NESTED GRID METHODS FOR AN OCEAN MODEL - A COMPARATIVE-STUDY

In this paper a comparison is carried out between three correction met hods for multigrid local mesh refinement in oceanic applications: FIC, LDC and the direct method (DM) proposed by Spall and Holland. This st udy is based on a nested primitive equation model developed by Laugier on the basis of the code OPA (LODYC). The external barotropic problem is solved using any of the three local grid correction algorithms yie lding an interactive nested grid model. The non-linear elliptic equati on for the barotropic streamfunction tendency is solved on two nested grids, called the global and the zoom grid, that interact between them selves. The zoom grid is entirely embedded within the global domain wi th a horizontal grid step ratio of 3:1. The computation on the global grid supplies the boundary conditions for the zoom grid region and the fine grid fields are used to correct the global coarse solution. The three local correction methods are tested on two problems relevant to oceanic circulation phenomena proposed by Spall and Holland: a barotro pic modon and an anticyclonic vortex. The results show that the nestin g technique is a very efficient way to solve these problems in terms o f a gain in precision compared with the required CPU time. The two-dom ain model with local mesh refinement allows one both to manage effecti vely the open boundary conditions for the local grid and to correct th e global solution thanks to the zoom solution, In the case of the modo n propagation the three local correction methods provide approximately the same results. For the baroclinic vortex it appears that the two i terative methods are more efficient than the direct one.