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Towards Dynamically Consistent Boundary Forcing in ROMS with One-Way Nesting Techniques

This study presents advances in dynamically consistent boundary forcing for the Regional Ocean Modeling System (ROMS), emphasizing one-way nesting approaches. It discusses the inherent limitations of one-way nesting, examining how it can still provide effective solutions when dynamics remain consistent and grid resolutions are matched adequately. The analysis leverages data from various global models and observations, including the World Ocean Atlas 2005, to evaluate performance. Results confirm the necessity of careful boundary mass flux matching and the relevance of observational data in achieving not only model accuracy but also realistic ocean dynamics.

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Towards Dynamically Consistent Boundary Forcing in ROMS with One-Way Nesting Techniques

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  1. Towards Dynamically Consistent Boundary forcing Jeroen Molemaker (UCLA) Evan Mason (ULPGC) Sasha Shchepetkin (UCLA) Francois Colat (UCLA)

  2. One way nesting • Obvious limitations: • One way will never be two way!

  3. The round peg and the square hole

  4. Testing lab: Canary Current system

  5. Forcing at side boundaries • Forcing of the outermost grid. • Something  ROMS • Forcing of (off line) one way nested grids • ROMS  ROMS

  6. Forcing the outermost grid • Output from other (global) models • Observations (such as World Ocean Atlas)

  7. Observation based forcing • World Ocean Atlas 2005 T, S monthly climatology • Absolute SSH (Rio, 2005) • Now, annual mean, but we should include at least monthly averaged perturbations

  8. Our ‘truth standard’ Drifter data SSH variance

  9. World Ocean Atlas 2005 • Using level of no motion (1300 m)

  10. World Ocean Atlas + absolute SSH

  11. World Ocean Atlas + absolute SSH ‘crude’ Ekman layer transport

  12. Assessing large scale, slow dynamics • Subtract geostrophic flow • - Scale vertical profiles with mixed layer depth, f and wind stress vector: • z’ = z/Hbl, (u’) = (u Hbl f)/t

  13. Roms’ Ekman spiral

  14. World Ocean Atlas + SSH ‘spiral’ Ekman transport

  15. Impervious to baroclinic structure? KPP spiral Ekman Patrick style Ekman

  16. How well did we do? Data: Model: SSH variance Drifters

  17. ROMS  ROMS • Expected consistency much higher • Only regime transition is unavoidable • Starting point: • Methods as existing in ROMS tools (Pierrick Penven, Patrick Marchesiello…. Many others)

  18. ROMS  ROMS • Sigma  z-levels  Sigma coordinate • No boundary mass flux correction

  19. Or not so reasonable?

  20. Horizontal-vertical interpolation

  21. Matching boundary mass flux with parent grid

  22. Matching grids at the boundary

  23. Summary • One way nesting can be good when: • Solutions have consistent dynamics • roms  roms • No enormous jumps in resolution • Interpolation does not destroy said consistency

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