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5 3D Level Modelling
5.13 Advanced Lateral Boundary Conditions
5.13.1 Background
Lateral boundary conditions do not only control the flow of fluid properties across a
boundary but also the dynamical behavior of waves as these meet a boundary. Partial
wave reflection is a common problem. The aim of this section is to introduce the
reader to different types of lateral boundary conditions that can be used to improve
the 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 a
boundary, and (b) conditions that allow for undisturbed propagation of waves and
flow across a boundary.
Conditions of the first kind can be referred to as inflow conditions, those of the
second 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 disturbances as these approach a downstream boundary.
5.13.2 Consistency
The horizontal pressure gradient force plays a dominant role in the dynamics of
oceanic flows. Hence, adequate choice of lateral boundary conditions for dynamic
pressure is uttermost crucial. In terms of lateral boundary conditions, “consistency”
means that the boundary condition used for dynamic pressure has to be consistent
with those set for velocity components. Often it is the best approach to employ
lateral boundary conditions for dynamic pressure only and, if possible, to use the
numerical code to predict boundary values of the velocity component normal to
the boundary. Prescription of boundary values for all variables is not recommended
since this can lead to inconsistency in the dynamics and unwanted side effects.
5.13.3 Inflow Conditions
For wave problems excluding Coriolis effects, it is often sufficient to prescribe
dynamic pressure variations at a lateral boundary and to calculate velocities inside
the prediction code. Steady geostrophic inflows such as those in Exercise 21 are best
realised via prescription of flow components normal to the boundary in conjunction
with vanishing gradients of dynamic pressure normal to the boundary which filters
away unwanted geostrophic flow running parallel to a boundary. Another commonly
applied method is the method of one-way nesting of a smaller model domain inside
a larger model domain and to use predictions from the larger domain as boundary
conditions for the smaller domain.
Lateral boundaries for combined geostrophic flow and wave problems are difficult to deal with. One solution would be to decompose dynamic pressure into
5 3D Level Modelling
5.13 Advanced Lateral Boundary Conditions
5.13.1 Background
Lateral boundary conditions do not only control the flow of fluid properties across a
boundary but also the dynamical behavior of waves as these meet a boundary. Partial
wave reflection is a common problem. The aim of this section is to introduce the
reader to different types of lateral boundary conditions that can be used to improve
the 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 a
boundary, and (b) conditions that allow for undisturbed propagation of waves and
flow across a boundary.
Conditions of the first kind can be referred to as inflow conditions, those of the
second 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 disturbances as these approach a downstream boundary.
5.13.2 Consistency
The horizontal pressure gradient force plays a dominant role in the dynamics of
oceanic flows. Hence, adequate choice of lateral boundary conditions for dynamic
pressure is uttermost crucial. In terms of lateral boundary conditions, “consistency”
means that the boundary condition used for dynamic pressure has to be consistent
with those set for velocity components. Often it is the best approach to employ
lateral boundary conditions for dynamic pressure only and, if possible, to use the
numerical code to predict boundary values of the velocity component normal to
the boundary. Prescription of boundary values for all variables is not recommended
since this can lead to inconsistency in the dynamics and unwanted side effects.
5.13.3 Inflow Conditions
For wave problems excluding Coriolis effects, it is often sufficient to prescribe
dynamic pressure variations at a lateral boundary and to calculate velocities inside
the prediction code. Steady geostrophic inflows such as those in Exercise 21 are best
realised via prescription of flow components normal to the boundary in conjunction
with vanishing gradients of dynamic pressure normal to the boundary which filters
away unwanted geostrophic flow running parallel to a boundary. Another commonly
applied method is the method of one-way nesting of a smaller model domain inside
a larger model domain and to use predictions from the larger domain as boundary
conditions for the smaller domain.
Lateral boundaries for combined geostrophic flow and wave problems are difficult to deal with. One solution would be to decompose dynamic pressure into