MPAS‐Ocean Simulation Quality for Variable‐Resolution North American Coastal Meshes. (25th March 2020)

• Record Type: Journal Article
• Authors: Hoch, Kristin E.; Petersen, Mark R.; Brus, Steven R.; Engwirda, Darren; Roberts, Andrew F.; Rosa, Kevin L.; Wolfram, Phillip J.
• Is Part Of: Journal of advances in modeling earth systems. Volume 12:Number 3(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-03-25
• Subjects: ocean modeling -- mesh -- variable resolution -- MPAS‐Ocean -- mesh quality -- Gulf Stream; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2019MS001848
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: Climate model components utilizing unstructured meshes enable variable resolution, regionally enhanced simulations within global domains. Here we investigate the relationship between mesh quality and simulation statistics using the JIGSAW unstructured meshing library and the Model for Prediction Across Scales‐Ocean (MPAS‐Ocean) with a focus on Gulf Stream dynamics. In the base configuration, the refined region employs 8 km cells that extend 400 km from the coast of North America. This coastal‐refined region is embedded within a low‐resolution global domain, with cell size varying latitudinally between 30 and 60 km. The resolution transition region between the refined region and background mesh is 600 km wide. Three sensitivity tests are conducted: (a) The quality of meshes is intentionally degraded so that horizontal cells are progressively more distorted; (b) the transition region from high to low resolution is steepened; and (c) resolution of the coastal refinement region is varied from 30 to 8 km. Overall, the ocean simulations are shown to be robust to mesh resolution and quality alterations. Meshes that are substantially degraded still produce realistic currents, with Southern Ocean transports within 0.4% and Gulf Stream transports within 12% of high‐quality mesh results. The narrowest transition case of 100 km did not produce any spurious effects. Refined regions with high‐resolution produce eddy kinetic energy and sea surface height variability that are similar to the high‐resolution reference simulation. These results provide heuristics for the design criteria of variable‐resolution climate model domains. Plain Language Summary: Computer simulations used to study the ocean use grids that cover the ocean's surface, and computations are conducted in each grid cell. The smaller these cells are, the more detailed the simulation is, but simulations with more cells are more expensive to run. We experiment with adding small cells in the region of interest, in this case the North American coast, and larger cells in the rest of the ocean. We conducted three series of tests and looked at the effects on the Gulf Stream, an ocean current off the East Coast of North America. (a) We wanted to know how much adding these small cells improved the simulation. We changed the size of the coastal cells from 30 km wide (less detailed) to 8 km wide (more detailed). Smaller cells improved the results along the North American coast. (b) We cannot go straight from the small to large cells and must have intermediate‐sized cells in between. We experiment with different numbers of these intermediate transition cells. The more intermediate cells we added, the better the results were. (c) We wanted to know whether the cells have to be a regular shape in order to get good results. We experimented with irregular cell shapes. The irregular cells produced results that were very similar to the regular cells. Key Points: Regionally refined MPAS‐Ocean simulations are comparable to global high‐resolution simulations for numerous metrics Variable‐resolution unstructured Voronoi meshes created using JIGSAW are evaluated for quality Simulation quality remains high for steep resolution transitions and intentionally degraded meshes