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HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data > wgrib2-new_grid
 

wgrib2: -new_grid

Introduction

Beginners are encouraged to read the new_grid introduction.

The -new_grid option interpolates the fields to a new grid. The default interpolation is bilinear but that can be changed using the -new_grid_interpolation option. This option uses scalar and vector interpolation as appropriate. In order for the vector interpolation to work, the vector quantities must be in a (U,V) order. For example: Z200, U200, V200, Z500, U500, V500 is good. If the data are not in (U,V) order, the inventory can be sorted into (U,V) order and the inventory can then be used to control the order of processing. If the vector quanties are not in (U,V) order, the vector quantites will not be interpolated. (See example 3 for converting the file into a (U,V) order.)

The option is not part of the default configuration but it included with many distributions. The interpolation code is written in fortran and combining fortran and C code can require some work. (gcc/gfortran and clang/gfortran are already handled by the makefile.) Getting the C and Fortran code to cooperate requires some system-specific knowledge and may not be possible in all cases. Consequently you are on your own in getting the -new_grid option installed.

Operations often has to use the -new_grid option to produce a large number of user grids. Fortunately the interpolation can be made embarrassingly parallel. A portable single-node solution is described in here. A multi-node solution is possible using MPI and the wgrib2 library.

Caution

The -new_grid option works in raw scan mode, so data are not converted to sn:we order. Conequently options that only work in sn:we order cannot work at the same time as the -new_grid option. Any option that uses geolocation (ex. -rpn, -lon) is incompatible with -new_grid.

Winds

Before you do an interpolation, you need to define the wind directions. Most people want the the V winds to be in the direction of the North Pole. With a verbose wgrib2 inventory, you will seee winds(N/S). However, some meteorologists want the V winds to go from grid point (i,j) to (i,j+1). The corresponding wgrib2 notation is "winds(grid)". See the -new_grid_winds option for more details.

Usage

-new_grid_winds W -new_grid A B C outfile
    W = earth or grid
        earth means that the U wind goes eastward, V goes northward
        grid means that U wind goes from grid (i,j) to (i+1,j)
          which is not eastward in a Lambert-conformal or polar stereographic grids
    A, B, C are the output grid description
    outfile is an output file.  The grib2 interpolated records are written in outfile

Grid description format


General Format

-new_grid A B C outfile General format A grid name with parameters B X or longitude description (ex lon0:nlon:dlon) C Y or latitude description (ex lat0:nlat:dlat)

Selected NCEP Grids (not in general format)

-new_grid ncep grid I outfile I = 2,3,4,45,98,126-129,170,173,184,194,221,230,242,249 (NCEP grid defintions) T62,T126,T170,T190,T254,T382,T574,T1148,T1534 (NCEP Gaussian grids definitions) Want more ncep grids? Modify ncep_grids.c

Latitude-Longitude Grid

-new_grid latlon lon0:nlon:dlon lat0:nlat:dlat outfile lat-lon grid 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

Rotated Latitude-Longitude Grid (requires ip2lib_d)

-new_grid rot-ll:sp_lon:sp_lat:sp_rot lon0:nlon:dlon lat0:nlat:dlat outfile rotated latitude-longitude grid sp_lon = longitude of the South pole (for rotation) sp_lat = latitude of the South pole (for rotation) sp_rot = angle of rotation (degrees) The grid is defined in the rotated coordinates. eat0, lon0 = degrees of lat/lon for 1st grid point nlat = number of longitudes nlon = number of latitudes dlon = grid cell size in degrees of longitude dlat = grid cell size in degrees of latitude

Lambert Conic Grid

-new_grid lambert:lov:latin1:latin2:lad lon0:nx:dx lat0:ny:dy outfile Lambert conic conformal -new_grid lambert:lov:latin1:latin2 lon0:nx:dx lat0:ny:dy outfile lad = latin2 -new_grid lambert:lov:latin1 lon0:nx:dx lat0:ny:dy outfile latin2 = latini lad = latin1 lov = longitude (degrees) where y axis is parallel to meridian latin1 = first latitude from pole which cuts the secant cone latin2 = second latitude from pole which cuts the secant cone lad = latitude (degrees) where dx and dy are specified lat0, lon0 = degrees of lat/lon for 1st grid point nx = number of grid points in X direction ny = number of grid points in Y direction dx = grid cell size in meters in x direction dy = grid cell size in meters in y direction note: if latin2 >= 0, the north pole is on proj plane if latin2 < 0, the south pole is on proj plane

Lambert Conic Grid with Centered Location

-new_grid lambertc:lov:latin1:latin2:lad lonc:nx:dx latc:ny:dy outfile Lambert conic conformal with centered position -new_grid lambertc:lov:latin1:latin2 lonc:nx:dx latc:ny:dy outfile like lambert except lonc and latc replace lon0 and lat0 -new_grid lambertc:lov:latin1 lonc:nx:dx latc:ny:dy outfile latc, lonc = degrees of lat/lon for center of the grid

Polar Stereographic Grid

-new_grid nps:lov:lad lon0:nx:dx lat0:ny:dy outfile north polar stereographic -new_grid sps:lov:lad lon0:nx:dx lat0:ny:dy outfile south polar stereographic lov = longitude (degrees) where y axis is parallel to meridian lad = latitude (degrees) where dx and dy are specified note: grib1 uses lad = 60N (nps) or 60S (sps) lad must be 60 (nps) or -60 (sps) (library limitation) lat0, lon0 = degrees of lat/lon for 1st grid point nx = number of grid points in X direction ny = number of grid points in Y direction dx = grid cell distance meters in x direction at lad dy = grid cell distance meters in y direction at lad

Global Gaussian Grid

-new_grid gaussian lon0:nx:dlon lat0:ny outfile global Gaussian grid lat0, lon0 = degrees of lat/lon for 1st grid point note: lon1 = -lon0, lat1 = lat0 + (nx-1)*dlon; nx = number of grid points in X direction ny = number of grid points in Y direction ny must be even dlon = degrees of longitude between adjacent grid points note: wgrib2 supports regional Gaussian grids, but the interpolation library doesn't.

Mercator Grid

-new_grid mercator:lad lon0:nx:dx:lonn lat0:ny:dy:latn outfile mercator grid lad = latitude (degrees) where dx and dy are specified lat0, lon0 = degrees of lat/lon for 1st grid point latn, lonn = degrees of lat/lon for last grid point nx = number of grid points in X direction ny = number of grid points in Y direction dx = grid cell distance in meters in x direction at lad dy = grid cell distance in meters in y direction at lad note: the mercator grid description is over specified User must make sure (nx,dy) is consistent with lonn as well as (ny,dy) is consistent with latn lambert, nps, sps, mercator only support we:sn ordering latlon, gaussian only support we:sn and we:ns ordering

Finding the Grid Format from Existing File

Sometimes one wants to convert grib files to a common grid. You already have a sample grib file and you have to figure out the grid format. For common grids, you can use the perl script grid_defn.pl to determine the grid format.

$ grid_defn.pl small.grb2
latlon 0.000000:2:10.000000 20.000000:2:8$ 

grid_defn.pl does not add a newline at the end of its output.
Not all grids are supported.

Examples

  wgrib2 IN.grb -set_grib_type same -new_grid_winds earth -new_grid latlon 100:10:1 30:20:1 OUT.grb
    Interpolates from IN.grb to OUT.grb
    Uses the same grib packing as in the input file
    Makes a 10x20, 1x1 degree lat-lon grid, lower left corner: 100E 30N


  wgrib2 IN.grb -set_grib_type same -new_grid_winds earth -new_grid ncep grid 221 OUT.grb
    Interpolates from IN.grb to OUT.grb
    Uses the same grib packing as in the input file
    Interpolates to NCEP grid 221.

  wgrib2 IN.grb -set_grib_type same -new_grid_winds earth -new_grid `grid_defn.pl OLD.grb` OUT.grb
    Interpolates from IN.grb to OUT.grb
    Uses the same grib packing as in the input file
    Interpolates using the grid format of OLD.grb (1st record)

 In this example, U and V are not in the required order.  This
shows a sorting to the required order for -new_grid to work.

    wgrib2 201201.A | sed -e 's/:UGRD:/:UGRDa:/' -e 's/:VGRD:/:UGRDb:/'  | \
    sort -t: -k3,3 -k5,8 -k4,4 | \
    wgrib2 201201.A -i -new_grid_winds earth -new_grid ncep grid 2 201201.A.grd2

    The first line creates an inventory with new variable names: UGRD -> UGRDa and VGRD -> UGRDb
    The second line sorts the inventory so that UGRDb follows UGRDB.
    The third line regrids the file, with the order of processing controlled by the inventory.

Type of Interpolation

The IPOLATES library (either iplib or ip2lib_d) supports a number of interpolation schemes including bilinear (default), bicubic, neighbor and budget. (In addition, ip2lib_d will support spectral.) The interpolation method can be selected by using the -new_grid_interpolation option before the -new_grid option. Some of the interpolation options need numeric parameters which are set by the -new_grid_ipopt option. IPOPT is defined in the iplib library documentation.

You can use different interpolations for different variables. For example, a bilinear interpolation of soil or vegetation type is meaningless. So nearest neighbor interpolation is used instead.

    wgrib2 IN.grb -new_grid_winds earth \
      -new_grid_interpolation bilinear \
      -if ":(VGTYP|SOTYP):" -new_grid_interpolation neighbor -fi \
      -new_grid latlon 0:360:1 90:181:-1 OUT.grb

   line 2: set default interpolation to bilinear
   line 3: if VGTYP or SOTYP then set the interpolation to nearest neighbor
   line 4: do the interpolation

Comments (4/2018): When you convert from a high resolution grid to a lower resolution grid, you have to be consider changing from the default interpolation (bilinear) to a budget interpolation. The budget gives a better estimate of the cell average (the default is 25 bilinear interpolations).

Changing from grid-relative to Earth-relative winds and vice versa

Most NCEP grib files use grid-relative winds. If you want to convert to Earth-relative winds or grid-relative winds, you can use the -new_grid option.

   To Earth relative:
     wgrib2 IN.grb -set_grib_type same -new_grid_winds earth -new_grid_interpolation neighbor \
      -new_grid `grid_defn.pl IN.grb` OUT.grb

   To Grid relative:
     wgrib2 IN.grb -set_grib_type same -new_grid_winds grid -new_grid_interpolation neighbor \
      -new_grid `grid_defn.pl IN.grb` OUT.grb

"-set_grib_type same" preserves the grib packing or compression
"-new_grid_interpolation neighbor" should be faster than the default bilinear
`grid_defn.pl IN.grb` returns the grid definition of the first grib message in IN.grb
The limitations of the above command are
  • IN.grb can only have one grid type
  • OUT.grb will have any submessages converted into messages

Changes from copygb

People may want to convert from copygb and copygb2 to wgrib2's -new_grid. Some differences to keep in mind.
  1. copygb default vectors: UGRD/VGRD
  2. wgrib2 default vectors: depends on version of wgrib2. See new_grid_vectors.
  3. copygb can have vectors in any order
  4. wgrib2 must have V follow U for vectors pairs
  5. copygb has bilinear, bicubic, nearest neighbor, budget, neighbor budget, and spectral interpolations.
  6. wgrib2 has bilinear, bicubic, nearest neighbor, and budget interpolations. (Spectral is in development.)
  7. wgrib2 can select the interpolation type depending on the variable (ex soil type)
  8. copygb uses fixed Earth's radius
  9. wgrib2 uses Earth's radius based on grib message
  10. wgrib2 doesn't have merging, mapthreshold or map files
  11. copygb by default, ignores the binary scaling and preserves decimal scaling
  12. wgrib2 by default, preserves binary and decimal scaling
  13. copygb does grib1.
  14. copygb2 does grib2.
  15. wgrib2 does grib2.

Speed: Interpolation Weights

The first step of the -new_grid interpolation is to calculate the interpolation weights. (Each grid point on the new grid is a weighted average of a small set of the old grid points.) To save time for future interpolations, the last set of weights is saved. Consequently interpolation is fastest when the input and output grids remain don't change. While one can have multiple -new_grid options on the command line, it is not recommended because the caching of the weights wouldn't work and weights would have to be recalculated every time.

Installation

In the makefile set USE_IPOLATES to 3 (wgrib2 v2.0.8) or 1 (older wgrib2)
Compile wgrib2.

Converting from WE:SN to WE:NS Grids

Many of wgrib2 grib2 writing options will write the grid in WE:SN order. This natural because geolocation is only enabled when the internal grids are in WE:SN order. However, some codes need the grid in WE:NS order. To convert a grib file from WE:SN order to WE:NS order, the simplest way is to use -new_grid. Lat0 and lon0 need to be lat/lon of the top left corner of the grid. Dlon will a positive number and dlat will be negative.

If you want to be tricky, you can do a variation of the "NDFD work arounds" technique. It will be faster and more generic.

Thinned Gaussian Grids to other Grids

Converting from a thinned Gaussian grid is a two step process using wgrib2. First you convert from the thinned grid to full grid using -reduced_gaussian_grid. Then you can use -new_grid to interpolate to your desired grid.

Quilting tiles - Merging files

Yes, it has been done using -new_grid, -import_grib -rpn, and the -grib_out options. Yes, I have done it.

Limitations of the iplib library

The iplib library has its limitations. Grids types used by NCEP get supported and others don't. The library has a grib1 interface, so new features of grib2 are not supported and the precision of the grid parameters are limited to the grib1 values. For example, latitude and longitude values are limited to millidegrees instead of microdegrees. Note: during the installation of ip2, I noticed that the longitude was reduced when reducing the range to [0,360). (longitude = amod(longitude + 3600.0, 360.0)) For single precision numbers, this reduces the precision to about two digits after the decimal place.
  1. not all grid types are supported by iplib
  2. only common grids are supported by the wgrib2 "wrapper" for iplib
  3. latitude, longitude values are nomially in millidegrees (affects interpolation)
  4. Single precision reals are used. The effective precision of the longitudes is to hunderdth of a degree.
  5. only grib1 scan order are supported (i.e., WE:SN and WE:NS)
  6. NDFD/Glahn scan order is not supported (i.e., WE|EW:SN)
  7. Earth is assumed to be spherical
  8. Lambert conformal: LatD must follow grib1 conventions
  9. Polar Stereographic: LatD must be 60 latitude (grib1 convention)
  10. nx, ny, npts must be ≤ 2147483647. (Grib2 standard is 4294967295.)

Limitations of the ip2lib_d library (wgrib2 v2.0.8+)

As of wgrib2 v2.0.8, ip2lib_d is the default interpolation library. Ip2lib_d is the double-precision grib2-interface IPOLATES library. The old ip2lib will remain a compile time option. The ip2lib_d is the new default IPOLATES library because is supports rotated lat-lon grids (Grid Definition Template 3.1) which will used in the regional FV3 model at NCEP. Canada is planning to release model forecasts using this grid, and the Europeans are already using this grid for their regional models.

The speed of the single precision iplib was originally faster than the double precision ip2lib_d. To remove this objection from converting to the new library, a effort was made to increase the OpenMP threading. Consequently the run time of ip2lib_d will be similar (bilinear) to much faster (budget) than iplib in a multiprocessor environment.

  1. supports more grids than iplib including WMO defined rotated lat-lon.
  2. latitude, longitude values are in microdegrees (affects interpolation)
  3. Double precision reals are used. (Wgrib2 stores is grid point values as single precision.)
  4. grib1 polar stereographic projection, LaD = 60N or 60S has been removed
  5. NDFD/Glahn scan order (i.e., WE|EW:SN) may be supported in the future.
  6. staggering will be supported unlike iplib
  7. Some interpolations support an ellipical earth.
  8. Lambert conformal: LatD must follow grib1 conventions
  9. nx, ny, npts must be ≤ 2147483647. (Grib2 standard is 4294967295.) This is a limitation of compiling using 4 byte integers.

NDFD work arounds

NDFD files are often written (WE|EW):SN order. This means that the odd rows are in WE order and the even rows are EW order. The rows go from south to north. If you are using wgrib2 that is using iplib, the simplest solution to convert the grid to WE:SN order is

    Version 0
  1. for data in input=WE|EW:SN scan order (wgrib2 IN -scan)
  2. read data, change order scan order of data, change flag table 3.4, save data
  3. wgrib2 IN.grb -rpn alt_x_scan -set table_3.4 64 -grib_out OUT.grb
    Version 1
  1. Read trick 55 in wgrib2 tricks
Version 1

A variation of the previous trick can be used to put data in to (WE|EW):SN packing.

See also: new_grid introduction, -new_grid_interpolation, -new_grid_winds, -new_grid_vectors, -lola, -bin, -import_bin, -rpn, -grib_out, new_grid multi-core usage,


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