Abstract
Abstract. We describe the Max Planck Institute Carbon Cycle Data Assimilation System (MPI-CCDAS) built around the tangent-linear version of the JSBACH land-surface scheme, which is part of the MPI-Earth System Model v1. The simulated phenology and net land carbon balance were constrained by globally distributed observations of the fraction of absorbed photosynthetically active radiation (FAPAR, using the TIP-FAPAR product) and atmospheric CO2 at a global set of monitoring stations for the years 2005 to 2009. When constrained by FAPAR observations alone, the system successfully, and computationally efficiently, improved simulated growing-season average FAPAR, as well as its seasonality in the northern extra-tropics. When constrained by atmospheric CO2 observations alone, global net and gross carbon fluxes were improved, despite a tendency of the system to underestimate tropical productivity. Assimilating both data streams jointly allowed the MPI-CCDAS to match both observations (TIP-FAPAR and atmospheric CO2) equally well as the single data stream assimilation cases, thereby increasing the overall appropriateness of the simulated biosphere dynamics and underlying parameter values. Our study thus demonstrates the value of multiple-data-stream assimilation for the simulation of terrestrial biosphere dynamics. It further highlights the potential role of remote sensing data, here the TIP-FAPAR product, in stabilising the strongly underdetermined atmospheric inversion problem posed by atmospheric transport and CO2 observations alone. Notwithstanding these advances, the constraint of the observations on regional gross and net CO2 flux patterns on the MPI-CCDAS is limited through the coarse-scale parametrisation of the biosphere model. We expect improvement through a refined initialisation strategy and inclusion of further biosphere observations as constraints.
Highlights
Estimates of the net carbon balance of the terrestrial biosphere are highly uncertain, because the net balance cannot be directly observed at large spatial scales (Le Quéré et al., 2015)
We demonstrate the capacity of the MPI-carboncycle data assimilation system (CCDAS) system to integrate atmospheric CO2 observations and the fraction of absorbed photosynthetically active radiation (FAPAR) recorded from satellites, which constrains the seasonality of the phenology and assesses the relative effect of the constraint from these two data streams on parameter values and modelled fluxes
Several statistics comparing the posterior model with observations for FAPAR and CO2 (Tables 4 and 5) show that the model performance of the JOINT experiment was comparable to the performance of the two single data-stream experiments relative to the assimilated quantity
Summary
Estimates of the net carbon balance of the terrestrial biosphere are highly uncertain, because the net balance cannot be directly observed at large spatial scales (Le Quéré et al., 2015). Studies aiming to quantify the contemporary global carbon cycle either infer the terrestrial carbon budget as a residual of the arguably better constrained other components of the global carbon budget (Le Quéré et al, 2015) or rely on measurements of atmospheric CO2 and the inversion of its atmospheric transport (Gurney et al, 2002). Both approaches have the caveat that they are not able to provide accurate estimates at high spatial resolution, and cannot utilise the broader set of Earth system observations that provide information on terrestrial carbon-cycle dynamics (Luo et al., 2012). Since they simulate all major aspects of the terrestrial carbon cycle, they
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