Interest of combining satellite altimeter data with temperature and salinity data on the new assimilation MERCATOR System
Benoît Tranchant(1) , Charles-Emmanuel Testut(1) , Nicolas Ferry(1) , and Pierre Brasseur(2)
Mercator-océan, 8/10 rue Hermes,
(2) LEGI, BP 53, 38041 Grenoble , France
The French MERCATOR project is developing several operational ocean forecasting systems to take part in the Global Ocean Data Assimilation Experiment (GODAE). Prototype systems are designed to simulate (1) the Atlantic and Mediterranean Sea (from 1/3° to 1/15°), and (2) the global ocean circulation (from 2° to 1/4°). A new generation of fully multivariate assimilation system referred to as SAM2v1 is being developed from the SEEK (Singular Evolutive Extended Kalman) algorithm (developed at LEGI, Grenoble). This scheme is a Reduced Order Kalman Filter using a 3D multivariate modal decomposition of the forecast error covariance as well as an adaptive scheme to specify parameters of the forecast error. The use of the SEEK filter and its 3D modal representation for the error statistic is intended to overcome some of the limitations of the previous OI scheme in highly inhomogeneous, anisotropic, and nonseparable regions of the world ocean such as shallow areas, as well as in the surface layer. Unlike the original SEEK filter, the variant of SAM2 which is considered here doesn’t evolve the error statistics according to the model dynamics [Testut et al., 2003]. This would require prohibitive costs given the size of the operational systems. However, some form of evolutivity of the background error is taken into account by considering different error sub-spaces related to the analysis day. As the previous system, it allows to assimilate vertical profiles and SST in addition to altimetry (JASON, ERS-2 and GFO), but in larger quantity at a lower cost. To minimise the computational requirements, the analysis kernel in SAM2 has been massively parallelized and integrated in a generic platform hosting the SAM1 and SAM2 kernel families [Tranchant et al., 2005]. This platform is driven by the PALM software [Lagarde et al., 2001; Piacentini et al., 2003] which makes the coupling between the model codes and the assimilation schemes more effective. Several Observing System Experiments have been performed with SAM2 for various prototypes (North Atlantic to Global configurations) and results will be presented and discussed.