tmass-conservation.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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tmass-conservation.rst (5183B)
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1 .. include:: ../../global.txt
2
3 .. default-role:: literal
4
5 .. _sec-mass-conservation:
6
7 Balancing the books
8 ===================
9
10 2D diagnostics
11 --------------
12
13 PISM provides a number of 2D diagnostics to keep track of mass conservation.\ [#]_
14
15 All of them are computed as time-averaged fluxes over requested reporting intervals.
16 Positive values correspond to mass gain.
17
18 For ice mass, at every grid point we have
19
20 .. literalinclude:: conservation/ice_mass_accounting_error.txt
21
22 .. only:: html
23
24 Click :download:`here <conservation/ice_mass_accounting_error.txt>` to download this
25 `ncap2` script.
26
27 All names on the right-hand side correspond to valid PISM diagnostic quantities.
28
29 To check that all changes in mass are accounted for, download the script above and run\ [#]_
30
31 .. code-block:: bash
32
33 ncap2 -v -S ice_mass_accounting_error.txt \
34 pism_output.nc mass_accounting_error.nc
35
36 The variable `ice_mass_accounting_error` in `mass_accounting_error.nc` will contain ice
37 mass accounting errors at each point. All values of this variable should be close to or
38 equal to zero. They are not zero (in general) due to rounding errors, though.
39
40 Use a shortcut
41
42 .. code-block:: none
43
44 pismr -extra_file ex.nc -extra_times N -extra_vars mass_fluxes,...
45
46 to save all fluxes needed to "balance the books" in terms of ice mass.
47
48 Alternatively, use fluxes in terms of "ice amount" (mass per unit area):
49
50 .. literalinclude:: conservation/ice_amount_accounting_error.txt
51
52 .. only:: html
53
54 Click :download:`here <conservation/ice_amount_accounting_error.txt>` to download this
55 `ncap2` script.
56
57 To save these, use the shortcut
58
59 .. code-block:: none
60
61 pismr -extra_file ex.nc -extra_times N -extra_vars amount_fluxes,...
62
63 Comments
64 ^^^^^^^^
65
66 - `tendency_of_ice_mass_due_to_flow` is the change in ice mass corresponding to flux
67 divergence
68 - `tendency_of_ice_mass_due_to_conservation_error` is the artificial change in ice mass
69 needed to preserve non-negativity of ice thickness. While PISM's mass transport scheme
70 is not proven to be mass-conserving *in every case* (see
71 :cite:`JaroschSchoofAnslow2013`), in most simulations the field is uniformly zero.
72 - `tendency_of_ice_mass_due_to_surface_mass_balance` is the change due to the surface
73 mass balance; note that this is *not* the same as the provided SMB: in ablation areas
74 this is the *effective* mass balance taking into account the amount of ice present
75 - `tendency_of_ice_mass_due_to_basal_mass_balance` is the *effective* change due to
76 basal (grounded and sub-shelf) melting
77 - `tendency_of_ice_mass_due_to_discharge` combines changes due to calving and frontal
78 melt
79
80 Scalar diagnostics
81 ------------------
82
83 Diagnostics listed above are also available as scalars, integrated over the whole
84 computational domain. The "integrated" mass accounting error can be computed using the
85 `ncap2` script below.
86
87 .. literalinclude:: conservation/scalar_accounting_error.txt
88
89 .. only:: html
90
91 Click :download:`here <conservation/scalar_accounting_error.txt>` to download this
92 `ncap2` script.
93
94 Comments
95 ^^^^^^^^
96
97 - `tendency_of_ice_mass_due_to_flow` is the integral of :math:`-\nabla \cdot Q` over the
98 computational domain. This should be zero (up to the effect of rounding errors) in
99 simulations that *do not* use Dirichlet boundary conditions for ice thickness.
100 Prescribing ice thickness creates sources and sinks, and this diagnostic describes their
101 influence.
102 - `tendency_of_ice_mass_due_to_conservation_error` should be zero (or close to zero) in
103 most simulations
104
105 .. _sec-mass-conservation-hydrology:
106
107 Mass accounting in subglacial hydrology models
108 ----------------------------------------------
109
110 PISM's hydrology models provide all the diagnostic fields needed to keep track of changes
111 in subglacial water thickness. [#]_
112
113 At every grid point we have
114
115 .. literalinclude:: conservation/water_mass_accounting_error.txt
116
117 .. only:: html
118
119 Click :download:`here <conservation/water_mass_accounting_error.txt>` to download this
120 `ncap2` script.
121
122 All names on the right-hand side correspond to valid PISM diagnostic quantities.
123
124 Use a shortcut
125
126 .. code-block:: none
127
128 pismr -extra_file ex.nc -extra_times N -extra_vars hydrology_fluxes,...
129
130 to save all diagnostics mentioned above.
131
132 See :ref:`sec-subhydro` for more information about hydrology models.
133
134 Mass accounting in the PDD model
135 --------------------------------
136
137 PISM's PDD model provides diagnostics needed to compare computed accumulation, melt, and
138 runoff to the effective mass balance. Use diagnostic quantities
139 `surface_accumulation_flux`, `surface_melt_flux`, and `surface_runoff_flux` (units of mass
140 per area per time) and `surface_accumulation_rate`, `surface_melt_rate`,
141 `surface_runoff_rate` (units of mass per time).
142
143 To save all these, use `-extra_vars` shortcuts `pdd_fluxes` and `pdd_rates`.
144
145 .. [#] See :ref:`sec-diagnostics-list` for the full list of diagnostics.
146
147 .. [#] `ncap2` is a part of NCO_.
148
149 .. [#] We keep track of :math:`W_{\text{till}} + W`, i.e. the sum of the effective
150 thickness of subglacial water stored in till *and* the effective thickness of
151 subglacial water in the transport layer (if applicable).