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On Tuesday, March 15th 2005, the 3D-Var
(3-Dimensional Variational) data assimilation system for the global forecasting
suite was extended to a 4D-Var scheme. This extension to include time as the
fourth dimension of the analysis system implies that the analysis is no longer a
simple snapshot of the atmospheric conditions at the time of the analysis (like
in the 3D-Var which does this for the primary synoptic hours).
Instead, the 4D-Var system provides a time evolution of the atmospheric
conditions during the assimilation time window (for the current implementation,
this window was chosen to maintain coherence with the 3D-Var, that is 6 hours
centered on the main synoptic hours). The 4D-Var analysis is done by using the
GEM forecast model itself as part of the assimilation process, and by using
observations that are well distributed during the analysis time window.
The 4D-Var then computes the best fit between these observations and the model
forecasts. The resulting analyses shows a much improved dynamical
consistency and are accompanied by a significant improvement in the global
system performance.
Further details about this implementation are
available in the official announcement note [English]
[French]. A more detailed technical document is available
in English.
Additional
technical information about the 4D-Var and the assimilation procedures can be
found on the MSC
web site.
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NOTIFICATION FROM
CANADA
4D-Var:
a major improvement to the Canadian global data assimilation
system
