2-WAY INTERACTIVE NESTING OF PRIMITIVE EQUATION OCEAN MODELS WITH TOPOGRAPHY

Two-way interactive nesting of primitive equation ocean models is inve stigated, with special attention to the problems encountered when ocea nic features, for example fronts, intersect the boundaries between the models, and also when topography is present. The model has two intera cting components, the coarse and fine grid regions. The fine grid regi on can have a finer resolution both in the horizontal and in the verti cal than the coarse grid model, allowing refinement of topographic fea tures in the vertical as well as the horizontal, although a model with nesting only in the horizontal is considered first, to clarify the ef fects of the lateral boundaries. An adaptation of the method of Spall and Holland is used. The interaction is two-way: the coarse grid field s are interpolated to provide boundary conditions for the fine grid re gion, and the variables on the fine grid are suitably averaged onto th e coarse grid in order to drive the coarse grid model. Nested calculat ions with 3:1 grid ratios are presented. Modeling of frontal features using nested models is addressed and topography is introduced, but wit hout refinement of the grid in the vertical in the fine grid region. I t is shown that the generation of noise at the interface of the fine g rid and coarse grid regions in the presence of topography can be dealt with by the use of a Newtonian damping scheme. Refinement of the fine grid region resolution in the vertical is implemented, and attention is drawn to problems arising when the fine and coarse grid topographie s are not identical. Care also needs to be taken to ensure that the in itial fields are resolved on both the coarse grid and the fine grid in the region of the interface in order to minimize the generation of di sturbances. Comparison of the results of the nested model with a fine grid everywhere reference calculation shows the nesting technique to b e working successfully over reasonably short periods of time integrati on (16 days) such as may be used operationally for ocean forecasting.