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Assimilation of float positions in general circulation and regional scale ocean models
A. Molcard, A. Griffa, T.M. Ozgokmen, L.I. Piterbarg, V. Taillandier, A. Mariano, T.M. Chin.
RSMAS UM, CNR
amolcard@rsmas.miami.edu(Abstract received 04/25/2005 for session D)
ABSTRACT
Because of the increase in the realism of ocean models and in the coverage of Lagrangian data sets in most of the world's oceans, assimilation of Lagrangian data emerges as a natural avenue to improve ocean state forecast. In this study, we provide an overview of recent work performed by the authors (Molcard et al., 2003; Ozgokmen et al. 2003; Molcard et al., 2005; Taillandier et al., 2005), where a Lagrangian data assimilation method for particle position is developed, implemented in a hierarchy of models, and tested using the twin-experiment approach. The first and main step consists of using the Lagrangian data to correct the Eulerian model velocity in the same layer where the data are collected. This is done by minimizing the distance between observed positions and positions of synthetic floats simulated by the model. The corrections obtained in a single layer are then projected to other layers using statistical correlations and the mass variables are corrected using a dynamical balancing technique based on geostrophy and mass conservation. An extensive set of twin experiments have been performed using an idealized double-gyre configuratonin. The results indicate that the assimilation is effective provided that the data sampling period Dt is smaller than the Lagrangian time scale TL. The performance of the Lagrangian assimilation technique is also compared to that of conventional methods of assimilating drifters as moving current meters, and assimilation of Eulerian data, such as fixed-point velocities. Overall, the results are very favorable for the assimilation of Lagrangian observations to improve the Eulerian velocity field in ocean models. Applications to realistic regional flows in the Mediteranean Sea have also been performed, further improving the method using an adjoint formulation to extend the Eulerian velocity corrections and to implement the mass correction.
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2005 LAPCOD Meeting, Lerici, Italy, June 13-17, 2005