A joint data assimilation system (Tan-Tracker) to simultaneously estimate surface CO₂ fluxes and 3-D atmospheric CO₂ concentrations from observations

Abstract. We have developed a novel framework ("Tan-Tracker") for assimilating observations of atmospheric CO₂ concentrations, based on the POD-based (proper orthogonal decomposition) ensemble four-dimensional variational data assimilation method (PODEn4DVar). The high flexibility and the high computational efficiency of the PODEn4DVar approach allow us to include both the atmospheric CO₂ concentrations and the surface CO₂ fluxes as part of the large state vector to be simultaneously estimated from assimilation of atmospheric CO₂ observations. Compared to most modern top-down flux inversion approaches, where only surface fluxes are considered as control variables, one major advantage of our joint data assimilation system is that, in principle, no assumption on perfect transport models is needed. In addition, the possibility for Tan-Tracker to use a complete dynamic model to consistently describe the time evolution of CO₂ surface fluxes (CFs) and the atmospheric CO₂ concentrations represents a better use of observation information for recycling the analyses at each assimilation step in order to improve the forecasts for the following assimilations. An experimental Tan-Tracker system has been built based on a complete augmented dynamical model, where (1) the surface atmosphere CO₂ exchanges are prescribed by using a persistent forecasting model for the scaling factors of the first-guess net CO₂ surface fluxes and (2) the atmospheric CO₂ transport is simulated by using the GEOS-Chem three-dimensional global chemistry transport model. Observing system simulation experiments (OSSEs) for assimilating synthetic in situ observations of surface CO₂ concentrations are carefully designed to evaluate the effectiveness of the Tan-Tracker system. In particular, detailed comparisons are made with its simplified version (referred to as TT-S) with only CFs taken as the prognostic variables. It is found that our Tan-Tracker system is capable of outperforming TT-S with higher assimilation precision for both CO₂ concentrations and CO₂ fluxes, mainly due to the simultaneous estimation of CO₂ concentrations and CFs in our Tan-Tracker data assimilation system. A experiment for assimilating the real dry-air column CO₂ retrievals (XCO₂) from the Japanese Greenhouse Gases Observation Satellite (GOSAT) further demonstrates its potential wide applications.