Abstract
Abstract. The presence of 3D cloud radiative effects in OCO-2 retrievals is demonstrated from an analysis of 2014–2019 OCO-2 XCO2 raw retrievals, bias-corrected XCO2bc data, ground-based Total Carbon Column Observation Network (TCCON) XCO2, and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud and radiance fields. In approximate terms, 40 % (quality flag – QF = 0, land or ocean) and 73 % (QF = 1, land or ocean) of the observations are within 4 km of clouds. 3D radiative transfer calculations indicate that 3D cloud radiative perturbations at this cloud distance, for an isolated low-altitude cloud, are larger in absolute value than those due to a 1 ppm increase in CO2. OCO-2 measurements are therefore susceptible to 3D cloud effects. Four 3D cloud metrics, based upon MODIS radiance and cloud fields as well as stand-alone OCO-2 measurements, relate XCO2bc–TCCON averages to 3D cloud effects. This analysis indicates that the operational bias correction has a nonzero residual 3D cloud bias for both QF = 0 and QF = 1 data. XCO2bc–TCCON averages at small cloud distances differ from those at large cloud distances by −0.4 and −2.2 ppm for the QF = 0 and QF = 1 data over the ocean. Mitigation of 3D cloud biases with a table lookup technique, which utilizes the nearest cloud distance (Distkm) and spatial radiance heterogeneity (CSNoiseRatio) 3D metrics, reduces QF = 1 ocean and land XCO2bc–TCCON averages from −1 ppm to near ±0.2 ppm. The ocean QF = 1 XCO2bc–TCCON averages can be reduced to the 0.5 ppm level if 60 % (70 %) of the QF = 1 data points are utilized by applying Distkm (CSNoiseRatio) metrics in a data screening process. Over land the QF = 1 XCO2bc–TCCON averages are reduced to the 0.5 (0.8) ppm level if 65 % (63 %) of the data points are utilized by applying Diastkm (CSNoiseRatio) data screening. The addition of more terms to the linear regression equations used in the current bias correction processing without data screening, however, did not introduce an appreciable improvement in the standard deviations of the XCO2bc–TCCON statistics.
Highlights
The Orbiting Carbon Observatory (OCO-2) measures the column-averaged atmospheric CO2 dry-air mole fraction, referred to as XCO2, on a global basis (Eldering et al, 2017)
We demonstrate that 3D cloud biases in XCO2bc–Total Carbon Column Observation Network (TCCON) remain after the current bias correction processing for both quality flag QF = 0 and QF = 1 data
We will calculate 3D cloud metrics based upon the Moderate Resolution Imaging Spectroradiometer (MODIS) files and stand-alone OCO-2 data, and we will investigate whether the application of the 3D metrics in a table lookup correction, or by data screening by the 3D metrics, leads to a reduction in the standard deviations and averages of TCCON–XCO2bc probability distribution functions (PDFs)
Summary
The Orbiting Carbon Observatory (OCO-2) measures the column-averaged atmospheric CO2 dry-air mole fraction, referred to as XCO2, on a global basis (Eldering et al, 2017). Comparisons of XCO2raw (the XCO2 that is produced by the operational retrieval) to TCCON measurements reveal that TCCON measurements are approximately 1 ppm larger than XCO2raw values, as discussed in the Version 9 Data Product User’s Guide (2018) Based upon these and other comparisons, the OCO-2 algorithm team applies multivariable linear regressions separately over land and ocean to bias-correct the XCO2raw retrievals to XCO2bc values. Auxiliary files (Cronk et al, 2018), not archived by the NASA Earthdata file system, contain MODIS radiances at 500 m spatial resolution, cloud mask, cloud fraction, cloud optical depth, and geolocation (based upon OCO-2 version 9 data) matched to the OCO-2 sounding ID We refer to these files as Colorado State University “CSU files”.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
