Assimilation of OCO-2 retrievals with WRF-Chem/DART: A case study for the Midwestern United States

The Data Assimilation Research Testbed (DART) has been extended to be able to assimilate the column-average dry-air mole fraction of CO2 (XCO2) retrievals from the Orbiting Carbon Observatory 2 (OCO-2) satellite. Atmospheric CO2 concentrations over the Midwestern United States were estimated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), with and without assimilating OCO-2 retrievals by the extended-DART. To focus on evaluating the effect of the assimilation, the study period was deliberately set to January 2016, the coldest month in the dormant season, to minimize the influence of biogenic CO2 flux. Independent ground-based and flight observations, as well as the CarbonTracker 2017 products (CT2017), were used to evaluate the results of two distinct approaches. Comparing to the estimated CO2 concentration distribution without assimilating the OCO-2 retrievals, the overall root mean square error (RMSE) and mean bias error (MBE) between the results with the assimilation and the observations were averagely reduced by 20.65% and 78.49%, the overall difference in the RMSE and MBE with respect to CT2017 were averagely reduced by 48.29% and 28.61%, respectively. Experiments showed that the assimilation of OCO-2 retrievals by the extended-DART could make the estimated CO2 concentration distribution significantly more consistent with the observations and the CarbonTracker products.