Toward Accurate Coastal Ocean Prediction

Several major problems, namely, uncertain surface forcing function, unknown open boundary conditions (OBC), and pressure gradient error using the acoordinate, affect the accuracy of coastal ocean prediction. At open lateral boundaries where the numerical grid ends, the fluid motion should be unrestricted. Ideal open boundaries are transparent to motions. The most popular and successful scheme is the adjoint method. The disadvantages that may restrict its use are ocean-model dependency and difficulty in deriving the adjoint equation when the model contains rapid (discontinuous) processes, such as change of ocean mixed layer from entrainment to shallowing regime. Development of a ocean-model independent algorithm for determining the OBC becomes urgent. Reduction of horizontal pressure gradient error is another key issue of using a-coordinate ocean models, especially of using coastal models. The error is caused by the splitting of the horizontal pressure gradient term into two parts and the subsequent incomplete cancellation of the truncation errors of those parts. As advances of the computer technology, use of highly accurate schemes for ocean models becomes feasible.