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Simplify logic for glc_renormalize_smb #133
Description
I think that this logic for glc_renormalize_smb in CMEPS:
CMEPS/mediator/med_phases_prep_glc_mod.F90
Lines 229 to 247 in 0315cae
differs from the equivalent logic in MCT:
In particular, note that the MCT side has a flag glc_coupled_fluxes. This flag is set by CISM based on whether CISM is operating in a fully-coupled sense. The need for this arises from the possibility that CISM can be running in a mode where it is doesn't send liquid/ice fluxes to the ocean (instead relying on CTSM to send the fluxes needed for conservation). (This is always the case when ice evolution is turned off, but it can also be the case even when ice evolution is turned on, depending on how a flag is set.) When we first implemented the renormalization, land ice scientists felt that the default behavior should be for renormalization to be off in this case.
@mvertens if it's straightforward to set the glc_coupled_fluxes logical in CISM's nuopc_cap, similarly to what's done in the MCT cap, then this could be made consistent. Note that this glc_coupled_fluxes flag potentially differs for each ice sheet when running with multiple ice sheets. If there's some challenge with doing this, then we could check with @whlipscomb and others if it's acceptable to change the default behavior.
Update (2021-10-18): See #133 (comment) for the latest plan.
Activity
mvertens commentedon Oct 15, 2021
@billsacks - where are we with this issue? Has this been resolved?
billsacks commentedon Oct 15, 2021
Thanks for checking. I believe this is still an issue: I think the logic here differs from the logic that was used in MCT. Before spending time changing the logic, though, it's probably worth talking with @whlipscomb and possibly others about whether we definitely want to maintain the MCT logic or use something different here.
mvertens commentedon Oct 15, 2021
@billsacks @whlipscomb - should we set up a quick meeting to talk about this. Since we have moved to CMEPS/CDEPS as the defaults - it would be important to sort this out as soon as possible.
whlipscomb commentedon Oct 15, 2021
@billsacks – I have a couple of questions. Could you please remind me how renormalization is handled for multiple ice sheets? For Greenland only, we do a global sum on the CTSM side, and another global sum on the CISM side, and when these disagree, we multiply the CISM fluxes by a renormalization term to force agreement. (Actually, I think there are two global sums – one for accumulation and one for ablation – but it's the same idea.) Are these sums now done independently for each ice sheet, with a different CTSM sum for each CISM overlap region?
If so, then it makes sense that the value of smb_renormalize for each ice sheet would be linked to the value of glc_coupled_fluxes for each ice sheet. Is this what's now done in CMEPS, or is glc_coupled_fluxes still a global parameter?
Assuming glc_coupled_fluxes is ice-sheet specific, then the only logical difference I see is that the MCT logic checks lnd_prognostic before checking glc_coupled_fluxes. I can imagine a case where we have Antarctic CISM coupled to an active ocean with a data land model. In that case, we could have glc_coupled_fluxes = T for Antarctica, while lnd_prognostic = F. The CMEPS logic implemented by @mvertens would then force renormalization over Antarctica for the case 'on_if_glc_coupled_fluxes'. I don't have a strong intuition about this, but my first thought is that renormalization will do no harm. @billsacks, do you agree, or have I missed something?
billsacks commentedon Oct 16, 2021
For multiple ice sheets, @mvertens has made this work so that the "global" sums are actually just done over each ice sheet independently (this is accomplished by using a multiplying factor of icemask for the given ice sheet). I have verified that, with renormalization on, the results for Greenland are identical between a two-ice sheet run and a single ice sheet run, and similarly for Antarctica, so this seems to be working correctly (assuming you agree that this is the correct way to do the renormalization).
However, what is not implemented is an ice-sheet specific determination of
smb_renormalize. In fact, even for a single ice sheet run, the logic forsmb_renormalizediffers from what it was for mct/cpl7: in mct/cpl7, the default value forsmb_renormalizedepended on whether CISM was operating in a fully-coupled sense, where it is sending fluxes to the ocean. Currently in CMEPS, the logic is just based on whether there is any land-glc coupling. This difference isn't obvious from looking at the code snippets in the original issue comment, and that's my fault for not being clearer there. In mct/cpl7,glc_coupled_fluxesis a flag set by CISM, and is based on whetherzero_gcm_fluxesis true or false:https://github.com/ESCOMP/CISM-wrapper/blob/6040f09b6f4baac7598ffe136de6b8829061238c/drivers/cpl/mct/glc_comp_mct.F90#L212-L221
In CMEPS, the logic is just based on whether there is any coupling between glc and lnd:
https://github.com/billsacks/CMEPS/blob/b9cdf1fde5c7b6fefa7c4c0c4294d0a84d032ed7/mediator/med_phases_prep_glc_mod.F90#L268
So there are two questions here:
For (1), I have personally found the current default rules confusing for myself and in helping others. I know there were good reasons behind the current rules, but I'm beginning to feel that it might be better to have a much simpler default: by default, this renormalization is always on, even if conservation isn't strictly necessary in this case. The reason I suggest this is that I remember there being some confusion in the past when people were comparing two different runs, where one had renormalization on and the other had renormalization off, as a result of the ice sheet being fully-coupled in one but not the other. But @whlipscomb, I'd like to hear your thoughts here (and possibly thoughts from others in the LIWG if appropriate).
For (2): the answer to (2a) will depend on what we do for (1).
For (2b) I assume we'd want this to be ice sheet-specific, though how to do this is not immediately clear to me: We have a mechanism for setting ice sheet-specific config values, but that's inside CISM. This namelist parameter is in the mediator. We could add an ice sheet-specific namelist setting to CMEPS, but that might be tricky. Alternatively, we could set this in the CISM config file and have each ice sheet pass this scalar flag to the mediator, but this could be ugly. I'm not sure what's best here. Honestly, for now, I'm sort of inclined to keep this a global flag until it becomes obvious that it's actually important to allow this to differ for ice sheets (unless, @whlipscomb, it's already pretty clear to you that this really will be needed).
whlipscomb commentedon Oct 18, 2021
@billsacks – Thanks for the clarifications. I think I understand the logic now.
I agree with what you say above. I think it's simpler to have renormalization always on (for all ice sheets) by default, for just the reason you mention.
For 2b, I'm having trouble thinking of a case where a user would want to turn on renormalization for (say) Greenland but not Antarctica. So let's go with the simpler global approach: either on or off for all instances.
billsacks commentedon Oct 18, 2021
Thanks, @whlipscomb
What's needed in this case is actually pretty simple – though will change answers for T compsets:
(1) Remove all logic related to the
glc_renormalize_smboptionon_if_glc_coupled_fluxes. This namelist flag will now just have two possible values,onandoff, with a default ofon.(2) Change the land ice documentation accordingly (there are a couple of mentions of
glc_renormalize_smbin the land ice documentation that will need to be updated with this new behavior).I am renaming this issue accordingly.
[-]Logic for glc_renormalize_smb differs from mct[/-][+]Simplify logic for glc_renormalize_smb[/+]Simplify logic for glc_renormalize_smb
Merge pull request #495 from billsacks/simplify_glc_renormalize_smb
Update CMEPS for 2-way ocean-wave coupling in HAFS (#133)
Squashed commit of the following: