Advanced Ocean Modelling: Using Open-Source Software

166 5 3D Level Modelling5.13 Advanced Lateral Boundary Conditions5.13.1 BackgroundLateral boundary conditions do not only control the flow of fluid properties across aboundary but also the dynamical behavior of waves as these meet a boundary. Partialwave reflection is a common problem. The aim of this section is to introduce thereader to different types of lateral boundary conditions that can be used to improvethe model performance. There are two different kind of open boundary conditions:(a) conditions that are used as forcing in order to create a certain inflow through aboundary, and (b) conditions that allow for undisturbed propagation of waves andflow across a boundary.Conditions of the first kind can be referred to as inflow conditions, those of thesecond kind as outflow conditions, noting that the latter also includes wave signals.Sponge layers are sometimes used in addition to this to filter away dynamical disturbancesas these approach a downstream boundary.5.13.2 ConsistencyThe horizontal pressure gradient force plays a dominant role in the dynamics ofoceanic flows. Hence, adequate choice of lateral boundary conditions for dynamicpressure is uttermost crucial. In terms of lateral boundary conditions, “consistency”means that the boundary condition used for dynamic pressure has to be consistentwith those set for velocity components. Often it is the best approach to employlateral boundary conditions for dynamic pressure only and, if possible, to use thenumerical code to predict boundary values of the velocity component normal tothe boundary. Prescription of boundary values for all variables is not recommendedsince this can lead to inconsistency in the dynamics and unwanted side effects.5.13.3 Inflow ConditionsFor wave problems excluding Coriolis effects, it is often sufficient to prescribedynamic pressure variations at a lateral boundary and to calculate velocities insidethe prediction code. Steady geostrophic inflows such as those in Exercise 21 are bestrealised via prescription of flow components normal to the boundary in conjunctionwith vanishing gradients of dynamic pressure normal to the boundary which filtersaway unwanted geostrophic flow running parallel to a boundary. Another commonlyapplied method is the method of one-way nesting of a smaller model domain insidea larger model domain and to use predictions from the larger domain as boundaryconditions for the smaller domain.Lateral boundaries for combined geostrophic flow and wave problems are difficultto deal with. One solution would be to decompose dynamic pressure into