Transition from NorESM2.5 to NorESM3.0 development

[TomasTorsvik]

The current development of NorESM is done in the noresm_develop branch and tagged with noresm2_5_alphaXX. For CMIP7 we intend to release a model NorESM3.0, which means that at some point there should be a transition from NorESM2.5 to NorESM3.0 development. To mark this transition, it would be useful to decide on a final tag/code release for NorESM2.5. This can be useful going forward, to avoid confusion about what model version we are really developing towards.

Technically, it is possible to compile and run the noresm_develop branch version of NorESM today, but we are still adding capability towards NorESM3.0, and the model is not tuned to produce a stable per-industrial climate state. For instance, we could decide that NorESM2_5_alpha10 will be the last NorESM2.5 tag, make that into a code release, and start naming subsequent tags on the noresm_develop branch as NorESM3_0_alpha01, etc.

If there are developments in NorESM2.3 that should be carried over to NorESM3.0, it would make sense to have these changes in NorESM2.5 as well, as not to "leapfrog" changes over a code version. This is perhaps an argument for why a NorESM2.5 release should wait until after NorESM2.3 is completed.

Please comment here if you have suggestions on how to make the transition from NorESM2.5 to NorESM3.0 development, especially if you see a reason to postpone the NorESM2.5 release to include specific development targets.

[TomasTorsvik]

Update after NorESM core meeting 16-01-2025:
It was decided that we should have a NorESM2.5 release rather soon, either using the existing noresm2_5_alpha08d tag or the upcoming noresm2_5_alpha09 tag. In this case, the NorESM2.5 version will not include updates from NorESM2.3, hence these updates will be included (if applicable) during the NorESM3 development stage.

[gold2718]

@MichaelSchulzMETNO has mentioned that noresm2_5_alpha08d is potentially a good candidate because there is a long run with this setup. If there is a desire to use the noresm2_5_alpha09 tag as the official NorESM2.5 release tag, are there restrictions on what can go into that tag? Do we need to ensure that the currently planned changes (see #582) do not change answers from the long run done using noresm2_5_alpha08d?

[MichaelSchulzMETNO]

What is planned to add for the alpha09 tag? Will these changes be answer changing? What would be the advantage of making the 09 tag rather than the tag 08 the 2.5 version?

[gold2718]

• The planned changes are described in noresm2_5_alpha09: Next tag #582.
• I am not sure what the answer changes will be but if we need to preserve the features of the long run, it will take some more effort to validate that.
• I do not currently see any advantage to using the 09 tag instead of the 08 tag.

[TomasTorsvik]

Changes in BLOM are answer changing. We held back on introducing BLOM changes in noresm2_5_alpha08, to make this a more clean test on CAM updates. Much of the BLOM/iHAMOCC code development during the latter half of 2024 is not present in noresm2_5_alpha08.