[Liwg-core] CISM settings for Greenland coupled runs
garmeson.lanl at gmail.com
Tue Jun 27 09:34:09 MDT 2017
Hi Bill et al from Chicago airport,
Ok let's wait until mid-July so we can get an update on your new tests
regarding BP/DIVA and the evolving basal sliding scheme.
More generally, while I am comfortable leading the overall spin up effort
I firmly believe the final decisions on (and responsibility for) CISM2
configuration in support of LIWG science as part of this exercise is yours,
not mine. For example, I would be hard pressed to choose DIVA at this point
given your clear expression last Thursday to switch to BP. As senior land
ice scientist and lead CISM2 developer you have the most understanding of
the CISM2 decisions and scientific trade offs. You also will presumably
be addressing CISM2-dynamics-specific discussions arising during manuscript
review. So (assuming the latter is correct) I will defer to your final
CISM2 choices after providing my input/opinions and motivating discussion
on topics that I sense are pertinent yellow light issues to LIWG science
Which is all to say I'm looking forward to the call in a few weeks to hear
about progress and your related thoughts! Lets reconnect then.
the choice to go with BP or DIVA is your
On Mon, Jun 26, 2017 at 10:05 AM William Lipscomb <lipscomb at ucar.edu> wrote:
> Hi Jeremy,
> Regarding DIVA and Blatter-Pattyn: I'm confident that both DIVA and BP are
> working correctly, in the sense that they're solving the equations they're
> intended to solve. The BP equations are more accurate approximations of
> Stokes flow, which is why I'd be inclined to use BP in a coupled simulation
> where the cost of either scheme is fairly minimal compared to other model
> components. But if you'd prefer to use the more extensively tested DIVA
> scheme, that's fine. It's your call.
> To help inform that decision, I'll be doing long Greenland spin-ups with
> identical parameter settings apart from DIVA v. BP. We can then compare
> output in detail and decide what further comparisons might be appropriate.
> Regarding basal sliding: I agree that we don't yet know to what extent the
> observed surge behavior in CISM is real versus numerical. For that reason
> I'm inclined, as you are, to try to minimize any numerical aspects of the
> scheme that would tend to promote surging. I'm now working on an effective-
> pressure option that builds in more inertia (in the form of a reservoir of
> stored basal water) so as to damp and/or lower the frequency of surging.
> The attached paper suggests that any sliding scheme allowing feedbacks
> between standard ice-flow dynamics/thermodynamics and subglacier melt
> storage/drainage will permit cyclic behavior. The only way to exclude
> cyclic behavior, then, would be to decouple subglacier water from the
> dynamics. This could be done by prescribing the basal traction coefficient
> or simply turning off the sliding (which_ho_babc = 5 and 4, respectively).
> Those options are available if you're not comfortable with cyclic surging.
> The no-sliding option would give slower, less realistic velocities over
> much of the ice sheet. The prescribed traction coefficient would give
> velocities in good agreement with observed values at the cost of internal
> physical consistency. Again, any approach will have its pros and cons.
> At this point I don't have much to add in a telecon, apart from what I've
> said in this email and the previous one. I suggest that we wait until there
> are more results from the simulations I'll be doing over the next couple of
> Bill L.
> On Sat, Jun 24, 2017 at 11:47 AM, Jeremy Fyke <garmeson.lanl at gmail.com>
>> 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
>> -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?
>> On Fri, Jun 23, 2017 at 9:38 AM, William Lipscomb <lipscomb at ucar.edu>
>>> 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
>>> 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
>>> Liwg-core mailing list
>>> Liwg-core at cgd.ucar.edu
> William Lipscomb
> Climate & Global Dynamics
> National Center for Atmospheric Research
> 1850 Table Mesa Drive
> Boulder, CO 80305
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