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Resolve "IAU bug fixes for RK3"

Mike Bell requested to merge 445-iau-bug-fixes-for-rk3 into main

Closes #445 (closed)


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?

      ORCA2_ICE_OBS results are different with respect to the main- this is consistent with the changes

    • 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?

Other testing

The code was tested using the ORCA2-ICE-OBS standard configuration. The assimilation increments are spatially uniform, T, S, u, v and SSH increments. There are no sea-ice increments.

These increments were assimilated over a 1-day period. The differences between restart files after 1 day of integration assimilating and not assimilating data were visually inspected. The differences using RK3 integration scheme match the increments for SSH, T and S fields quite well and just slightly better than they do using the MLF scheme. For both MLF and RK3 the velocity increments fail to have any significant impact on the velocity fields.

Tests assimilating T and S fields only on the 3rd sub-stage of RK3 were similarly successful - but have not been included in this issue/update.


  • 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 doc (manual, guide, web, ...) now updated or completed following the proposed summary in preview section?
Edited by Daley Calvert

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