Version 3 (modified by davestorkey, 9 months ago) (diff)

Name and subject of the action

Last edition: 01/21/20 15:08:25 by davestorkey

The PI is responsible to closely follow the progress of the action, and especially to contact NEMO project manager if the delay on preview (or review) are longer than the 2 weeks expected.

  1. Summary
  2. Preview
  3. Tests
  4. Review


Action ENHANCE-01_davestorkey_fix_3D_momentum_trends
PI(S) Dave Storkey
Digest Fix 3D momentum trends diagnostics to work properly with split-explicit timestepping
Dependencies If any
Branch source:/NEMO/branches/{YEAR}/dev_r{REV}_{ACTION_NAME}
Previewer(s) Names
Reviewer(s) Names
Ticket #2344


There are various issues with the 3D momentum trends diagnostics (ln_dyn_trd=T) when using the split-explicit solution of the momentum equation (dynspg_ts - default from NEMO 4.0 onwards):

  1. The "SPG" trend is not the trend due to the surface pressure gradient. It is the change in the velocities due to the solution of the barotropic part of the equations in dyn_spg_ts.
  1. More generally, for trends which are allowed to vary over the barotropic subcycling (PVO, BFR), there is a small inaccuracy because currently this variation is not taken into account.
  1. At various points in the code the barotropic part of the velocities is subtracted or added back in (eg. at the beginning of dyn_spg_ts, or at the beginning of dyn_zdf for the implicit bottom friction option). This subtraction/addition of the barotropic component is sometimes incorrectly included in the trend associated with that module.

These problems mean that it is currently not possible to close the momentum budget, as already noted in tickets #1584 and #1984. The 3D momentum trends are used to calculate various other diagnostics such as 2D vorticity trends (ln_vor_trd=T) or kinetic energy trends (ln_KE_trd=T) so the problems also affect these derived diagnostics.


  1. Momentum trends diagnostics should correspond to terms in the primitive equations (rather than NEMO modules).
  1. We should have a small number of 3D trends which are sufficient to close the budget: a) Each 3D trend should include the barotropic and baroclinic part. (We can output the barotropic part separately if we want). Where the barotropic part of the trend varies over the subcycling this part can be calculated accurately using the time-averaging weights already available in dynspg_ts.F90. b) The horizontal pressure gradient trend (HPG) should include the surface pressure gradient (SPG). The SPG trend can be output as an extra diagnostics not required to close the budget. c) The vertical diffusion trend (ZDF) already includes the contribution of the wind stress, top stress (in ice shelf cavities) and implicit bottom stress, but it should also include the contribution from the explicit bottom stress terms. The explicit bottom stress can still be output as an extra diagnostic not required to close the budget.
  1. We should make sure that the subtraction or addition of the barotropic component of velocities or velocity trends is not included in the momentum trend diagnostics. This particularly applies to the ZDF trend. (This is similar to a fix proposed but not implemented by Christoph in ticket #1584).

Documentation updates

Using previous parts, define the main changes to be done in the NEMO literature (manuals, guide, web pages, …).


Since the preview step must be completed before the PI starts the coding, the previewer(s) answers are expected to be completed within the two weeks after the PI has sent the request to the previewer(s).
Then an iterative process should take place between PI and previewer(s) in order to find a consensus

Possible bottlenecks:

  • the methodology
  • the flowchart and list of routines to be changed
  • the new list of variables wrt coding rules
  • the summary of updates in literature

Once an agreement has been reached, preview is ended and the PI can start the development into his branch.


Once the development is done, the PI should complete the tests section below and after ask the reviewers to start their review.

This part should contain the detailed results of SETTE tests (restartability and reproducibility for each of the reference configuration) and detailed results of restartability and reproducibility when the option is activated on specified configurations used for this test

Regular checks:

  • Can this change be shown to produce expected impact (option activated)?
  • Can this change be shown to have a null impact (option not activated)?
  • Results of the required bit comparability tests been run: are there no differences when activating the development?
  • If some differences appear, is reason for the change valid/understood?
  • If some differences appear, is the impact as expected on model configurations?
  • Is this change expected to preserve all diagnostics?
  • If no, is reason for the change valid/understood?
  • Are there significant changes in run time/memory?


A successful review is needed to schedule the merge of this development into the future NEMO release during next Merge Party (usually in November).


  • Is the proposed methodology now implemented?
  • Are the code changes in agreement with the flowchart defined at preview step?
  • Are the code changes in agreement with list of routines and variables as proposed at preview step?
    If, not, are the discrepancies acceptable?
  • Is the in-line documentation accurate and sufficient?
  • Do the code changes comply with NEMO coding standards?
  • Is the development documented with sufficient details for others to understand the impact of the change?
  • Is the project literature (manual, guide, web, …) now updated or completed following the proposed summary in preview section?


Is the review fully successful? If not, please indicate what is still missing

Once review is successful, the development must be scheduled for merge during next Merge Party Meeting.