Nonlinear Processes in Geophysics www.nonlin-processes-geophys.net 1023-5809 1607-7946 15 4 2008 10.5194/npg-15-503-2008 http://www.nonlin-processes-geophys.net/15/503/2008/ http://www.nonlin-processes-geophys.net/15/503/2008/npg-15-503-2008.html http://www.nonlin-processes-geophys.net/15/503/2008/npg-15-503-2008.pdf 503 521 2008-07-01 Controlling instabilities along a 3DVar analysis cycle by assimilating in the unstable subspace: a comparison with the EnKF A. Carrassi a.carrassi@oma.be A. Trevisan L. Descamps O. Talagrand F. Uboldi Royal Meteorological Institute of Belgium – RMI, Bruxelles, Belgium Dept. of Physics – University of Ferrara, Ferrara, Italy Istituto di Scienze dell'Atmosfera e del Clima (ISAC) – Consiglio Nazionale delle Ricerche (CNR), Largo Gobetti 101, Bologna, Italy Laboratoire de Météorologie Dynamique, École Normale Supérieure, Paris, France Consultant, Novate Milanese, Italy A hybrid scheme obtained by combining 3DVar with the Assimilation in the Unstable Subspace (3DVar-AUS) is tested in a QG model, under perfect model conditions, with a fixed observational network, with and without observational noise. The AUS scheme, originally formulated to assimilate adaptive observations, is used here to assimilate the fixed observations that are found in the region of local maxima of BDAS vectors (Bred vectors subject to assimilation), while the remaining observations are assimilated by 3DVar. The performance of the hybrid scheme is compared with that of 3DVar and of an EnKF. 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