tbed-roughness.rst - pism - [fork] customized build of PISM, the parallel ice sheet model (tillflux branch)
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       tbed-roughness.rst (1895B)
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            1 .. include:: ../../../global.txt
            2 
            3 .. _sec-bedsmooth:
            4 
            5 Parameterization of bed roughness in the SIA
            6 --------------------------------------------
            7 
            8 Schoof :cite:`Schoofbasaltopg2003` describes how to alter the SIA stress balance to model
            9 ice flow over bumpy bedrock topgraphy. One computes the amount by which bumpy topography
           10 lowers the SIA diffusivity. An internal quantity used in this method is a smoothed version
           11 of the bedrock topography. As a practical matter for PISM, this theory improves the SIA's
           12 ability to handle bed roughness because it parameterizes the effects of "higher-order"
           13 stresses which act on the ice as it flows over bumps. For additional technical description
           14 of PISM's implementation, see :ref:`sec-bed-roughness`.
           15 
           16 There is only one associated option: :opt:`-bed_smoother_range` gives the half-width of
           17 the square smoothing domain in meters. If zero is given, ``-bed_smoother_range 0`` then
           18 the mechanism is turned off. The mechanism is on by default using executable ``pismr``,
           19 with the half-width set to 5 km (``-bed_smoother_range 5.0e3``), giving Schoof's
           20 recommended smoothing size of 10 km :cite:`Schoofbasaltopg2003`.
           21 
           22 This mechanism is turned off by default in executables ``pisms`` and ``pismv``.
           23 
           24 Under the default setting ``-o_size medium``, PISM writes fields :var:`topgsmooth` and
           25 :var:`schoofs_theta` from this mechanism. The thickness relative to the smoothed bedrock
           26 elevation, namely :var:`topgsmooth`, is the difference between the unsmoothed surface
           27 elevation and the smoothed bedrock elevation. It is *only used internally by this
           28 mechanism*, to compute a modified value of the diffusivity; the rest of PISM does not use
           29 this or any other smoothed bed. The field :var:`schoofs_theta` is a number `\theta`
           30 between `0` and `1`, with values significantly below zero indicating a reduction in
           31 diffusivity, essentially a drag coefficient, from bumpy bed.