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.