tenergy-balance.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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       tenergy-balance.rst (2516B)
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            1 .. include:: ../../../global.txt
            2 
            3 .. _sec-energy:
            4 
            5 Modeling conservation of energy
            6 -------------------------------
            7 
            8 In normal use PISM solves the conservation of energy problem within the ice, the thin
            9 subglacial layer, and a layer of thermal bedrock. For the ice and the subglacial layer it
           10 uses an enthalpy-based scheme :cite:`AschwandenBuelerKhroulevBlatter` which allows the energy
           11 to be conserved even when the temperature is at the pressure-melting point.
           12 
           13 Ice at the melting point is called "temperate" ice. Part of the thermal energy of
           14 temperate ice is in the latent heat of the liquid water stored between the crystals of the
           15 temperate ice. Part of the thermal energy of the whole glacier is in the latent heat of
           16 the liquid water under the glacier. The enthalpy scheme correctly models these storehouses
           17 of thermal energy, and thus it allows polythermal and fully-temperate glaciers to be
           18 modeled :cite:`AschwandenBlatter`.
           19 
           20 The state of the full conservation of energy model includes the 3D ``enthalpy`` variable
           21 plus the 2D ``bwat`` and ``tillwat`` subglacial hydrology state variables (subsection
           22 :ref:`sec-subhydro`), all of which are seen in output files. The important basal melt rate
           23 computation involves all of these energy state variables, because the basal melt rate
           24 (``bmelt`` in output files) comes from conserving energy across the ice-bedrock layer
           25 :cite:`AschwandenBuelerKhroulevBlatter`. Fields ``temp``, ``liqfrac``, and ``temp_pa`` seen in
           26 output files are all actually diagnostic outputs because all of these can be recovered
           27 from the enthalpy and the ice geometry.
           28 
           29 Because this part of PISM is just a conservation law, there is little need for the user to
           30 worry about controlling it. If desired, however, conservation of energy can be turned off
           31 entirely with :opt:`-energy none`. The default enthalpy-based conservation of energy model
           32 (i.e. ``-energy enthalpy``) can be replaced by the temperature-based (i.e. "cold ice")
           33 method used in :cite:`BBssasliding` and verified in :cite:`BBL` by setting option :opt:`-energy
           34 cold`.
           35 
           36 The thermal bedrock layer model is turned off by setting ``-Mbz 1`` (i.e. zero spaces)
           37 while it is turned on by choosing a depth and number of points, as in ``-Lbz 1000 -Mbz
           38 21``, for example, which gives a layer depth of 1000 m and grid spaces of 50 m (=
           39 1000/20). The input geothermal flux (``bheatflx`` in output files) is applied at the
           40 bottom of the bedrock thermal layer if such a layer is present and otherwise it is applied
           41 at the base of the ice.