Can/Should we handle time-evolving orography

Migrated Jira Issue CDDSO-71
Jira creation date 21/04/2022 15:24
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The UKESM1-ice-LL model includes the BISICLES ice sheet model, in which the orography over Greenland varies annually. The ISMIP6 data request appears to avoid requested data on model levels from their experiments, but VIACSAB does request `loaddust` and `concdust` (in `Emon`) from their experiments.In #2529 this was discovered and I've dug into how MIP Convert struggles with this issue; the error raised is```iris.exceptions.ConcatenateError: failed to concatenate into a single cube. An unexpected problem prevented concatenation. Expected only a single cube, found 5.```Note that this was attempting to process 5 years of data, and the orography is updated at the end of each year. Processing a single year of data at once works fine.There are potential ways past this such as promote orography to be a lat-lon-time varying coordinate should a concatenate operation such as this fail, but CMOR may not handle this at present (see [https://github.com/PCMDI/cmor/issues/649 here]).UKESM2 is expected to include an ice sheet model as standard -- will this mean that all variables on model levels will have to include time varying orography?
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Start by reviewing the failure described for five years of data and compare it with the successful single-year processing case. Read the related discussion in #2529 and the linked CMOR issue, then define how time-evolving orography should be handled for the affected model-level variables and how completion will be verified.

Written by the indexing model from the issue text.