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DIVA is inaccurate when flwa varies strongly vertically #15
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From @gunterl on October 4, 2016 20:46 Hi Matt, Thank you for sharing these results, it's nice to see more work is done Hope all is well. Gunter On Tue, Oct 4, 2016 at 2:34 PM, Matt Hoffman notifications@github.com
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From @matthewhoffman on October 4, 2016 23:4 @gunterl , thanks for the feedback. Yes, this was interesting to discover under a more realistic configuration and seems like an important detail for practical application. My model domain is very simple. Here is a longitudinal profile of thickness (this is a "plastic" glacier shape where Tau_d=10^5 Pa everywhere): |
From @gunterl on October 5, 2016 17:9 Hi Matt, Thanks for continuing the conversation. My next question is if your test On Tue, Oct 4, 2016 at 5:04 PM, Matt Hoffman notifications@github.com
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From @gunterl on October 5, 2016 17:16 Or SSA for that matter? On Tue, Oct 4, 2016 at 5:04 PM, Matt Hoffman notifications@github.com
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From @matthewhoffman on October 5, 2016 20:14 No, this was totally grounded. I have not tried any other solvers on it. The specific setup was using the Schoof basal friction law with a prescribed time series of effective pressure. There was significant amounts of both sliding and deformation. It was kind of an ad hoc setup that I was using for something else, and I stumbled onto the problem unintentionally. @DanFMartin and I have been discussing ideas about setting up a more standardized test that could be used to assess these differences. I was thinking a very simple geometry might be the "slab" test case I set up in CISM a couple years ago but never quite fully verified: It is described in sections 5.1-2 of: Then we would impose a temperature (or alternatively flwa) field that varies strongly vertically, but is horizontally uniform. I was using essentially the borehole temperature data from here: @gunterl , @DanFMartin , let me know if you have any other suggestions about how to set it up. |
From @gunterl on October 5, 2016 20:50 Hi Matt, So I believe C (the powerlaw coefficient from Schoof's law) is spatially On Wed, Oct 5, 2016 at 2:14 PM, Matt Hoffman notifications@github.com
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From @matthewhoffman on October 5, 2016 21:39 In my test, the Coulomb friction coefficent C is a constant, but effective pressure is a time varying 2d field that came from my subglacial hydro model. I don't think any of that is relevant to producing the inaccurate DIVA velocities - I think that is all due to a depth-varying temperature field. So I'd suggest we start with something like no sliding or a moderate beta (say, 1000) and confirm we can reproduce this behavior in a simpler setup. |
From @gunterl on October 5, 2016 21:49 Yes I agree. I was just trying to get a better sense of the dynamics. On Wed, Oct 5, 2016 at 3:39 PM, Matt Hoffman notifications@github.com
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From @DanFMartin on October 5, 2016 23:46 I agree -- Let's start with a moderate beta and go from there. We can add complexity if needed later. Sent from my iPhone
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From @matthewhoffman on October 25, 2016 22:31 I happened on this discussion in a Doug Brinkerhoff paper:
Brinkerhoff, D. J., and J. V Johnson (2015), Dynamics of thermally induced ice streams simulated with a higher-order flow model, J. Geophys. Res. Earth Surf., 120, doi:10.1002/2015JF003499.Received. |
From @whlipscomb on October 25, 2016 22:58 @matthewhoffman, Thanks for the lead. I'll re-read the paper and follow up with Doug or Jesse if needed. |
From @matthewhoffman on October 4, 2016 20:33
The DIVA and BP velocity solvers calculate quite similar results for standard test cases (e.g. ISMIP-HOM), but recent testing has revealed that they generate quite different results when flwa varies significantly vertically.
Here is an example of two runs with identical setup except for which solver is used:
I isolated the issue - it only occurs when flwa varies vertically as in the above example. If flwa is made vertically constant, then the two solvers give similar answers:
It makes sense that the results of DIVA would be sensitive to the details of how flwa (or the effective viscosity) is integrated vertically. This may take some careful thought and testing to resolve in a way that allows DIVA to yield results comparable to BP. For example, flwa can vary vertically by two or more orders of magnitude, meaning a straight arithmetic average may be in appropriate. Similarly, consideration may be needed for the vertical arrangement of flwa values - presumably soft ice at the bed shuold affect the depth-integrated effective viscosity much more than soft ice near the surface.
For the record, I was using this flwa profile (with uniform vertical levels):
which comes from the temperature profile in this paper:
Ryser, C., M. P. Lüthi, L. C. Andrews, M. J. Hoffman, G. A. Catania, R. L. Hawley, T. A. Neumann, and S. S. Kristensen (2014), Sustained high basal motion of the Greenland ice sheet revealed by borehole deformation, J. Glaciol., 60(222), 647–660, doi:10.3189/2014JoG13J196.
and using the updated Cuffey and Paterson flwa formula.
Copied from original issue: E3SM-Project/cism-piscees#61
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