The new computational kernel of ROMS


Alexander Shchepetkin

University of California at Los Angeles



This talk presents new class of elementary time stepping algorithms for the system
which is the prototype of either shallow-water external mode, or three-dimensional coupled density-momentum internal mode equations. All known time-stepping algorithms for this system may be subdivided into two big groups: synchronous (where r.h.s. of both equations are computed at the same time, and then both variables are updated together) and forward-backward (step one equation, then the other immediately using the newest available information). It is shown that one can generalize forward-backward approach, which result in increased range of numerical stability without, if desired, sacrificing numerical accuracy.

Build around these new time-stepping algorithms is the updated kernel of ROMS - essentially a coupled split-explicit system, which uses optimized forward-backward predictor-corrector methods for both subsystems. Consideration will also be given to aspects of numerical stability of mode splitting exposing the link between splitting error in pressure gradient terms and model stability limits (cf., Higdon - de Szoeke, 1997; Hallberg, 1997) and a new definition of definition of barotropic mode suitable for sigma-coordinates is introduced.

Consideration of parallelization issues (including comparison of efficiency of shared-memory and MPI options coexisting in within the same source code) completes the discussion the the new engine.