A Semi-Implicit Free Surface Formulation for the Semi-Collocated Grid Diecast Ocean Model
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MISSISSIPPI STATE UNIV STENNIS SPACE CENTER CENTER FOR AIR SEA TECHNOLOGY
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Under funding from the Office of Naval Research ONR, the Mississippi State University Center for Air Sea Technology CAST presents a semi-implicit free-surface formulation for the DieCAST Ocean model that retains the accurate, low dissipation numerics of its latest and best semi collocated rigid lid version. The approach involves integrating the time implicit trapezoidal shallow water equations on a staggered Arakawa c grid, with vertically averaged baroclinic forcing terms determined on a non-staggered Arakawa a grid, including a fourth-order-accurate baroclinic pressure gradient. The shallow water equation numerics are virtually equivalent to standard sigma coordinate approaches for the model barotropic mode. In application to transient wind forced lake Kelvin waves, the new free-surface version gives virtually identical results to the corresponding strongly validated rigid lid DieCAST version reflecting the fact that the rigid lid barotropic mode numerics are also a sigma-like approach, and requires less than 50 percent more computing. Thus, the numerics used by both rigid lid and free-surface DieCAST versions combines the best of z-level and sigma coordinate numerics, as well as, the best of a and c grid numerics.
- Physical and Dynamic Oceanography