6TH-ORDER DIFFERENCE SCHEME FOR SIGMA-COORDINATE OCEAN MODELS

How to reduce the horizontal pressure gradient error is a key issue of using a-coordinate ocean models, especially of using primitive equati on models for coastal regions. The error is caused by the splitting of the horizontal pressure gradient term into two parts and the subseque nt incomplete cancellation of the truncation errors of those parts. Du e to the fact that the higher the order of the difference scheme, the less the truncation error and the more complicated the computation, a sixth-order difference scheme for the a-coordinate ocean models is pro posed in order to reduce error without increasing complexity of the co mputation. After the analytical error estimation, the Semi-spectral Pr imitive Equation Model is used to demonstrate the benefit of using thi s scheme. The stability and accuracy are compared with those of the se cond-order and fourth-order schemes in a series of calculations of unf orced flow in the vicinity of an isolated seamount. The sixth-order sc heme is shown to have error reductions by factors of 5 compared to the fourth-order difference scheme and by factors of 50 compared to the s econd-order difference scheme over a wide range of parameter space as well as a great parametric domain of numerical stability.