| Home > Research > Ifsdocs > WAVES > |
Chapter 2. The kinematic part of the energy balance equation
Chapter 3. Parametrization of source terms and the energy balance in a growing wind sea
Chapter 4. An optimal interpolation scheme for assimilating altimeter data into the WAM model
Chapter 5 Numerical scheme
Chapter 6 The WAM-model software
Chapter 7 Wind-wave interaction at ECMWF
REFERENCES
Previous Section
5.4 Boundary conditions and grid nesting
Normally the wave model grid is surrounded by land points. Therefore, the natural boundary conditions are no energy flux into the grid and free advection of energy out of the grid at the coastline.
The generation and propagation of ocean waves covers a wide range of space and time scales. In the open ocean, the scale of a wind sea system is determined by the size of a depression, which typically has scales of the order of 1000 km. On the other hand, near the coast, the scale of a wave system is determined by the coastal geometry and bottom topography, which have usually much smaller scales. A wave model which covers all scales uniformly is not practicable because of computer limitations. In addition, running a high-resolution wave model for the open ocean seems a waste of computer time.
There are several ways out of this problem. One approach would be to run a wave model with a variable grid, having a high resolution whenever needed (for example near the coast) and having a coarse resolution in the open ocean. So far this approach has not been followed. The WAM model was developed with the practical application in mind of running a global ocean wave model at ECMWF and running limited area models at the European National Weather Centres. Therefore, preference was given to another solution, in which one has the option to run the model on nested grids. This gives the opportunity to use results of a coarse mesh model from a large region in a fine mesh regional model. Several successive levels of nesting may be necessary. The two-dimensional spectra computed by the coarse mesh model are saved at grid points which are on the boundary of the limited area, high-resolution grid. These spectra are then interpolated in space and time to match the high resolution at the grid boundaries. It should be pointed out, however, that a straightforward linear interpolation of spectra gives problems because the interpolated spectra are usually not well balanced, resulting in their rejection when used as boundary conditions for a fine mesh run. To circumvent this problem, the following interpolation procedure is used. Instead of linearly interpolating the spectra from the adjacent points of the coarse grid directly, we rescale these spectra in such a way that the rescaled spectrum has the same mean frequency, mean wave direction and wave energy as found from a linear interpolation of these mean quantities to the fine mesh grid point. The wave spectrum at the fine mesh grid point is then found by linearly interpolating the rescaled spectra. This procedure seems to give satisfactory results.
Next Section
Previous Section
 |
23.04.2002 |
 |
© ECMWF |
