tmismip.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch) (HTM) git clone git://src.adamsgaard.dk/pism (DIR) Log (DIR) Files (DIR) Refs (DIR) LICENSE --- tmismip.rst (6300B) --- 1 .. include:: ../../global.txt 2 3 .. _sec-MISMIP: 4 5 MISMIP 6 ------ 7 8 This intercomparison addresses grounding line dynamics by considering an idealized 9 one-dimensional stream-shelf system. In summary, a flowline ice stream and ice shelf 10 system is modeled, the reversibility of grounding line movement under changes in the ice 11 softness is tested, different sliding laws are tested, and the behavior of grounding lines 12 on reverse-slope beds is tested. The intercomparison process is described at the website 13 14 |mismip-url| 15 16 Find a full text description there, along with the published report on the results 17 :cite:`MISMIP2012`; that paper includes results from PISM version 0.1. These documents are 18 essential reading for understanding MISMIP results generally, and for appreciating the 19 brief discussion in this subsection. 20 21 PISM's version of MISMIP includes an attached ice shelf even though modeling the shelf is 22 theoretically unnecessary in the flow line case. The analysis in :cite:`SchoofMarine1` shows 23 that the only effect of an ice shelf, in the flow line case, is to transfer the force 24 imbalance at the calving front directly to the ice column at the grounding line. Such an 25 analysis does not apply to ice shelves with two horizontal dimensions; real ice shelves 26 have "buttressing" and "side drag" and other forces not present in the flow line 27 :cite:`Goldbergetal2009`. See the next subsection on MISMIP3d and the Ross ice shelf example in 28 section :ref:`sec-ross`, among other examples. 29 30 We must adapt the usual 3D PISM model to two horizontal dimensions, i.e. to do flow-line 31 problems (see section :ref:`sec-flowline-modeling`). The flow direction for MISMIP is 32 taken to be "`x`". We periodize the cross-flow direction "`y`", and use the minimum number 33 of points in the `y`-direction. This number turns out to be "``-My 3``"; fewer points than 34 this in the cross-flow direction confuses the finite difference scheme. 35 36 PISM can do MISMIP experiments with either of two applicable ice dynamics models. Model 1 37 is a pure SSA model; "category 2" in the MISMIP classification. Model 2 combines SIA and 38 SSA velocities as described in :cite:`Winkelmannetal2011`; "category 3" because it resolves 39 "vertical" shear (i.e. using SIA flow). 40 41 There are many runs for a complete MISMIP intercomparison submission. Specifically, for a 42 given model there are `62` runs for each grid choice, and three (suggested) grid choices, 43 so a full suite is `3 \times 62 = 186` runs. 44 45 The coarsest grid ("mode 1") has 12 km spacing. The finest grid, "mode 2" with 1.2 km 46 spacing, accounts for all the compute time, however; in the MISMIP description it is 1500 47 grid spaces in the flow line direction (= 3001 grid *points* in PISM's doubled 48 computational domain). In between is "mode 3", a mode interpretable by the intercomparison 49 participant, and here we just use a 6 km grid. 50 51 The implementation of MISMIP in PISM conforms to the intercomparison description, but that 52 document specifies 53 54 ... we require that the rate of change of grounding line position be `0.1` m/a or 55 less, while the rate of change of ice thickness at each grid point at which ice 56 thickness is defined must be less than `10^{-4}` m/a... 57 58 as a standard for "steady state". The scripts here do not implement this stopping 59 criterion. However, we report enough information, in PISM output files with scalar and 60 spatially-variable time-series, to compute a grounding line rate or the time at which the 61 thickness rate of change drops below `10^{-4}` m/a. 62 63 See 64 65 .. code-block:: none 66 67 examples/mismip/mismip2d/README.md 68 69 for usage of the scripts that run MISMIP experiments in PISM. For example, as described in 70 this ``README.md``, the commands 71 72 .. code-block:: none 73 74 ./run.py -e 1a --mode=1 > experiment-1a-mode-1.sh 75 bash experiment-1a-mode-1.sh 2 >& out.1a-mode-1 & 76 ./plot.py ABC1_1a_M1_A7.nc -p -o profileA7.png 77 78 first generate a bash script, then use it to do a run which takes about 20 minutes, and 79 then generate an image in ``.png`` format. Note that step 7 is in the middle of the 80 experiment. It is shown in :numref:`fig-MISMIPmodel1exper1aA7` (left). 81 82 83 .. figure:: figures/mismip-resolution.png 84 :name: fig-MISMIPmodel1exper1aA7 85 86 A marine ice sheet profile in the MISMIP intercomparison; PISM model 1, experiment 1a, 87 at step 7. Left: grid mode 1 (12 km grid). Right: grid mode 3 (6 km grid). 88 89 .. figure:: figures/SM-1a-A1.png 90 :name: fig-SMexper1aM1A1 91 92 Analytical profile for steady state of experiment 1a, step 1, from theory in 93 :cite:`SchoofMarine1`. This is a boundary layer asymptotic matching result, but not the 94 exact solution to the equations. 95 96 The script ``MISMIP.py`` in ``examples/mismip/mismip2d`` has the ability to compute the 97 profile from the Schoof's :cite:`SchoofMarine1` asymptotic-matching boundary layer theory. This 98 script is a Python translation, using ``scipy`` and ``pylab``, of the `provided MATLAB 99 codes <mismip-code_>`_. For example, 100 101 .. code-block:: none 102 103 python MISMIP.py -o mismip_analytic.png 104 105 produces a ``.png`` image file with :numref:`fig-SMexper1aM1A1`. By default 106 ``run.py`` uses the asymptotic-matching thickness result from the :cite:`SchoofMarine1` theory 107 to initialize the initial ice thickness, as allowed by the MISMIP specification. 108 109 .. figure:: figures/profileA7-M2.png 110 :name: fig-MISMIPmode2results 111 112 Results from MISMIP grid mode 2, with 1.2 km spacing, for steady state of experiment 113 1a: profile at step 7 (compare :numref:`fig-MISMIPmodel1exper1aA7`). 114 115 Generally the PISM result does not put the grounding line in the same location as Schoof's 116 boundary layer theory, and at least at coarser resolutions the problem is with PISM's 117 numerical solution, not with Schoof's semi-analytic theory. The result improves under grid 118 refinement, however. Results from grid mode 3 with 6 km spacing, instead of 12 km in mode 119 1, are the right part of :numref:`fig-MISMIPmodel1exper1aA7`. The corresponding 120 results from grid mode 2, with 1.2 km spacing, are in Figure 121 :numref:`fig-MISMIPmode2results`. Note that the difference between the numerical grounding 122 line location and the semi-analytical location has been reduced from 76 km for grid mode 1 123 to 16 km for grid mode 2 (a factor of about 5), by using a grid refinement from 12 km to 124 1.2 km (a factor of about 10). 125