tevolutionary-vs-diagnostic.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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       tevolutionary-vs-diagnostic.rst (1977B)
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            1 .. include:: ../../global.txt
            2 
            3 .. _sec-basicmodes:
            4 
            5 Evolutionary versus diagnostic modeling
            6 ---------------------------------------
            7 
            8 The main goal of a numerical ice sheet model like PISM is to be a dynamical system which
            9 evolves as similarly as possible to the modeled ice sheet. Such a goal assumes one has the
           10 "right" climate inputs and parameter choices at each time step. It also assumes one has
           11 the "right" initial conditions, such as an adequate description of the present state of
           12 the ice sheet, but this assumption is rarely satisfied. Instead a variety of heuristics
           13 must be used to minimally-initialize an ice sheet model. For options associated to
           14 establishing mathematical initial conditions when first starting PISM, see section
           15 :ref:`sec-initboot`.
           16 
           17 Inside PISM are evolution-in-time partial differential equations which are solved by
           18 taking small time steps. "Small" may vary from thousandths to tens of model years, in
           19 practice, depending primarily on grid resolution, but also on modeled ice geometry and
           20 flow speed. Time steps are chosen adaptively in PISM, according to the stability criteria
           21 of the combined numerical methods :cite:`BBssasliding`, :cite:`BBL`.
           22 
           23 However, especially for ice streams and shelves, non-time-stepping "diagnostic" solution
           24 of the stress balance partial differential equations might be the desired computation, and
           25 PISM can also produce such "diagnostic" velocity fields. Such computations necessarily
           26 assume that the ice geometry, viscosity, and boundary stresses are known. Because of the
           27 slowness of the ice, in the sense that inertia can be neglected in the stress balance
           28 :cite:`Fowler`, such computations can determine the ice velocity.
           29 
           30 Sections :ref:`sec-start` and :ref:`sec-ross` give examples illustrating evolutionary and
           31 diagnostic modes of PISM, respectively. The first describes time-stepping evolution models
           32 for the Greenland ice sheet, while the second describes a diagnostic SSA model for the
           33 Ross ice shelf.
           34