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Data assimilation concepts
and methods
March 1999
By F. Bouttier and P.
Courtier
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9 . 1D-Var and other variational analysis systems
The essence of the 3D-Var algorithm is to rewrite a least-squares problem as the minimization of a cost-function. The method was introduced in order to remove the local data selection in the OI algorithm, thereby performing a global analysis of the 3-D meteorological fields, hence the name. Of course, the technique has been applied equally well to other problems in which the control variable is much smaller. A very successful example is the satellite data retrieval problem, in which the 1D-Var algorithm performs a local analysis of one atmospheric column (the model state) at the location of each satellite sounding such as TOVS radiances or microwave measurements. Similar variational techniques have been applied to the retrieval of surface wind fields from a collection of scatterometer ambiguous wind measurements or to the analysis of land surface properties in a numerical weather prediction model (in this case the control variable is more or less a column of the 3-D model, but the time dimension is taken into account as in 4D-Var). Except 1D-Var, these methods have no established name yet.
ref: Eyre 1987.
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