Weak constraint and long window 4D-Var

English

Séminaire Modèles et Algorithmes Déterministes: EDP-MOISE

10/05/2012 - 11:00 Mr Yannick Tremolet Salle 1 - Tour IRMA

Experiments in a two-layer quasi-geostrophic system confirm the results obtained by Fisher et al. (2005) for the Lorenz (1995) model. The experiments show that increasing the length of the analysis window in 4D-Var improves the analysis. Moreover, this benefit is present even if the model error is induced by systematic error in the model parameters, and is imperfectly described by the analysis system. We present results using the full ECMWF analysis system showing that the analysis window can usefully be extended to 24 hours. We also present results using a degraded observation network comprising surface pressure observations only. We find that a 24-hour analysis window gives better results than a 12-hour analysis window, and the improvement is even more significant when the 24-hour windows are “overlapped” by performing an analysis every 12 hours.

A major potential benefit of weak-constraint 4D-Var is that it allows parallelization of the inner loops of 4D-Var. We discuss how this benefit might be realized, and present a new parallel algorithm which we call the “saddle point” formulation of 4D-Var. We demonstrate the algorithm using the two-layer quasi-geostrophic system.