Regridding CORe Files
CORe files are generated on a 512x256 Gaussian grid. This
uncommon grid is used because it is the data assimilation (DA) grid and
the primary output from the model. We are archiving the
analyses on this grid in order avoid potential double interpolation errors
to users of other grids.
We recommend that you use the program wgrib2 to regrid the CORe files.
Wgrib2 handles both scalar and vector interpolations, several grid
interpolation schemes and many common grids.
Wgrib2 is used in NCEP Operations and is free and open source.
Output Grid
Wgrib2 can write files in many grid formats. Some common output grids are
- latitude-longitude grid
- Lambert conic grid
- polar stereographic grid
- Mercator grid
Wgrib2 can write to some model grids such as
- rotated latitude longitude grid
- Gaussian grid
The most commonly used grid is the latitude-longitude grid. It
is converted by the option
-new_grid latlon lon0:nlon:dlon lat0:nlat:dlat outfile
lat0, lon0 = degrees of lat/lon for 1st grid point
nlon = number of longitudes
nlat = number of latitudes
dlon = grid cell size in degrees of longitude
dlat = grid cell size in degrees of latitude
for example -new_grid latlon 0:360:1.0 -90:181:1.0
produces a 1x1 degree lat-lon grid
Type of Interpolation
The CORe data needs 3 different types of interpolation
- bilinear (most data)
- nearest neighbor for integer values like soil type
- budget for precipitation so that large scale means are preserved
The bilinear interpolation is divided into scalar for variables like temperature, and
vector for variables like the zonal and meridional wind. The effects of vector
interpolation are only noticeable near the pole.
Variables require different interpolations. Wgrib2 allows you to specify the
interpolation by an option string using the -if/-elseif/-else/-endif options.
Suggested Interpolation for CORe
$ wgrib2 IN_FILE.grb \
-if ':(SLTYP|VGTYP|SOTYP):' -new_grid_interpolation neighbor \
-elseif ':(PRATE|CPRAT|CPOFP):' -new_grid_interpolation budget \
-else -new_grid_interpolation bilinear -endif \
-new_grid_winds earth \
-new_grid latlon 0:360:1 -90:181:1 OUT_FILE.grb
line 1. Defines IN_FILE.grb to be the input (grib) file
line 2. Sets surface slope, vegetation and soil types to nearest neighbor
interpolation (these fields are integers)
line 3. Sets precipitation, convective precipitation and % frozen precipitation
to budget interpolation
line 4. Sets other fields to bilinear interpolation
line 5. Defines the V wind to be pointing to the north pole
rather to the upper grid point
line 6. Interpolates to the 1x1 global grid
Spherical Earth
CORe uses a 6371.229 km spherical earth. The -new_grid routine only supports
spherical geometries with a realistic earth radius.
CORe home page
-new_grid
-new_grid_interpolation
-new_grid_winds
Comments: Wesley.Ebisuzaki@noaa.gov, Leigh.Zhang@noaa.gov
Created Sep 12, 2025
|
www.nws.noaa.gov 
