A regional carbon data assimilation system and its preliminary evaluation in East Asia

Abstract. In order to optimize surface CO₂ fluxes at grid scales, a regional surface CO₂ flux inversion system (Carbon Flux Inversion system and Community Multi-scale Air Quality, CFI-CMAQ) has been developed by applying the ensemble Kalman filter (EnKF) to constrain the CO₂ concentrations and applying the ensemble Kalman smoother (EnKS) to optimize the surface CO₂ fluxes. The smoothing operator is associated with the atmospheric transport model to constitute a persistence dynamical model to forecast the surface CO₂ flux scaling factors. In this implementation, the "signal-to-noise" problem can be avoided; plus, any useful observed information achieved by the current assimilation cycle can be transferred into the next assimilation cycle. Thus, the surface CO₂ fluxes can be optimized as a whole at the grid scale in CFI-CMAQ. The performance of CFI-CMAQ was quantitatively evaluated through a set of Observing System Simulation Experiments (OSSEs) by assimilating CO₂ retrievals from GOSAT (Greenhouse Gases Observing Satellite). The results showed that the CO₂ concentration assimilation using EnKF could constrain the CO₂ concentration effectively, illustrating that the simultaneous assimilation of CO₂ concentrations can provide convincing CO₂ initial analysis fields for CO₂ flux inversion. In addition, the CO₂ flux optimization using EnKS demonstrated that CFI-CMAQ could, in general, reproduce true fluxes at grid scales with acceptable bias. Two further sets of numerical experiments were conducted to investigate the sensitivities of the inflation factor of scaling factors and the smoother window. The results showed that the ability of CFI-CMAQ to optimize CO₂ fluxes greatly relied on the choice of the inflation factor. However, the smoother window had a slight influence on the optimized results. CFI-CMAQ performed very well even with a short lag-window (e.g. 3 days).