EISMINT II

There are seven experiments described in the published EISMINT II writeup [19]. They are named A, B, C, D, F, G, and H. They have these common features:

  • runs are for \(2\times 10^5\) years, with no prescribed time step;
  • a \(61\times 61\) horizontal grid on a square domain (\(1500\) km side length) is prescribed;
  • surface inputs (temperature and mass balance) have angular symmetry around the grid center;
  • the bed is flat and does not move (no isostasy);
  • the temperature in the bedrock is not modeled;
  • only the cold (not polythermal) thermomechanically-coupled SIA is used [19]; and
  • basal melt rates do not affect the evolution of the ice sheet.

The experiments differ from each other in their various combinations of surface temperature and mass balance parameterizations. Experiments H and G involve basal sliding, under the physically-dubious SIA sliding rubric ([22], Appendix B), while the others don’t. Four experiments start with zero ice (A,F,G,H), while the other experiments (B,C,D) start from the final state of experiment A.

In addition to the seven experiments published in [19], there were an additional five experiments described in the EISMINT II intercomparison description [102], labeled E, I, J, K, and L. These experiments share most features listed above, but with the following differences. Experiment E is the same as experiment A except that the peak of the accumulation, and also the low point of the surface temperature, are shifted by 100 km in both \(x\) and \(y\) directions; also experiment E starts with the final state of experiment A. Experiments I and J are similar to experiment A but with non-flat “trough” bed topography. Experiments K and L are similar to experiment C but with non-flat “mound” bed topography.

See Table 38 for how to run all EISMINT II experiments in PISM. Experiments E – L are only documented in [102].

Table 38 Running the EISMINT II experiments in PISM. Use -skip -skip_max 5, on the \(61\times 61\) default grid, for significant speedup.
Command: “pisms + Relation to experiment A
-eisII A -Mx 61 -My 61 -Mz 61 -Lz 5000 -y 2e5 -o eisIIA.nc  
-eisII B -i eisIIA.nc -y 2e5 -o eisIIB.nc warmer
-eisII C -i eisIIA.nc -y 2e5 -o eisIIC.nc less snow (lower accumulation)
-eisII D -i eisIIA.nc -y 2e5 -o eisIID.nc smaller area of accumulation
-eisII F -Mx 61 -My 61 -Mz 81 -Lz 6000 -y 2e5 -o eisIIF.nc colder; famous spokes [23]
-eisII G -Mx 61 -My 61 -Mz 201 -Lz 5000 -y 2e5 -o eisIIG.nc sliding (regardless of temperature)
-eisII H -Mx 61 -My 61 -Mz 201 -Lz 5000 -y 2e5 -o eisIIH.nc melt-temperature activated sliding
-eisII E -i eisIIA.nc -y 2e5 -o eisIIE.nc shifted climate maps
-eisII I -Mx 61 -My 61 -Mz 61 -Lz 5000 -y 2e5 -o eisIII.nc trough topography
-eisII J -i eisIII.nc -y 2e5 -o eisIIJ.nc trough topography and less snow
-eisII K -Mx 61 -My 61 -Mz 61 -Lz 5000 -y 2e5 -o eisIIK.nc mound topography
-eisII L -i eisIIK.nc -y 2e5 -o eisIIL.nc mound topography and less snow

The vertical grid is not specified in EISMINT II, but a good simulation of the thermomechanically-coupled conditions near the base of the ice requires relatively-fine resolution there. We suggest using the default unequally-spaced grid. With 61 levels it gives a grid spacing of \(\sim 20 m\) in the ice layer closest to the bed, but more vertical levels are generally better. Alternatively these experiments can be done with an equally-spaced grid; in this case we suggest using enough vertical levels to give 20 m spacing, for example. When there is sliding, even more vertical resolution is recommended (see Table 38). Also, the vertical extent must be sufficient so that when the ice thickness grows large, especially before thermo-softening brings it back down, the vertical grid is tall enough to include all the ice. Table 38 therefore includes suggested settings of -Lz; experiment F is different because ice thickness increases with colder temperatures.

These SIA-only simulations parallelize well. Very roughly, for the standard \(61\times 61\) horizontal grid, wall-clock-time speedups will occur up to about 30 processors. Runs on finer (horizontal) grids will benefit from even more processors. Also, the “skip” mechanism which avoids updating the temperature at each time step is effective, so options like -skip -skip_max 5 are recommended.

The EISMINT II experiments can be run with various modifications of the default settings. For instance, a twice-finer grid in the horizontal is “-Mx 121 -My 121”. Table 39 lists some optional settings which are particular to the EISMINT II experiments.

Table 39 Changing the default settings for EISMINT II
Option Default values [experiments] Units Meaning
-eisII A   Choose single character name of EISMINT II [19] simplified geometry experiment. See Table 38.
-Mmax

0.5 [ABDEFGHIK],

0.25 [CJL]

\(m / year\) max value of accumulation rate
-Rel

450 [ABEFGHIK],

425 [CDJL]

km radial distance to equilibrium line
-Sb \(10^{-2}\) [all] \((m/year)/km\) radial gradient of accumulation rate
-ST \(1.67 \times 10^{-2}\) [all] K/km radial gradient of surface temperature
-Tmin

238.15 [ACDEGHIJKL],

243.15 [B],

223.15 [F]

K max of surface temperature
-bmr_in_cont     Include the basal melt rate in the mass continuity computation; overrides EISMINT II default.

See subdirectory examples/eismintII/ for a simple helper script runexp.sh.


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