Improving spatial localization in 4DEnVar
Desroziers, Gérald ; Arbogast, Etienne ; Berre, Loïk
Année de publication
2016
<font color="#000000"><font face="Calibri, sans-serif"><font style="font-size: 10pt" size="2">Abstract</font></font></font></h2><p align="justify"><span style="color:#800080;">The four-dimensional ensemble-variational (4DEnVar) formulation is a credible alternative to the 4D-Var formulation, especially for numerical weather prediction centres that have invested a lot in this latter formulation during the last decades. First implementations of this technique, however, rely on a simplified form for the localization of the 4D covariances inside the assimilation period. It is shown in this article that the use of a unique localization for all cross-covariances between perturbations at different times can be a crude approximation, especially in areas where the mean flow speed is large. To overcome this problem, a Lagrangian advection of the localization is proposed. It is first tested in the simplified Burgers' model and then introduced in the real-size system associated with the French global model Action de Recherche Petite Echelle Grande Echelle (ARPEGE). The test of this advection in both environments shows a significant positive impact in regions where the advection is large. The possibility of using such a Lagrangian advection to evolve the static initial covariance matrix in a flow-dependent way inside the assimilation period, in a hybrid 4DEnVar formulation, is also investigated.</span></p>
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