A method for evaluating bias in global measurements of CO<sub>2</sub> total columns from space

Abstract

We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO₂ (X_CO₂) from space, and we illustrate the method by applying the method to the Atmospheric CO₂ Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) v2.8 data. The approach exploits the lack of large gradients in X_CO₂ 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 correlation between free-tropospheric temperature and X_CO₂ 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

Effects of atmospheric light scattering on spectroscopic observations of greenhouse gases from space. Part 2: Algorithm intercomparison in the GOSAT data processing for CO2 retrievals over TCCON sites

“…Along with the noticeable deviation of the slope a from unity (0.81-0.90), this could hinder carbon flux inversions as noted by O'Dell et al [2011]. However, as demonstrated in Wunch et al [2011b], employing a suitable bias correction based on a multi-linear regression against a small number of geophysical variables, such as signal level and surface albedo, leads to a value of a consistent with unity for ACOS B2.9. The values of global bias and standard deviations between GOSAT ACOS B2.9 and TCCON X CO2 generally agree with those previously reported by Wunch et al [2011b] when using other coincident GOSAT-TCCON criteria (mean data from a much larger grid box of AE10 latitude  AE30 longitude combined with a potential temperature constraint of AE2 K at 700 hPa and a 10 day temporal constraint).…”

“…However, as demonstrated in Wunch et al [2011b], employing a suitable bias correction based on a multi-linear regression against a small number of geophysical variables, such as signal level and surface albedo, leads to a value of a consistent with unity for ACOS B2.9. The values of global bias and standard deviations between GOSAT ACOS B2.9 and TCCON X CO2 generally agree with those previously reported by Wunch et al [2011b] when using other coincident GOSAT-TCCON criteria (mean data from a much larger grid box of AE10 latitude  AE30 longitude combined with a potential temperature constraint of AE2 K at 700 hPa and a 10 day temporal constraint). We should also note that although ACOS B2.9 and UoL-FP: 3G algorithms provide the largest number of observations; they still only utilizẽ 60% of the total number of individual GOSAT soundings available from all data products.…”

“…[10] The basic specifications of the GOSAT instrument [Hamazaki et al, 2005;Kuze et al, 2009;Nakajima et al, 2010] and TCCON measurements [Washenfelder et al, 2006;Wunch et al, 2011a], as well as the atmospheric transport model (NIES-08.1i) [Belikov et al, 2012] were outlined in Part 1. In this section, we specify observation conditions and GOSAT-TCCON coincidence criteria used in this study.…”

“…According to several theoretical studies, the user requirements for the GOSAT retrieval quality are rather stringent [Rayner and O'Brien, 2001;Houweling et al, 2004;Miller et al, 2007;Chevallier et al, 2007]; for example, regional biases of a few tenths of a part per million could hamper surface flux inversions [Miller et al, 2007;Chevallier et al, 2007]. An important aspect for validating the GOSAT retrievals and meeting the user requirements is the use of the ground-based high-resolution Fourier transform spectrometer (FTS) measurements from the Total Carbon Column Observing Network (TCCON) [Washenfelder et al, 2006;Wunch et al, 2011a], whose uncertainties in X CO2 are expected to be within 0.8 ppm (2s) [Wunch et al, 2010;Deutscher et al, 2010;Messerschmidt et al, 2010]. Several studies have compared GOSAT retrieval gas products from different algorithms against TCCON measurements Butz et al, 2011;Wunch et al, 2011bOshchepkov et al, 2012;Cogan et al, 2012].…”

“…An important aspect for validating the GOSAT retrievals and meeting the user requirements is the use of the ground-based high-resolution Fourier transform spectrometer (FTS) measurements from the Total Carbon Column Observing Network (TCCON) [Washenfelder et al, 2006;Wunch et al, 2011a], whose uncertainties in X CO2 are expected to be within 0.8 ppm (2s) [Wunch et al, 2010;Deutscher et al, 2010;Messerschmidt et al, 2010]. Several studies have compared GOSAT retrieval gas products from different algorithms against TCCON measurements Butz et al, 2011;Wunch et al, 2011bOshchepkov et al, 2012;Cogan et al, 2012]. Most of these studies find encouraging validation results using different periods of GOSAT observations, GOSAT-TCCON coincident criteria, and equations to represent statistical characteristics of the comparison.…”