A study of overflow simulations using MPAS-Ocean: Vertical grids, resolution, and viscosity

• Published in: Ocean modelling (Oxford) 2015-12, Vol.96 (P2), p.291-313
• Main Authors: Reckinger, Shanon M., Petersen, Mark R., Reckinger, Scott J.
• Format: Article
• Language: English
• Subjects: Buoyancy; Computer simulation; Domes; Dynamics; Dynamics of overflow mixing and entrainment (DOME); GEOSCIENCES; Horizontal; Marine; Mathematical models; Model resolution; MPAS-Ocean; Sigma vertical coordinate; Slopes; Viscosity; Z-star with partial bottom cells
• ISSN: 1463-5003; 1463-5011
• DOI: 10.1016/j.ocemod.2015.09.006

•Horizontal resolutions of 50 km produce excessive mixing regardless of other parameters.•A horizontal resolution of 10 km and a vertical resolution of 60 m are recommended.•Mixing and final buoyancy are strongly sensitive to vertical viscosity.•Eddies occur at lower horizontal resolution as long as the vertical resolution is high enough.•Sigma coordinates are less sensitive to changes in resolution and viscosity than z-coordinates. MPAS-Ocean is used to simulate an idealized, density-driven overflow using the dynamics of overflow mixing and entrainment (DOME) setup. Numerical simulations are carried out using three of the vertical coordinate types available in MPAS-Ocean, including z-star with partial bottom cells, z-star with full cells, and sigma coordinates. The results are first benchmarked against other models, including the MITgcm’s z-coordinate model and HIM’s isopycnal coordinate model, which are used to set the base case used for this work. A full parameter study is presented that looks at how sensitive overflow simulations are to vertical grid type, resolution, and viscosity. Horizontal resolutions with 50 km grid cells are under-resolved and produce poor results, regardless of other parameter settings. Vertical grids ranging in thickness from 15 m to 120 m were tested. A horizontal resolution of 10 km and a vertical resolution of 60 m are sufficient to resolve the mesoscale dynamics of the DOME configuration, which mimics real-world overflow parameters. Mixing and final buoyancy are least sensitive to horizontal viscosity, but strongly sensitive to vertical viscosity. This suggests that vertical viscosity could be adjusted in overflow water formation regions to influence mixing and product water characteristics. Lastly, the study shows that sigma coordinates produce much less mixing than z-type coordinates, resulting in heavier plumes that go further down slope. Sigma coordinates are less sensitive to changes in resolution but as sensitive to vertical viscosity compared to z-coordinates.