tclimate-inputs.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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tclimate-inputs.rst (7040B)
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1 .. include:: ../../global.txt
2
3 .. _sec-climate-inputs:
4
5 Climate inputs, and their interface with ice dynamics
6 -----------------------------------------------------
7
8 Because PISM's job is to approximate ice flow, its "world view" is centered around ice
9 dynamics. The discussion of boundary conditions in this Manual is thus
10 ice-dynamics-centric. On the other hand, there is no constraint on the nature of, or
11 completeness of, climate models which could be coupled to PISM. This section therefore
12 explains a PISM organizing principle, namely that *climate inputs affect ice dynamics by a
13 well-defined interface*.
14
15 Almost no attempt is made here to describe the physics of the climate around ice sheets,
16 so see :cite:`massbalanceglossary` for terminology and :cite:`Hock05` for a review of how
17 surface melt can be modeled. See the :ref:`Climate Forcing Manual <sec-climate-forcing>`
18 for much more information on PISM's climate-coupling-related options and on the particular
19 fields which are shared between the ice dynamics core and the climate model.
20 :numref:`tab-ice-dynamics-bc` lists fields which are needed as boundary conditions at the
21 interfaces.
22
23 All PISM ice sheet models have some kind of interface green in
24 :numref:`fig-climate-inputs`) to a subaerial surface processes layer containing snow,
25 firn, and liquid (or refrozen) runoff. The surface layer is assumed to cover the whole
26 surface of the ice, and all grounded areas that the ice might occupy, including ablation
27 areas and ice-free land. We also always have an interface (blue) to the ocean, but this
28 interface is inactive if there is no floating ice.
29
30 .. figure:: figures/climate-cartoon.png
31 :name: fig-climate-inputs
32
33 PISM's view of interfaces between an ice sheet and the outside world
34
35 .. list-table:: Boundary conditions required by PISM's ice dynamics core; see
36 :numref:`fig-climate-inputs`. The optional :red:`red` interface is absent
37 if PISM does not "own" the surface processes layer.
38 :name: tab-ice-dynamics-bc
39 :header-rows: 1
40
41 * - Boundary surface
42 - Fields (conditions)
43
44 * - upper surface of the surface processes layer (:red:`red`)
45 - *optional*; typically: air temperature, precipitation
46
47 * - top ice surface, but below firn (:green:`green`)
48 - *required*: boundary temperature (or enthalpy), mass flux (SMB) into the ice
49
50 * - ice shelf basal surface (:blue:`blue`)
51 - *required*: mass flux into the ocean, boundary temperature
52
53 * - bottom surface of thermally-modeled bedrock layer (not shown)
54 - *required*: geothermal flux
55
56 The surface processes layer might be very simple. It might either read the important
57 fields from a file or otherwise transfer them from a separate (non-PISM) climate model.
58 If, however, the surface processes layer is "owned" by the PISM model then there is an
59 additional interface (red) to the atmosphere above. In no case does PISM "own" the
60 atmosphere; if it has an interface to the atmosphere at all then it reads atmosphere
61 fields from a file or otherwise transfers them from a climate model.
62
63 Regarding the base of the ice, the temperature of a layer of bedrock in contact with
64 grounded ice is generally included in PISM's conservation of energy model; see subsections
65 :ref:`sec-coords` and :ref:`sec-grid`. Also, as described in section
66 :ref:`sec-beddef`, PISM can apply an optional bed deformation component approximating
67 the movement of the Earth's crust and upper mantle in response to changing ice load. In
68 these senses everything below the black dashed line in :numref:`fig-climate-inputs` is
69 always "owned" by PISM.
70
71 The PISM ice dynamics core would like to get the required fields listed in
72 :numref:`tab-ice-dynamics-bc` directly from observations or measurements, or directly from
73 a GCM. In many realistic modeling situations, however, PISM code must be used for all or
74 part of the surface processes modeling necessary to provide the ice-dynamics core with the
75 needed fields. Due to differences in model resolutions and required down-scaling, this
76 need for some PISM-based boundary-processes modelling may occur even in some cases where
77 PISM is coupled to a GCM. Thus, to be able to use the data that is available, a PISM run
78 might use components that are responsible for modeling surface (snow) processes or
79 sub-shelf/ocean interaction. These components might be very minimal, merely turning data
80 that we already have into data in the right units and with the right metadata.
81
82 .. figure:: figures/data-flow.png
83 :name: fig-climate-input-data-flow
84 :width: 75%
85 :align: center
86
87 PISM climate input data flow. Colored arrows correspond to interfaces in
88 :numref:`fig-climate-inputs`.
89
90 Thus we have PISM's design: the ice-dynamics PISM core does not contain any
91 parameterization or other model for boundary mass or energy fluxes into or out of the ice.
92 These boundary parameterizations and models are present in the PISM source code, however,
93 as instances of ``pism::Component`` classes. This simplifies customizing and
94 debugging PISM's climate inputs, and it promotes code reuse. It isolates the code that
95 needs to be changed to couple PISM to different climate models.
96
97 The classes ``pism::SurfaceModel``, ``pism::AtmosphereModel``, and
98 ``pism::OceanModel`` are all derived from ``pism::Component``. Corresponding to
99 the red dashed line in :numref:`fig-climate-inputs`, a ``pism::AtmosphereModel``
100 might not even be present in some PISM configurations. While they are required,
101 ``pism::SurfaceModel`` and ``pism::OceanModel`` may contain (hide) anything from
102 nearly-trivial parameterizations of ice surface temperatures and mass fluxes to a GCM of
103 great complexity.
104
105 The "modifiers" in :numref:`fig-climate-input-data-flow` adjust the climate model inputs.
106 Modifiers can be chained together so that multiple modifications are made to the outputs
107 of the original component. For example, ice-core-derived air temperature offsets, used to
108 model the space-time distribution of paleo-climatic surface temperature, is an example of
109 an implemented modifier. Please see the :ref:`Climate Forcing Manual
110 <sec-climate-forcing>` for a list of climate components and modifiers included in PISM
111 source code and other details. Users wishing to customize PISM's climate inputs and/or
112 couple PISM to a climate model should additionally see the `PISM Source Browser
113 <pism-browser_>`_ and the documentation therein.
114
115 :numref:`fig-climate-input-data-flow` illustrates the data flow needed by the ice dynamics
116 core. The data flow in the other direction, i.e. needed by the model to which PISM is
117 coupled, depends on particular modeling choices, but great flexibility is allowed.
118
119 Why describe all this structure here? On the one hand, some users may be interested in
120 coupling PISM to other models. On the other hand, the PISM authors do not claim expertise
121 in modeling atmosphere, ocean, or even snow processes. This separation has a definite
122 code-reliability purpose. PISM users are ultimately responsible for providing the climate
123 inputs they intend.