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