Exploring a hybrid ensemble–variational data assimilation technique (4DEnVar) with a simple ecosystem carbon model

The study presented here evaluates the ability of the 4DEnVar data assimilation technique to estimate the parameters from synthetically generated observations from a simple carbon model. The method is particularly attractive in its speed and ease of use, and its avoidance in construction of adjoint or tangent linear model code. Additionally, the assimilation analysis step can be performed independently of ensemble generation; there is no need to integrate the 4DEnVar code with that of the underlying model, assuming parameters are static in time. The 4DEnVar method is capable of closely estimating the model parameters with increased certainty given that the ensemble produces a sufficient number of trajectories exhibiting behaviour seen in the observations. We find that the root mean squared error between trajectories and observations is significantly reduced when compared with the prior — in one case a 96% and 99% reduction in the biomass and soil pools respectively.