Simultaneous global and limited-area ensemble data assimilation using joint states

We propose a data assimilation scheme that simultaneously produces the analyses for a global model and an embedded limited-area model, considering forecast information from both models. The purpose of the proposed approach is twofold. First, we expect that the global analysis will benefit from incorporation of information from the higher resolution limited-area model. Second, our method is expected to produce a limited-area analysis that is more strongly constrained by the large-scale flow than a conventional limited-area analysis. The proposed scheme minimises a cost function in which the control variable is the joint state of the global and the limited-area models. In addition, the cost function includes a constraint term that penalises large differences between the global and the limited-area state estimates. The proposed approach is tested by idealised experiments, using ‘toy’ models introduced by Lorenz in 2005. The results of these experiments suggest that the proposed approach improves the global analysis within and near the limited-area domain and the regional analysis near the lateral boundaries. These analysis improvements lead to forecast improvements in both the global and the limited-area models.