Real-data assimilation experiment with a joint data assimilation system: assimilating carbon dioxide mole fraction measurements from the Greenhouse gases Observing Satellite

The performance of a joint data assimilation system (Tan-Tracker), which is based on the PODEn4Dvar assimilation method, in assimilating Greenhouse gases Observing SATellite (GOSAT) carbon dioxide (CO2) data, was evaluated. Atmospheric 3D CO2 concentrations and CO2 surface fluxes (CFs) from 2010 were simulated using a global chemistry transport model (GEOS-Chem). The Tan-Tracker system used the simulated CO2 concentrations and fluxes as a background field and assimilated the GOSAT column average dry-air mole fraction of CO2 (X-CO2) data to optimize CO2 concentrations and CFs in the same assimilation window. Monthly simulated X-CO2 (X-CO2, Sim) and assimilated X-CO2 (X-CO2, TT) data retrieved at different satellite scan positions were compared with GOSAT-observed X-CO2 (X-CO2, Obs) data. The average RMSE between the monthly X-CO2, TT and X-CO2, Obs data was significantly (30%) lower than the average RMSE between X-CO2, Sim and X-CO2, Obs. Specifically, reductions in error were found for the positions of northern Africa (the Sahara), the Indian peninsula, southern Africa, southern North America, and western Australia. The difference between the correlation coefficients of the X-CO2, Sim and X-CO2, Obs and those of the X-CO2, TT and X-CO2, Obs was only small. In general, the Tan-Tracker system performed very well after assimilating the GOSAT data.