Impact of the digital filter as a weak constraint in the pre-operational 4D-Var assimilation system of Météo-France
Gauthier, P. ; Thépaut, J.-N.
Année de publication
2001
In this paper, a weak constraint formulation of the digital filter based<br> on the DolphChebyshev window is introduced in a preoperational version<br> of the 4DVAR analysis of Météo-France. The constraint is imposed only <br>on the analysis increments to damp spurious fast oscillations associated<br> with gravityinertia waves. In the incremental formulation of 4DVAR, <br>the analysis increments are obtained from a global model at a uniform <br>low resolution with a simplified set of physical parameterizations, <br>while the high-resolution forecast is obtained with a model that uses a <br>variable-resolution grid having a higher resolution over France and the <br>complete set of physical parameterizations. Both models have the same <br>vertical resolution. In a set of preliminary experiments using the same <br>background field and the same set of observations, it is shown that the <br>weak constraint imposed only on the low-resolution increments manages to<br> control efficiently the emergence of fast oscillations in the resulting<br> high-resolution forecast while maintaining a closer fit to the <br>observations than is possible if the digital filter initialization is <br>applied externally on the final analysis increments. It is also shown <br>that this weak constraint does not add any significant computer cost to <br>the 4DVAR analysis. Finally, 4DVAR has been cycled over a period of 2 <br>weeks and the results show that, compared to 3DVAR, the initial <br>dynamical imbalances are significantly less in 4DVAR even if no <br>constraint is imposed at all. However, it has been noted that the <br>innovation statistics show a positive impact when a constraint is <br>applied.</div>
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