Abstract
The nested grid is used in a barotropic free surface model. Two grids are nested; one is a coarse mesh grid that covers a large region and the other a fine mesh grid set in a limited area inside the larger region. The interaction between the two grids is one-directional; the boundary condition for the smaller domain is taken from the solutions of the larger domain. The two major problems are how to select appropriate boundary settings for the limited area fine mesh grid and to evaluate how quickly the boundary error grows and invades the inner domain. Two methods of boundary setting are proposed; one is to specify a set of well-posed “physical” boundary conditions as well as to provide the “computational” boundary conditions by using “upwind” extrapolation and “pseudo-characteristic” extrapolation methods. The other is to specify all variables at all boundaries as they are taken from the solutions of the larger area coarse mesh grid and to apply “boundary smoothing” in order to suppress the computational modes. Tests indicate that the solutions for the nested fine mesh appear satisfactory with both methods up to 6.5 days. The semi-implicit difference scheme proves to be particularly efficient for the nested grid calculation.
