trun-3.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 --- trun-3.rst (2797B) --- 1 .. include:: ../../global.txt 2 3 .. _sec-higherresrun: 4 5 Third run: higher resolution 6 ---------------------------- 7 8 Now we change one key parameter, the grid resolution. Model results differ even when the 9 only change is the resolution. Using higher resolution "picks up" more detail in the bed 10 elevation and climate data. 11 12 If you can let it run overnight, do 13 14 .. literalinclude:: scripts/run-3.sh 15 :language: bash 16 :lines: 3- 17 18 This run might take 4 to 6 hours. However, supposing you have a larger parallel computer, 19 you can change "``mpiexec -n 4``" to "``mpiexec -n N``" where ``N`` is a substantially 20 larger number, up to 100 or so with an expectation of reasonable scaling on this grid 21 :cite:`BBssasliding`, :cite:`DickensMorey2013`. 22 23 .. figure:: figures/g10km-10ka-hy-usurf-csurf-cbase.png 24 :name: fig-secondoutputfiner 25 26 Fields from output file ``g10km_10ka_hy.nc``. 27 28 Compare to :numref:`fig-secondoutputcoarse`, which only differs by resolution. 29 30 :Left: :var:`usurf` in meters. 31 :Middle: :var:`velsurf_mag` in m/year. 32 :Right: :var:`velbase_mag` in m/year. 33 34 Some fields from the result ``g10km_10ka_hy.nc`` are shown in 35 :numref:`fig-secondoutputfiner`. :numref:`fig-csurfvsobserved` also compares observed 36 velocity to the model results from 20 km and 10 km grids. As a different comparison, 37 :numref:`fig-ivolboth` shows ice volume time series ``ice_volume_glacierized`` for 20 km and 38 10 km runs done here. We see that this result depends on resolution, in particular because 39 higher resolution grids allow the model to better resolve the flux through 40 topographically-controlled outlet glaciers (compare :cite:`Pfefferetal2008`). However, 41 because the total ice sheet volume is a highly-averaged quantity, the 42 ``ice_volume_glacierized`` difference from 20 km and 10 km resolution runs is only about one 43 part in 60 (about 1.5%) at the final time. By contrast, as is seen in the near-margin ice 44 in various locations shown in :numref:`fig-csurfvsobserved`, the ice velocity at a 45 particular location may change by 100\% when the resolution changes from 20 km to 10 km. 46 47 Roughly speaking, the reader should only consider trusting model results which are 48 demonstrated to be robust across a range of model parameters, and, in particular, which 49 are shown to be relatively-stable among relatively-high resolution results for a 50 particular case. Using a supercomputer is justified merely to confirm that 51 lower-resolution runs were already "getting" a given feature or result. 52 53 .. figure:: figures/ivol-both-g20km-g10km.png 54 :name: fig-ivolboth 55 56 Time series of modeled ice sheet volume ``ice_volume_glacierized`` on 20km and 10km grids. 57 The present-day ice sheet has volume about `2.9\times 10^6\,\text{km}^3` 58 :cite:`BamberLayberryGogenini`, the initial value seen in both runs.