The current version of CANDIE uses the Arakawa A grid formulation. It was developed by Dan Wright. The main reason for choosing the A grid CANDIE as our standard model code is based on the finding that the C-grid model does not reproduce well the bottom trapped baroclinic topographic Rossby waves in a periodic channel with a uniformly slopping bottom. The governing equations and time discretization used on the A grid CANDIE are identical to the C grid (see Sections A.1 and A.2). The main differences between the A-grid and C-grid formations are solely associated with the spatial discretization of horizontal velocity components u and v. On the A grid u and v are defined at the center of each cell (i.e., the p points, see Figure 1). On the C grid, on the other hand, u and v are defined on the cell faces normal to the x and y directions, respectively.
The A grid formulation overcomes the difficulty of the C-grid in estimating the Coriolis terms since the state variables u and v are defined at the same location. The conservation equations for heat, salt and momentum, however, each require knowledge of the velocity components on the cell faces, i.e. on the C grid. Thus interpolations of the horizontal velocity components on the A grid onto the C grid are still required. The trial velocity components on the C grid can be calculated from the A-grid velocity components. Using two-point averaging, for example, we have:
The surface pressure correction can then be calculated from the depth integrated horizontal divergence of over each water column. The barotropic velocity corrections can readily be calculated from through (33) and (34). Interpolations of back to the p-points yield the total horizontal velocity components on the A grid at the end of the time step: