filter4dTechnical Memorandum No.347 1 Developments in 4D-Var and Kalman Filtering by Mike Fisher and Erik Andersson Abstract We discuss the status and the performance of the reduced rankTechnical Memorandum No.347 1 Developments in 4D-Var and Kalman Filtering by Mike Fisher and Erik Andersson Abstract We discuss the status and the performance of the reduced rank