Documentation for PISM, the Parallel Ice Sheet Model

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history [2015/12/16 18:51]
Ed Bueler add subheading in 2008
history [2016/08/18 19:11]
Ed Bueler minor fiddles
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 In September 2006 PISM was for the first time hosted publicly, [[http://​gna.org/​projects/​pism/​|on GNA]] with a [[http://​svn.gna.org/​viewcvs/​pism/​trunk/​COPYING?​view=log|GNU General Public License]]. ​ (//We benefited greatly from using SVN and having free GNA hosting, even though we eventually moved happily to git and github.//) In September 2006 PISM was for the first time hosted publicly, [[http://​gna.org/​projects/​pism/​|on GNA]] with a [[http://​svn.gna.org/​viewcvs/​pism/​trunk/​COPYING?​view=log|GNU General Public License]]. ​ (//We benefited greatly from using SVN and having free GNA hosting, even though we eventually moved happily to git and github.//)
  
-The next idea, circa mid-2007, that went into PISM was that the SSA should be solved //​everywhere//​. ​ This is because, in a Coulomb or near-Coulomb basal drag regime, the solution simply returns zero sliding where the base is sufficiently strong. ​ Solving everywhere thus defines the ice stream regions organically. ​ This idea arose because Bueler actually read C. Schoof'​s isothermal paper [C. Schoof (2006). //A variational approach to ice stream flow//, J. Fluid Mech. 556, 227--251], and realized this made just as much sense in a thermocoupled context. ​ Additionally,​ solving the SIA everywhere would do no harm because in low-angle ​streams ​and shelves it produced low predicted velocities. ​ Furthermore,​ a convex combination of two reasonable stress balance solutions (i.e. SIA+SSA) was reasonable.+=== 2007PISM gets ice streams ​===
  
-Almost as importantin driving the solve-SSA-everywhere model, was the failure of PISM, and every other model, to produce anything sensible from the [[http://www.ingentaconnect.com/content/igsoc/​jog/​2010/​00000056/​00000197/​art00001|ISMIP-HEINO modeling assumptions/​requirements and boundary conditions]].  ​The issue seen in that project ​is that, in essence, there is no way to switch sliding on or off, in a physically-based thermomechanically-coupled mannerentirely within ​the SIA paradigm.  ​One needs to balance ​the transitions in boundary shear stress with //membrane stresses// within the ice The SIA is a good modelbut not of sliding (or ice shelvesfor that matter).+The next ideacirca mid-2007that went into PISM was that the SSA should be solved ​//everywhere//​.  ​This is because, in a Coulomb ​or near-Coulomb basal drag regime, the solution simply returns zero sliding where the base is sufficiently strong.  ​Solving everywhere thus defines ​the ice stream regions organically. ​ This idea arose because Bueler actually read C. Schoof'​s isothermal paper [C. Schoof (2006). ​//A variational approach to ice stream flow//, JFluid Mech. 556227--251]and realized this made just as much sense in a thermocoupled context.
  
-These ideas, and Jed's work on the PISM SSA implementation,​ led to actually having [[http://​pism.github.io/​uaf-iceflow/​talkagu.pdf|ice ​streams ​in the model for the right reasons by 2007]].  ​The paper E. Bueler and J. Brown, (2009)//Shallow shelf approximation as a “sliding law” in a thermomechanically coupled ice sheet model//, JGeophys. Res. 114 (F3) was the result. ​ This paper turned out to be the core of PISM, and it is the most-cited of the PISM-related papers.+Solving ​the SIA everywhere would do no harm because in low-angle streams ​and shelves ​the SIA produces low velocities.  ​Furthermorea convex combination of two reasonable stress balance solutions ​(i.eSIA+SSA) was reasonable.
  
 +Almost as important, in driving the creation of the solve-SSA-everywhere model, was the failure of PISM, and every other model, to produce anything sensible from the [[http://​www.ingentaconnect.com/​content/​igsoc/​jog/​2010/​00000056/​00000197/​art00001|ISMIP-HEINO modeling assumptions/​requirements and boundary conditions]]. ​ The issue seen in that project is that, in essence, there is no way to switch sliding on or off, in a physically-based thermomechanically-coupled manner, entirely within the SIA paradigm. ​ One needs to balance transitions in boundary shear stress with //membrane stresses// within the ice.
 +
 +In other words, the SIA is a good model, but not of sliding (//or ice shelves, for that matter//).
 +
 +These ideas, and Jed's work on the PISM SSA implementation,​ led to actually having [[http://​pism.github.io/​uaf-iceflow/​talkagu.pdf|ice streams in the model for the right reasons by 2007]]. ​ The paper E. Bueler and J. Brown, (2009). //Shallow shelf approximation as a “sliding law” in a thermomechanically coupled ice sheet model//, J. Geophys. Res. 114 (F3) was the result. ​ This paper turned out to be the core of PISM, and it is the most-cited of the PISM-related papers.
  
 === 2008: new team === === 2008: new team ===
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 Without needing excess tuning based on generally-unavailable data (i.e. inversion of measured surface velocities) the results for surface velocity have the right look.  Indeed, by early 2009 we saw that nearly-untuned results for Greenland [[http://​pism.github.io/​uaf-iceflow/​BKAJS_submit2_twocolumn.pdf|matched observations reasonably well]]. Without needing excess tuning based on generally-unavailable data (i.e. inversion of measured surface velocities) the results for surface velocity have the right look.  Indeed, by early 2009 we saw that nearly-untuned results for Greenland [[http://​pism.github.io/​uaf-iceflow/​BKAJS_submit2_twocolumn.pdf|matched observations reasonably well]].
 +
 +=== 2008: PIK collaboration ===
  
 Fall 2008 involved another big change: Anders Levermann and students (Maria Martin and Ricarda Winkelmann) came to Fairbanks to propose a collaboration in which they would add what PISM needed. ​ To be continued ... Fall 2008 involved another big change: Anders Levermann and students (Maria Martin and Ricarda Winkelmann) came to Fairbanks to propose a collaboration in which they would add what PISM needed. ​ To be continued ...
history.txt · Last modified: 2016/08/18 19:27 by Ed Bueler
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