[Ncep.list.emc.fv3gfs_tickets] FV3GFS Ticket #1: Update global_chgres program for FV3 core

[FV3GFS Trac Ticket]

#1: Update global_chgres program for FV3 core
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  Reporter:  george.gayno@…  |      Owner:  fanglin.yang@…
      Type:  task            |     Status:  assigned
  Priority:  major           |  Milestone:
 Component:  component1      |    Version:
Resolution:                  |   Keywords:  global_chgres
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Comment (by george.gayno@…):

 As Fanglin noted in comment:42, the output from the updated
 `global_chgres` developed under this ticket is not bit identical to that
 from the `global_chgres` as modified by GFDL. There are several reasons
 for this:

 * GFDL modified the May 2016 GFS version (13.0.5) of `global_chgres` for
 FV3.  The new `global_chgres` developed under this ticket added GFDL's
 changes to the Q3FY17 version (v14.1.0).  Numerous changes were made to
 global_chgres for v14.1.0.

 * The GFDL version interpolated winds to the FV3 grid using IPOLATES,
 which is for scalar fields.  The updated version uses IPOLATEV, which is
 for vectors. IPOLATEV moves the vectors along a great circle route to the
 target point. This results in differences near the poles and very minor
 differences elsewhere.

 * The GFDL version did not set soil type correctly.  It looked like it was
 interpolated using the bi-linear method.  That is not correct for a
 categorical field.  The new global_chgres uses nearest neighbor.

 * The sfcsub.f routine, used to set several static fields on the FV3 grid,
 is not the same between the GFDL and updated versions.  For example, the
 z0 for vegetation type 12 is 3.5 cm in the GFDL version and 12.5 cm in the
 updated version.

 * GFDL added their own logic to search for valid data at FV3 water points
 not located near input GFS water points (i.e., isolated lakes).  The
 updated version uses the search logic in IPOLATES.  The GFDL search radius
 is much larger, which results in differences in SST and ice along
 coastlines and inland water bodies.

 In summary, there are more differences in the surface files than the sigma
 files.

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Ticket URL: <https://svnemc.ncep.noaa.gov/trac/fv3gfs/ticket/1#comment:43>
fv3gfs <https://svnemc.ncep.noaa.gov/trac/fv3gfs>
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