tbed-deformation.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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---
tbed-deformation.rst (5224B)
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1 .. include:: ../../../global.txt
2
3 .. _sec-beddef:
4
5 Earth deformation models
6 ------------------------
7
8 The option :opt:`-bed_def` ``[iso, lc]`` turns one of the two available bed deformation
9 models.
10
11 .. _sec-bed-def-iso:
12
13 Point-wise isostasy
14 ===================
15
16 The first model ``-bed_def iso``, is instantaneous pointwise isostasy. This model assumes
17 that the bed at the starting time is in equilibrium with the load. Then, as the ice
18 geometry evolves, the bed elevation is equal to the starting bed elevation minus a
19 multiple of the increase in ice thickness from the starting time:
20
21 .. math::
22
23 b(t,x,y) = b(0,x,y) - f \left[H(t,x,y) - H(0,x,y)\right].
24
25 Here `f` is the density of ice divided by the density of the mantle, so its value is
26 determined by the values of :config:`bed_deformation.mantle_density` and
27 :config:`constants.ice.density` in the configuration file; see :ref:`sec-pism-defaults`.
28 For an example and verification, see Test H in :ref:`sec-verif`.
29
30 .. _sec-bed-def-lc:
31
32 Lingle-Clark
33 ============
34
35 The second model ``-bed_def lc`` is much more physical. It is based on papers by Lingle
36 and Clark :cite:`LingleClark` and Bueler and others :cite:`BLKfastearth`. It generalizes
37 and improves the most widely-used earth deformation model in ice sheet modeling, the flat
38 earth Elastic Lithosphere Relaxing Asthenosphere (ELRA) model :cite:`Greve2001`. It
39 imposes essentially no computational burden because the Fast Fourier Transform is used to
40 solve the linear differential equation :cite:`BLKfastearth`. When using this model in
41 PISM, the rate of bed movement (uplift) and the viscous plate displacement are stored in
42 the PISM output file and then used to initialize the next part of the run. In fact, if
43 gridded "observed" uplift data is available, for instance from a combination of actual
44 point observations and/or paleo ice load modeling, and if that uplift field is put in a
45 NetCDF variable with standard name ``tendency_of_bedrock_altitude`` in the input file,
46 then this model will initialize so that it starts with the given uplift rate.
47
48 Parameters controlling the Lingle-Clark model are listed below.
49
50 .. list-table:: Parameters controlling the Lingle-Clark bed deformation model
51 :header-rows: 1
52 :Widths: 2,1
53
54 * - Parameter
55 - Description
56
57 * - :config:`bed_deformation.lc.elastic_model`
58 - if "on" (the default), include the elastic part of the model
59
60 * - :config:`bed_deformation.lc.update_interval`
61 - time interval (years) between updates
62
63 * - :config:`bed_deformation.lc.grid_size_factor`
64 - ratio of the size of the grid used by this model to the size of PISM's physical
65 computational grid
66
67 * - :config:`constants.ice.density`
68 - density of ice (used to compute ice-equivalent load thickness)
69
70 * - :config:`bed_deformation.mantle_density`
71 - mantle density
72
73 * - :config:`bed_deformation.mantle_viscosity`
74 - mantle viscosity
75
76 * - :config:`bed_deformation.lithosphere_flexural_rigidity`
77 - flexural rigidity of the lithosphere
78
79 Here are minimal example runs to compare these models:
80
81 .. code-block:: none
82
83 mpiexec -n 4 pisms -eisII A -y 8000 -o eisIIA_nobd.nc
84 mpiexec -n 4 pisms -eisII A -bed_def iso -y 8000 -o eisIIA_bdiso.nc
85 mpiexec -n 4 pisms -eisII A -bed_def lc -y 8000 -o eisIIA_bdlc.nc
86
87 Compare the :var:`topg`, :var:`usurf`, and :var:`dbdt` variables in the resulting output
88 files. See also the comparison done in :cite:`BLKfastearth`.
89
90 To include "measured" uplift rates during initialization, use the option
91 :opt:`-uplift_file` to specify the name of the file containing the field :var:`dbdt` (CF
92 standard name: ``tendency_of_bedrock_altitude``).
93
94 Use the :opt:`-topg_delta_file` option to apply a correction to the bed topography field
95 read in from an input file. This sets the bed topography `b` at the beginning of a run as
96 follows:
97
98 .. math::
99 :label: eq-bedcorrection
100
101 b = b_{0} + \Delta b.
102
103 Here `b_{0}` is the bed topography (:var:`topg`) read in from an input file and `\Delta b`
104 is the :var:`topg_delta` field read in from the file specified using this option.
105
106 A correction like this can be used to get a bed topography field at the end of a
107 paleo-climate run that is closer to observed present day topography. The correction is
108 computed by performing a "preliminary" run and subtracting modeled bed topography from
109 present day observations. A subsequent run with this correction should produce bed
110 elevations that are closer to observed values.
111
112 .. warning::
113
114 The variable :var:`viscous_bed_displacement` does not correspond to any measured
115 physical quantity. Do not even attempt to analyze it without a careful reading of
116 :cite:`BLKfastearth`.
117
118 Trying to provide a "hand-crafted" :var:`viscous_bed_displacement` field to PISM is not
119 a good idea.
120
121 Keep in mind that zero :var:`viscous_bed_displacement` does *not* mean that the bed
122 deformation model is in equilibrium.
123
124 .. FIXME: Document regridding viscous_bed_displacement when switching to a finer grid in a
125 long bootstrapping simulation. This is the one and only reasonable example of using a
126 viscous_bed_displacement field provided by the user. (Re-starting from a file created
127 by PISM does not count.)