A STRATEGY FOR OPERATIONAL IMPLEMENTATION OF 4D-VAR, USING AN INCREMENTAL APPROACH

An order of magnitude reduction in the cost of four-dimensional variat ional assimilation (4D-Var) is required before operational implementat ion is possible. Preconditioning is considered and, although it offers a significant reduction in cost, it seems that it is unlikely to prov ide a reduction as large as an order of magnitude. An approximation to 4D-Var, namely the incremental approach, is then considered and is sh own to produce the same result at the end of the assimilation window a s an extended Kalman filter in which no approximations are made in the assimilating model but in which instead a simplified evolution of the forecast error is introduced. This approach provides the flexibility for a cost-benefit trade-off of 4D-Var to be made.