5. Computational options¶
5.1. Barotropic mode solver¶
As part of the implicit solution of the barotropic mode, a
two-dimensional elliptic equation for the surface pressure is solved.
Two solver methods are available, both iterative: preconditioned
conjugate gradient and a Chronopouos-Gear form of pcg requiring fewer
global reductions. Convergence of the iterative solvers is governed by
the two parameters convergenceCriterion and maxIterations as
shown in the table below. The convergence criterion
convergenceCriterion should be chosen small enough such that the
pressure balance (printed as part of the model global diagnostics)
agrees to 3-4 digits. The parameter maxIterations must be chosen
large enough to allow the solver to converge (typically a few hundred),
but small enough so that the code will terminate in a reasonable time if
the solver is unable to converge.
Todo
Add link to namelist group &solvers for Barotropic solver namelist
Occasionally, when benchmarking the code, it is useful to fix the
number of iterations to give a consistent iteration count between
runs. In this case, convergenceCriterion is set to exactly zero
and the solver will iterate maxIterations and continue with the
simulation without terminating. This feature should only be used for
benchmarking and not for actual ocean simulations.
The convergenceCheckFreq provides a means to improve performance by
checking for convergence every convergenceCheckFreq iterations, thus
eliminating an extra global sum on most iterations. Another means for
improving performance is to supply a preconditioner to improve
convergence. The preconditioner must be computed off-line and must be in
the form of a nine point stencil operator. The preconditioner is then
supplied in a file named preconditionerFile containing the nine
operator weights.
5.2. Advection methods¶
Currently, advection of momentum is always done by leapfrog centered advection with periodic ‘mixing’ steps. For tracer advection, three options are available. The first is standard leapfrog centered advection; the second is a 3rd-order upwinding [Leonard] which, although not monotone, will improve monotonicity at a somewhat increased computational cost, and the third, available only in CESM1 POP2, is a one-dimensional Lax-Wendroff with one-dimensional flux limiters.
Todo
Add link to namelist group &advect for Advection namelist
5.3. Pressure gradient options¶
The pressure-averaging technique was explained in a previous section. Because it increases the timestep, it should always be enabled. The option to turn it off is provided only to permit comparisons with and without pressure-averaging or between POP and other codes that do not incorporate this technique.
The pressure gradient term includes a density factor which is assumed to be a constant reference density in Boussinesq models. The depth-dependent pressure effects on this density can be corrected for using simple depth-dependent factors.
Todo
Add link to namelist group &advect for Pressure averaging namelist