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.
|Control of iterative solver for barotropic mode|
|solverChoice||'pcg'||'ChronGear'||'pcg', 'ChronGear'||preconditioned conjugate gradient or Chronopoulis-Gear preconditioned conjugate-gradient|
|preconditionerChoice||'diagonal'||'diagonal'||'diagonal', 'file'||If 'diagonal', then no preconditioner is used. If 'file', then a preconditioner is used to reduce number of iterations to convergence|
|preconditionerFile||'unknownPrecondFile'||'unknownPrecondFile'||string ≤ 256 characters||file containing preconditioner coefficients for solver|
|convergenceCriterion||1.0e-12||1.0e-13||< 1.0||convergence criterion: |δX/X| < convergenceCriterion|
|maxIterations||1000||1000||integer > 1||upper limit on number of iterations allowed|
|convergenceCheckFreq||10||10||integer ≥ 0||check for convergence every convergenceCheckFreq iterations|
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.