Comparison of SUNTANS results of the velocity field in Monterey Bay with ADCP data (in the box between t/T=5.5 and 7) in the Monterey Submarine Canyon showing how, unlike previous simulations of internal waves in the Bay, SUNTANS can capture the correct magnitude and phase of the internal waves. Image by S. Jachec; data courtesy of E. Petruncio, L. Rosenfeld, and J. Paduan of the Naval Postgraduate School.

Overview

SUNTANS is a nonhydrostatic, unstructured-grid, parallel, coastal ocean simulation tool that solves the Navier-Stokes equations under the Boussinesq approximation with a large-eddy simulation of the resolved motions. The formulation is based on the method outlined by Casulli in his 1999 papers, where the free-surface and vertical diffusion are discretized with the theta-method, which eliminates the Courant condition associated with fast free-surface waves and the friction term associated with small vertical grid spacings at the free-surface and bottom boundaries. The grid employs z-levels in the vertical and triangular cells in the planform. Advection of momentum is accomplished with the second-order accurate unstructured-grid scheme of Perot (2000), and scalar advection is accomplished semi-implicitly using the method of Gross (1999), in which continuity of volume and mass are guaranteed when wetting and drying is employed. The wetting and drying capabilities of SUNTANS enable its use for coastal as well as estuarine domains. The theta-method for the free-surface yields a two-dimensional Poisson equation, and the nonhydrostatic pressure is governed by a three-dimensional Poisson equation. These are both solved with the preconditioned conjugate gradient algorithm with diagonal preconditioning. SUNTANS is written in the C programming language, and the message-passing interface (MPI) is employed for use in a distributed memory parallel computing environment. Load balancing and grid-partitioning are being managed with the ParMETIS package.

The grids for SUNTANS are unstructured in the planform and employ z-levels in the vertical. We employ stair-stepped grids in the vertical in order to eliminate errors in computing the baroclinic pressure gradients as well as to enable volume and scalar conservation when computing integrals over each water column. Accurate resolution of complex topography will be accomplished with the use of the immersed boundary method (IBM).

16 processor load-balanced partitioning of Monterey Bay with 250,000 planform grid cells and 120 vertical levels (18.2 million total). The partitions in the deep (red, 3000 m) regions have less surface area than those in the shallow (blue, 200 m) regions because inactive cells are not stored. This yields a 37% savings in memory for this particular grid.