Semi-Lagrangian advection in the NEMO ocean model
Bibliographic record
Abstract
Abstract. As model resolutions increase, the Courant-Frederichs-Lewy (CFL) number based on advective motion becomes the limiting factor in setting the timestep of time-explicit circulation models. Some atmospheric models escape this limit by using an implicit or semi-implicit semi-Lagrangian formulation of advection. This formulation calculates fluid properties along parcel trajectories which follow the fluid motion and end, for each timestep, at prescribed grid-points. This work is the first application of the semi-Lagrangian method to an operational ocean model. In this context, we solve the difficulty posed by the ocean's irregular, interior boundaries by calculating parcel trajectories using a time-exponential formulation. This formulation ensures that all trajectories that are solutions to a fixed-point iteration have an origin point in the valid domain, and it does not require any prescribed extrapolation of the fluid velocities into the invalid (land) portion of the domain. We derive this method in a way that is compatible with the leapfrog timestepping scheme used in the NEMO-OPA (Nucleus for European Modelling of the Ocean, Océan Parallélisé) ocean model, and we present simulation results for a simplified test-case of flow past a model island and for 10-year free runs of the global ocean on the quarter-degree ORCA025 grid.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".