tREADME - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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       tREADME (1330B)
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            1 This is a place to put under-development "tracer module" stuff.
            2 
            3 A "tracer module" is going to be a PISM class which uses
            4 the velocity field produced by the stress balance parts of PISM.
            5 That velocity field advects tracers like
            6   * 3D ice age (relevant to validation-by-isochrone and also grain size)
            7   * 3D isotopic ratios (see S. Jamieson email 6 Nov. 2012)
            8   * 2D fracture density (compare Albrecht & Levermann, 2012)
            9 
           10 A key idea is that these tracers *do not* diffuse.
           11 
           12 This PISM class will presumably take advantage of the fact that the
           13 ice velocity field in 3D is incompressible, namely
           14   div (u,v,w) = 0.
           15 Because of that, the equations
           16   A_t + (u,v,w) . grad A = r.h.s.
           17 and
           18   A_t + div((u,v,w) A) = r.h.s.
           19 for some tracer with value A(t,x,y,z).  So the code can "take advantage"
           20 of the fact that we can imagine
           21   q = (u,v,w) A
           22 is a "conserved flux", up to changes from the r.h.s.
           23 
           24 A closely-related class might also address the 3D enthalpy problem
           25 in which the there is vertical conduction and advection but only advection
           26 in horizontal directions.
           27 
           28 A derived class might be appropriate to a 2D (map-plane) advection problem
           29 like that in Albrecht & Levermann (2012).  Here the map-plane 2D velocity field,
           30 i.e. the vertical average of the 3D velocity field, is not incompressible;
           31 generally div (bar u,bar v) is not zero.
           32