Simplified geometry experiments¶
There have been three stages of ice sheet model intercomparisons based on simplified geometry experiments since the early 1990s .
EISMINT I (European Ice Sheet Modeling INiTiative) 1 was the first of these and involved only the isothermal shallow ice approximation (SIA). Both fixed margin and moving margin experiments were performed in EISMINT I, and various conclusions were drawn about the several numerical schemes used in the intercomparison. EISMINT I is superceded, however, by verification using the full variety of known exact solutions to the isothermal SIA . The “rediscovery”, since EISMINT I, of the Halfar similarity solution with zero accumulation , and verification runs using that solution, already suffices to measure the isothermal SIA performance of PISM more precisely than would be allowed by comparison to EISMINT I results.
EISMINT II  pointed out interesting and surprising properties of the thermocoupled SIA. References , , , , ,  each interpret the EISMINT II experiments and/or describe attempts to add more complete physical models to “fix” the (perceived and real) shortfalls of ice sheet model behavior on EISMINT II experiments. We believe that the discussion in , ,  adequately explains the “spokes” in EISMINT II experiment F as a genuine fluid instability, while  and Appendix B of  adequately cautions against the continuum model that generates the “spokes” in EISMINT II experiment H. Thus we can move on from that era of controversy. In any case, PISM has built-in support for all of the published and unpublished EISMINT II experiments; these are described in the next subsection.
The ISMIP (Ice Sheet Model Intercomparison Project) 2 round of intercomparisons covers 2008–2013 (at least). There are four components of ISMIP substantially completed, namely HOM = Higher Order Models , , HEINO = Heinrich Event INtercOmparison , , MISMIP (below), and MISMIP3d (also below).
PISM did not participate in ISMIP-HOM but does support most of prescribed experiments (see ISMIP-HOM).
PISM participated in HEINO, but this ability is unmaintained. We believe the continuum problem described by HEINO, also used in EISMINT II experiment H (above), is not meaningfully approximate-able because of a required discontinuous jump in the basal velocity field. The continuum problem predicts infinite vertical velocity because of this jump (, Appendix B). Details of the numerical schemes and their results are irrelevant if the continuum model makes such a prediction. PISM offers the physical continuum model described in , an SIA+SSA hybrid, as an alternative to the continuum model used in ISMIP-HEINO and EISMINT II experiment H. Indeed the SIA+SSA hybrid is offered as a unified shallow model for real ice sheets (section Ice dynamics, the PISM view).
A third and fourth ISMIP parts are the two parts of the Marine Ice Sheet Model Intercomparison Project, MISMIP  and MISMIP3D . These experiments are supported in PISM, as described in subsections MISMIP and MISMIP3d below.