[Met_help] [rt.rap.ucar.edu #89609] History for interpolation options in point_stat

[John Halley Gotway via RT]

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  Initial Request
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I thought point_stat doesn't do any smoothing, right?
Is it possible that I missed something or did something wrong that smoothed our model data accidentally?

The 2 red lines are from MET and my reading of the model output (with markers). It seems MET does smoothing of the spikes…
The black line is the obs. from MET. 



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  Complete Ticket History
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Subject: interpolation options in point_stat
From: John Halley Gotway
Time: Thu Apr 04 13:14:20 2019

Ka Yee,

The Point-Stat tool does whatever you tell it to do in the config
file.  The option that comes into play here is "interp".  That tells
Point-Stat how to map forecast data to each observation lat/lon
location.  There are many, many ways of doing this, and I'll cut-and-
paste the long description of it from the MET User's Guide.  In order
to do the least amount of smoothing, I'd recommend using the nearest
neighbor, like this:
interp = {
   vld_thresh = 1.0;
   shape      = SQUARE;
   type = [ { method = NEAREST; width  = 1; } ];
}

You are not limited to a single option.  The "type" entry is an array,
and you can specify as many interpolation techniques as you'd like.
The INTERP_MTHD and INTERP_PNTS columns of the output indicate the
method used for the current line of output.

Thanks,
John

//
// The "interp" entry is a dictionary that specifies what
interpolation or
// smoothing (for the Grid-Stat tools) methods should be applied.
// This dictionary may include the following entries:
//
//    - The "field" entry specifies to which field(s) the
interpolation method
//      should be applied.  This does not apply when doing point
verification
//      with the Point-Stat or Ensemble-Stat tools:
//       - "FCST" to interpolate/smooth the forecast field.
//       - "OBS" to interpolate/smooth the observation field.
//       - "BOTH" to interpolate/smooth both the forecast and the
observation.
//
//    - The "vld_thresh" entry specifies a number between 0 and 1.
When
//      performing interpolation over some neighborhood of points the
ratio of
//      the number of valid data points to the total number of points
in the
//      neighborhood is computed. If that ratio is less than this
threshold,
//      the matched pair is discarded. Setting this threshold to 1,
which is the
//      default, requires that the entire neighborhood must contain
valid data.
//      This variable will typically come into play only along the
boundaries of
//      the verification region chosen.
//
//    - The "shape" entry may be set to SQUARE or CIRCLE to specify
the shape
//      of the smoothing area.
//
//    - The "type" entry is an array of dictionaries, each specifying
an
//      interpolation method.  Interpolation is performed over a N by
N box
//      centered on each point, where N is the width specified.  Each
of these
//      dictionaries must include:
//
//      - The "width" entry is an integer which specifies the size of
the
//        interpolation area. The area is either a square or circle
containing
//        the observation point. The width value specifies the width
of the
//        square or diameter of the circle. A width value of 1 is
interpreted
//        as the nearest neighbor model grid point to the observation
point.
//        For squares, a width of 2 defines a 2 x 2 box of grid points
around
//        the observation point (the 4 closest model grid points),
while a width
//        of 3 defines a 3 x 3 box of grid points around the
observation point,
//        and so on. For odd widths in grid-to-point comparisons
//        (i.e. Point-Stat), the interpolation area is centered on the
model
//        grid point closest to the observation point. For grid-to-
grid
//        comparisons (i.e. Grid-Stat), the width must be odd.
//
//      - The "method" entry specifies the interpolation procedure to
be
//        applied to the points in the box:
//         - MIN         for the minimum value
//         - MAX         for the maximum value
//         - MEDIAN      for the median value
//         - UW_MEAN     for the unweighted average value
//         - DW_MEAN     for the distance-weighted average value
//                         where weight = distance^-2
//         - LS_FIT      for a least-squares fit
//         - BILIN       for bilinear interpolation (width = 2)
//         - NEAREST     for the nearest grid point (width = 1)
//         - BEST        for the value closest to the observation
//         - UPPER_LEFT  for the upper left grid point (width = 1)
//         - UPPER_RIGHT for the upper right grid point (width = 1)
//         - LOWER_RIGHT for the lower right grid point (width = 1)
//         - LOWER_LEFT  for the lower left grid point (width = 1)
//
//         The BUDGET and FORCE regridding options are not valid for
//         interpolating. For grid-to-grid comparisons, the only valid
options
//         are MIN, MAX, MEDIAN, and UW_MEAN.
//
interp = {
   field      = BOTH;
   vld_thresh = 1.0;
   shape      = SQUARE;

   type = [
      {
         method = UW_MEAN;
         width  = 1;
      }
   ];
}






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