Nesting a nonhydrostatic model in a hydrostatic model: The boundary interface

A method is developed for adjusting the values of the prognostic variables near the interface between a nonhydrostatic, high resolution model embedded in a hydrostatic, coarser resolution model. It incorporates a method of conditioning the outer domain lateral boundary values to enforce conservation of volume when the variables are interpolated onto the inner domain grid. This is accomplished by adjusting the baroclinic normal velocities at the open boundaries after interpolation. The method also includes a relaxation scheme which matches the values of the prognostic variables across a narrow zone near the open boundaries of the submesoscale inner domain. This prevents the development of discontinuities, reflections and perimeter currents at the periphery of the inner domain. Submesoscale hindcasts are conducted in areas of high Nonlinear Internal Wave (NLIW) activity. Since NLIWs have amplitudes, O(100 m), which are not small relative to their wavelengths, O(100 – 1000 m) these hindcasts require a nonhydrostatic model in the inner domain. Since the hydrostatic model lacks the physics and resolution to support NLIWs there will be discrepancies between the values of the hydrostatic and nonhydrostatic prognostic variables at the boundaries. It is shown that the application of these method for adjusting the values of the prognostic variables near the interface allows the computation of a nonhydrostatic, submesoscale hindcast forced by a nested hydrostatic forecasting system.