John Dukowicz, Los Alamos National Lab
HYPOP is a new type of hybrid ocean model in which the momentum equations are solved on an Eulerian z-coordinate grid, and the continuity and tracer equations are solved on a vertically-Lagrangian, isopycnal grid. This combination avoids the principal problems of isopycnal and z-coordinate models: the discretization of the pressure gradient on a vertically sloping grid and the associated difficulty of treating the interaction with topography on an isopycnal grid, and the spurious diapycnal mixing in the continuity and tracer equations on an Eulerian grid. The hybrid equations involve a pressure interpolated from the Lagrangian grid for use in the momentum equation pressure gradient and a velocity interpolated from the Eulerian grid for fluxing Lagrangian layer thickness and total layer tracer distribution. The interpolation of pressure and velocity is a special kind of remapping because it must satisfy an energy consistency condition whereby the interchange of kinetic and potential energy is correctly accounted for. The Eulerian part of the code is to be based on the Los Alamos POP model; hence the name HYPOP.