A study of overflow simulations using MPAS-Ocean: Vertical grids,resolution, and viscosity |
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| Institution: | 1. Fairfield University School of Engineering, Department of Mechanical Engineering, 1073 North Benson Rd., Fairfield, CT 06824, United States;2. Computer, Computational, and Statistical Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States;3. Brown University, Department of Earth, Environmental, and Planetary Sciences, Providence, RI 02912, United States;1. University of Virginia School of Medicine, United States;2. University of Virginia, Department of Orthopedics, United States;3. University of Virginia, Department of Radiology and Medical Imaging, United States;1. Virginia Institute of Marine Science, College of William & Mary, 1375 Greate Road, Gloucester Point, VA 23062, USA;2. California Department of Water Resource, Sacramento, CA 95814, USA;3. University of Florida, Civil and Coastal Engineering Department, 365 Weil Hall, PO Box 116580, Gainesville, FL 32611-6580, USA;4. Tsinghua University, Beijing, China;1. Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey, USA;2. Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Canada;3. Earth and Space Research, Corvallis, Oregon, USA;4. Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA |
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| Abstract: | 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. |
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