tmodel-hierarchy.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 --- tmodel-hierarchy.rst (3301B) --- 1 .. include:: ../../global.txt 2 3 .. _sec-model-hierarchy: 4 5 A hierarchy of simplifying assumptions for grounded ice flow 6 ------------------------------------------------------------ 7 8 :numref:`tab-modelhierarchy` describes a hierarchy of models, listed roughly in order of 9 increasing effectiveness in modeling grounded ice sheets with fast flow features. This is 10 also the order of increasing need for data to serve as boundary and initial conditions, 11 however, as also described in the Table. 12 13 .. list-table:: Hierarchy of flow models in PISM for the grounded parts of ice sheets. 14 Listed from most to fewest simplifying assumptions *and* from least to 15 greatest need for boundary data. The *italicized* models are planned for 16 future versions of PISM but are not implemented so far. 17 :name: tab-modelhierarchy 18 :header-rows: 1 19 20 * - Model 21 - Assumptions 22 - Required data 23 24 * - *perfectly-plastic ice* 25 - *steady state*; ice has shear stresses below a pre-determined ice "yield stress"; 26 also needs pre-determined location of ice sheet margin 27 - - bed elevation 28 29 * - *balance velocities* 30 - *steady state*; ice flows down surface gradient :cite:`JoughinetalGrBal97` 31 - *same as above, plus:* 32 33 - surface mass balance 34 - (initial) ice thickness 35 36 * - isothermal SIA 37 - non-sliding lubrication flow, fixed softness :cite:`BLKCB`, :cite:`EISMINT96` 38 - *same as above, but time-dependence is allowed* 39 40 * - thermo-coupled SIA 41 - non-sliding lubrication flow, temperature-dependent softness :cite:`BBL`, 42 :cite:`EISMINT00` 43 - *same as above, plus:* 44 45 - surface temperature 46 - geothermal flux 47 48 * - polythermal SIA 49 - allows liquid water fraction in temperate ice; conserves energy better 50 :cite:`AschwandenBuelerKhroulevBlatter`, :cite:`Greve` 51 - *same as above* 52 53 * - SIA + SSA hybrid 54 - SSA as a sliding law for thermo-coupled SIA :cite:`BBssasliding`, 55 :cite:`Winkelmannetal2011`; polythermal by default 56 - *same as above, plus:* 57 58 - model for subglacial water 59 - model for basal resistance 60 61 * - *Blatter-Pattyn* 62 - "higher-order", bridging stresses :cite:`Blatter`, :cite:`Pattyn03`, 63 :cite:`SchoofCoulombBlatter` 64 - *same as above* 65 66 It may also be helpful to view the implemented stress balances as PISM software components 67 (C++ classes). :numref:`fig-stressbalance` shows the sequences of actions taken by the 68 SIA-only, SSA-only, and SIA+SSA hybrid model components. In each case a membrane stress 69 solution is generated first, then a distribution of vertical shear in the column of ice is 70 generated second, and finally a use of incompressibility computes the vertical component 71 of the velocity. The nonsliding SIA-only model has a trivialized membrane stress solution. 72 The SSA-only model has a trivialized computation of vertical shear. 73 74 .. figure:: figures/stressbalance.png 75 :name: fig-stressbalance 76 :width: 75% 77 :align: center 78 79 The SIA-only, SSA-only, and SIA+SSA hybrid models represent different "routes" through 80 stress balance PISM components. In each case the inputs are ice geometry and boundary 81 stresses, and the final output is a three-dimensional velocity field within the ice.