Observing Satellite (GOSAT) was successfully launched on 23 January 2009, with the goal of measuring total column abundances of CO2 and CH4 with unprecedented precision from space (Yokota et al, 2004). We discuss the evaluation of the ACOS-GOSAT XCO2 data product by comparing it with more precise and accurate XCO2 measurements from the ground-based Total Carbon Column Observing Network (TCCON, Wunch et al, 2011). Our technical approach for evaluating the XCO2 product from the ACOS-GOSAT retrievals makes use of the relatively spatially uniform CO2 in the Southern Hemisphere to identify systematic errors, including large-scale biases and other artifacts caused by the retrieval algorithm or errors in the instrument calibration. Once identified, these biases are removed and the success of this modification to the data is evaluated through comparisons with the Northern Hemisphere TCCON data. The spectral ranges used in the ACOS algorithm match those of the OCO and future OCO-2 instrument
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