bilingrb - A bilinear GRIB interpolator & subsampler

bilingrb is a program for extraction, interpolation, or subsampling of
geographic subdomains from GRIB 2D-fields in lat-lon format. The records
are interpolated using bilinear interpolation, not the most subtle of
techniques, but frequently close enough for .gov work.

Comments are appreciated, future versions may appear.


      Syntax

  % bilingrb gribfile newgribfile westlon southlat dlon dlat nlon nlat

where gribfile & newgribfile are the old and new GRIB
files,westlon,southlat are the coords of the SW corner in your new grid,
dlon & dlat define the east and north resolution in degrees, and nlon &
nlat define the grid size in the east and north direction, respectively.


      Facts & caveats

    * The grid must be in geographic coordinates (lat-lon). The grid
      scan mode is arbitrary.
    * Do not use bilingrb with directional data, it will mess up your
      data seriously near the branch cut (where the wind direction
      shifts from 0 to 359 degrees). Such GRIB records must be weeded
      out in advance, for instance with wgrib, an excellent piece of
      software available at wesley.wwb.noaa.gov
      <http://wesley.wwb.noaa.gov>.
    * Vector components near the poles should be handled with care.
    * Cyclic grids (periodic in longitude) are allowed, selecting a
      subdomain that crosses the branch cut (0/360 or -180/180) is also
      allowed.
    * Subsampling is achieved by selecting the starting coordinates to
      coincide with the grid points in the old grid and choosing dlon
      and dlat, the resolution, to be multiples of the original resolution.
    * Undefined points (e.g. land in an ocean grid) are tackled by
      bilingrb.The new grid points become undefined if any of their
      neighbours in the old grid are undefined (except when the new grid
      point coincides exactly with a grid point in the old grid, this to
      avoid unnecessary swelling of coastlines when subsampling).
    * Specifying bounding coords that go beyond the original grid domain
      is verboten and will cause  bilingrb to quit, so make sure you
      weed out unwanted records from your GRIB file in advance.
    * The interpolation is weighted with the cosine of the latitude.


      Examples

1. If world.grb is a global file with resolution 1.5 x 1.5 deg, then

  % bilingrb world.grb smallworld.grb -10.0 45.0 0.5 0.5 51 71

produces a 51x71 interpolated subdomain with bounding coordinates
(45N,10W) and (80N,15E) with resolution 0.5 x 0.5 deg.

2. If a subsampled version of world.grb with resolution 3.0 x 3.0 deg is
desired, then

  % bilingrb world.grb sparseworld.grb 0.0 -90.0 3.0 3.0 121 61

yields a 121x61 subsampled grid of the whole domain, assuming the
original grid has a node at (90S,0E). Note that when subsampling it is
recommended that you select westlon, southlat to coincide with a grid
point in the old grid and dlon, dlat to be multiples of the old grid
resolution. This circumvents interpolation error and avoids bloated
coastlines if dealing with undef'd points (dry points in a wet dataset).


      Installation

% gunzip bilingrb.tar.gz

% cd Gribw

% make -f gribwlib.make

% cd ../Bilingrb

% make -f bilingrb.make

The TAR-ball includes a Linux binary of bilingrb, which makes
compilation unnecessary for those fortunate owners of Linux boxes.


      See also

ggrib, <http://wesley.wwb.noaa.gov/ggrib.html> a program for extracting
subdomains without interpolating.
wgrib <http://wesley.wwb.noaa.gov/wgrib.html>, a program for viewing and
extracting records.

v1.1, 2004-01-22, oyvind.breivik@met.no

-- 
Øyvind Breivik, Norwegian Meteorological Institute, www.met.no