Evaluation of a terrestrial carbon cycle submodel in an earth system model using networks of eddy covariance observations

Improvement of terrestrial submodels in earth system models (ESMs) is important to reduce uncertainties in future projections of global carbon cycle and climate. Since these submodels lack detailed validation, evaluation of terrestrial submodels using networks of field observations is necessary. The purpose of this study is to improve an ESM by refining a terrestrial submodel using eddy covariance observations. To evaluate and improve the terrestrial submodel included in the UVic-ESCM, we conducted two experiments: an off-line experiment and an ESM experiment. In the off-line experiment, we used the terrestrial submodel forced by observed climate inputs (off-line model run). We evaluated and refined the model at a point scale using eddy covariance observations. In this process, we first tested the default terrestrial submodel with eddy covariance observations. Next, we refined the terrestrial submodel using eddy covariance observations. Then, in the ESM experiment (carbon cycle-climate coupled model run), we used the default and refined terrestrial submodel as a terrestrial submodel in the ESM. We tested the effects of the terrestrial submodel improvements on the ESM simulations at both site and global scales. First, we evaluated the terrestrial submodel as an off-line mode (the terrestrial submodel was extracted from ESM) at point scales using 48 eddy covariance observation data, and improved it through fixing model parameters and structures. The modifications were conducted to reproduce the seasonal simulation of the terrestrial carbon cycle by removing several biases in the terrestrial submodel through fixing parameters and models related to such as snow melting process, rooting depth, and photosynthesis efficiency. The terrestrial submodel was improved with the reduction of the root mean square error and the closer simulation of the seasonal carbon fluxes. Second, using the UVic-ESCM with the improved terrestrial submodel, we confirmed model improvement at most observation sites. The ESM with the default terrestrial submodel mostly underestimates the monthly ET and carbon fluxes at all sites. Once we replaced the default terrestrial submodel with the improved one in the ESM, the simulation results of ET and carbon fluxes were greatly improved. The terrestrial submodel refinement also affected future projections; the UVic-ESCM with the improved terrestrial submodel simulated 100 ppmv lower atmospheric CO2 concentration in 2100 compared with the default UVic-ESCM. Our study pointed out the importance of refinement of terrestrial submodels in ESM simulations, and effectiveness of our model improvement procedure. However, further analysis such as use of spatial validation data, assessment of biomass and soil carbon amount and more objective procedure for model refinement are needed.