[Liwg-core] CISM settings for Greenland coupled runs

Jeremy Fyke garmeson.lanl at gmail.com
Sat Jun 24 09:47:55 MDT 2017


Hi Bill L.,

Thanks for the update on these topics related to GrIS within upcoming
coupled simulations.  Some thoughts below, which conclude with a suggestion
for a telecon early next week.

Regarding isostasy: this is exciting that it seems to be working.  I'll
definitely leave it to you/Sarah/Miren to make the go/no go call, and then
go with your decision (and restart bedrock/ice geometry).

Regarding basal sliding scheme: I'll reiterate my conservative opinion as
it pertains to surging CISM outlet glaciers in our first coupled
simulations.  Although surge behavior is potentially interesting/real,
because it's cutting-edge/controversial I believe it requires a separate
ISM-specific study.  This is analogous to CAM/CLM/CICE/POP approaches which
explore major new model behavior in dedicated papers prior to inclusion in
coupled simulations.  I think first CESM2/CISM2 simulations are not
appropriate places to demonstrate surging for the first time.  This is
because reviewer's concerns about unprecedented/undocumented CISM dynamics
could complicate acceptance of the coupled model description (and papers
based on descendent simulations).  I'd argue this viewpoint something to
consider your development efforts and the sensitivity testing you/Sarah are
working on.

Regarding a potential switch to Blatter-Pattyn: DIVA has been an integral
part of recent LIWG planning/work, so your direction to me on Thursday
after the meeting to use Blatter-Pattyn (BP) for the JG/BG simulations
honestly really surprised me.  This strikes me as an unexpected major
decision for both technical, scientific and manpower reasons.  Based on
presentations, discussions and simulation experience my assumption was that
you/we had good trust in DIVA.  Clearly there's some basic confusion on my
part here - which I feel needs clarification before I and all LIWG
stakeholders give a green light to coupled CESM/CISM control/transient
experiments, with either DIVA or BP.

Motivated by these thoughts I suggest an open LIWG telecon early next week
in the spirit of finalizing CISM aspects of the coupled GrIS CESM/CISM
configuration.  I'm definitely not thinking a final decision on all things
CISM will emerge during this call (although this would be ideal of
course!).  Rather, given the central importance of CISM to LIWG efforts, at
this stage I consider it important for the LIWG to get an updated and
consistent understanding of:

-what your ongoing CISM implementation plan is for the newly-understood
stages of the Aschwanden basal traction scheme
-how/why you predict surge dynamics to evolve during this implementation
plan

-remaining DIVA characteristics that you consider uncertain enough to
consider a switch to BP for upcoming CESM runs
-DIVA-BP comparison procedures you have planned
-how existing DIVA-based knowledge will relate to BP tuning (if the DIVA-BP
comparison suggests the need to switch to BP)
-what the DIVA->BP switch (if necessary) will possibly entail in terms of
effort and timelines
-a default fallback CISM configuration

Tuesday a.m. would work for me for a telecon.  I'd imagine inviting
non-liwg-core folks associated with CISM work as well since they may have
useful input.  I'd be happy to arrange telecon logistics if required.  Most
importantly, does that work for you, Bill?

Thanks,

Jeremy






On Fri, Jun 23, 2017 at 9:38 AM, William Lipscomb <lipscomb at ucar.edu> wrote:

> Hi Jeremy,
>
> I was in a rush to head home with Matthew yesterday afternoon, before we
> had time to talk in detail about CISM config settings.  So I wanted to
> follow up in an email.  My understanding is that there are three main
> questions to settle:
>
> (1) Is the isostasy scientifically validated?
> (2) DIVA or Blatter-Pattyn?
> (3) Which effective pressure parameterization for basal sliding?
>
> On (1), I would defer to Sarah.  I'd just add that CISM isostasy is
> numerically the same as Glimmer isostasy (with roundoff-level
> differences).  The main difference is that now we can run it on multiple
> processes with exact restart.
>
> A question for Sarah: How would you like to handle the relx field?  Right
> now we're assuming that the modern Greenland topography is in equilibrium
> given the load.  Do you think this is a good enough assumption, or should
> we try to acquire or compute a more accurate version of the relaxed
> topography?
>
> On (2), there are pros and cons each way, and it may come down to your
> personal preference.  The main advantage of B-P is that it's formally a
> more accurate approximation of Stokes flow, in that vertical variations in
> membrane stresses are included in the momentum balance.  These vertical
> variations can be important where the bed is frozen and the basal
> topography is rough.  See Goldberg (2011, JGlac) Fig. 1 for an illustration
> based on ISMIP-HOM Expt. B (the 2D version of Expt A).  Where there is fast
> basal sliding as in Expts C/D, BP and depth-integrated results are very
> similar (Goldberg Fig. 2).
>
> The main advantages of DIVA are that it's faster (by an order of
> magnitude) and has been run successfully for longer.  I and others have now
> run DIVA for hundreds of thousands of years for Greenland, compared to just
> ~20,000 for BP.  There's no reason to expect one approximation to be more
> numerically robust than the other, but we'd need to run BP for longer
> before having the confidence we now have in DIVA.  As for cost, if you're
> planning to run for ~1 M cpu-hr, then the cost of a 10,000-year BP run,
> though greater than DIVA, would still be a small fraction of the total cost.
>
> I suggest that you hold off on a decision until I can show you a clean
> comparison of DIVA v. BP for a long Greenland spin-up with the parameter
> settings we settle on.  Also, I can give you more precise numbers for
> throughput.  I'll be working on this in the next couple of weeks.
>
> I'm still working on (3).  My runs with the new RACMO2.3 SMB have a
> Greenland volume that's too high by ~10%.  I talked yesterday with Jan and
> Miren about this, and I think an important issue is that the new SMB has
> positive values for peripheral glaciers that should be made negative for
> purposes of an ice sheet spin-up exercise.  Once this issue is resolved, I
> can go back to comparing effective pressure parameterizations, including
> the fraction of the bed that is thawed and the frequency and magnitude of
> oscillations.
>
> On a closer look at Aschwanden et al. (2016), I see that the basal model
> described in (6) is very simple, with a fixed, spatially uniform drainage
> term (and no routing scheme). I think it would not be hard to implement
> this formulation in CISM, so I'd like to try it and compare it to the
> bmlt-based scheme I'm testing now.  The bmlt-based scheme is similar in
> that it leads to a slippery bed where the basal melt rate exceeds 1 mm/yr,
> but Andy's scheme may have more inertia (since you have to refreeze water
> to increase the basal friction) and thus suppress oscillations.
>
> The summary message is that I'm continuing to work to provide plots and
> numbers to inform your decisions on config settings.  Please don't hesitate
> to ask if you have more questions.
>
> Bill L.
>
> --
> *****
> William Lipscomb
> Climate & Global Dynamics
> National Center for Atmospheric Research
> 1850 Table Mesa Drive
> Boulder, CO 80305
>
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>
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