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Impact of the ROMS Pressure Gradient Formulation and Vertical Grid Generator Improvements on HYCOM Performance
George R. Halliwell, Rainer Bleck, Valerie Garnier
RSMAS/University of Miami
(Abstract received 07/15/2007 for session X)
ABSTRACT
The impact on model performance of two additions to the HYCOM code is evaluated. First, the ROMS pressure gradient formulation originally tested by V. Garnier was implemented in the latest HYCOM code base, allowing the user to choose between the original Montgomery potential and new ROMS formulations. Comparison of z, sigma, and isopycnic coordinate performance in the classic seamount problem using both pressure gradient formulations demonstrates that for sigma coordinates, the ROMS formulation reduces the pressure gradient error as measured by spurious velocity to a magnitude comparable to that achieved by the ROMS code. Isopycnic coordinates using the Montgomery potential formulation result in errors that are two orders of magnitude smaller than this. Use of z coordinates also results in smaller errors, being 50% smaller for Montgomery potential and 75% smaller for ROMS. Sigma coordinates used with Montgomery potential and isopycnic coordinates used with the ROMS produce much larger errors. The impact of these choices on West Florida Shelf simulations will be presented. Second, the HYCOM vertical grid generator, which presently uses piecewise constant (PCM) remapping in the isopycnic interior and piecewise linear (PLM) remapping elsewhere, was improved by adding a third option, the higher-order piecewise parabolic (PPM) remapping developed by R. Bleck. Tests of model sensitivity to different remapping choices are underway in Atlantic basin simulations and will be presented at the meeting, emphasizing ocean features sensitive to numerical diffusion such as the thickness of the seasonal thermocline in the tropical ocean and the depth of the Equatorial Undercurrent.
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2007 LOM Workshop, Bergen, Norway, August 20-22, 2007