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

wgrib2: -grid

Introduction

The -grid option prints out the grid information.

$ wgrib2 gep19.t00z.pgb2af180 -grid -d 1
1:0:grid_template=0:winds(N/S):
	lat-lon grid:(360 x 181) units 1e-06 input WE:NS output WE:SN res 48
	lat 90.000000 to -90.000000 by 1.000000
	lon 0.000000 to 359.000000 by 1.000000 #points=65160
$ wgrib2 nam.683 -grid
1:0:grid_template=30:winds(grid):
	Lambert Conformal: (1473 x 1025) input WE:SN output WE:SN res 8
	Lat1 12.190000 Lon1 226.541000 LoV 265.000000
	LatD 25.000000 Latin1 25.000000 Latin2 25.000000
	LatSP 0.000000 LonSP 0.000000
$ wgrib2 .t00z.master.grb2f048 -grid -d 1
ebis@landing2:~/grib2_examples$ wgrib2 gfs.t00z.master.grb2f048 -grid | more
1:0:grid_template=40:winds(N/S):
	Gaussian grid: (3072 x 1536) units 1e-06 input WE:NS output WE:SN
	number of latitudes between pole-equator=768 #points=4718592
	lat 89.909340 to -89.909340
	lon 0.000000 to 359.882813 by 0.117188
$ wgrib2 merc.g2 -grid -d 1
1:0:grid_template=10:winds(N/S):
	Mercator grid: (73 x 23) LatD 22.500000 input WE:SN output WE:SN res 48
	lat -48.090000 to 48.090000 by 513669.000000 m
	lon 0.000000 to 0.000000 by 513669.000000 m
	orientation 0.000000

The four previous examples are for a grid definitions of a lat-lon, Lambert Conformal, Gaussian and Mercator grids. These are the most common grids that are commonly distributed from NCEP. Other commonly used grids are: polar stereographic, rotated lat-lon and thinned Gaussian. Radar and satellites often use different grids.

Understanding the grid definitions

The grid definitions are based on the grib grib defintions as published by the WMO and copied by the NCEP's grib documentation. For example, the lat-lon grid defintion is given by https://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_doc/grib2_temp3-0.shtml. Wgrib2's version of the lat-lon grid is given by

$ wgrib2 gep19.t00z.pgb2af180 -grid -d 1
1:0:grid_template=0:winds(N/S):
	lat-lon grid:(360 x 181) units 1e-06 input WE:NS output WE:SN res 48
	lat 90.000000 to -90.000000 by 1.000000
	lon 0.000000 to 359.000000 by 1.000000 #points=65160

template=N      .. grid defintion template 3.N
winds(N/S)      .. winds are earth relative
winds(grid)     .. winds are grid relative
(NX x NY)       .. grid dimensions (for rectangular grid)
units 1e-06     .. scaling of angles, scaled angles are stored as 4-byte integers
input WE:NS     .. grib has the data in WE:NS scan order
output WE:SN    .. wgrib2 internally has the data is in WE:SN scan order
                   if wgrib2 writes a bin/ieee/text file, it will be in the output scan order
                   The output scan order has to be WE:SN for geolocation to work.
                   The default output scan order is WE:SN.
output WE:NS    .. wgrib2 internally has the data is in WE:NS scan order
                   The is enabled by -order we:ns, and is used for writing binary data
                   in WE:NS order.
output raw      .. wgrib2 internally has the data in the input scan order
                   This scan order is needed for -new_grid to work.  
res N           .. value of the resolution and component flags octet (byte)
lat X to Y by Z .. latitudes start at X goes to Y by steps of Z
lon X to Y by Z .. longitudes start at X goes to Y by steps of Z
                   note: grib2 uses longitude is in [0,360) model
#points N       .. number of points in the grid including grid points with undefined values

Staggered Grids, wgrib2 2.0.8+

Staggered grids are often used in grid point (as opposed to spectral) atmospheric models. (Arakawa, A.; Lamb, V.R. (1977). "Computational design of the basic dynamical processes of the UCLA general circulation model". Methods in Computational Physics: Advances in Research and Applications. 17: 173–265.) There are advantages in storing the model grids in grib for both the modeler and the user. The advantages are compactness, and a standard format. The user also has the advantage of eliminating an extra interpolation step. The staggering information is stored in the last 4 bits of flag table 3.4. If these bits are all zero, there is no staggering.

Staggered grids in GRIB work by

  1. A "fundamental" grid is defined
  2. staggered grid could have an 0 dx offset in the X direction
  3. or staggered grid could have an 1/2 dx offset in the X direction
  4. or staggered grid could have 0 dx offset for odd rows and 1/2 dx for even rows (all in the X direction)
  5. staggered grid could have an 0 dy offset in the Y direction
  6. or staggered grid could have an 1/2 dy offset in the Y direction
  7. staggered grid could have fewer points in the row or column if the offset is non-zero
This scheme allows encoding the Arakawa A-E Egrids. The wgrib2 -grid will show the staggering. The storage description was updated in v2.0.8 to be
  1. nx*ny: length of the row is nx, there are ny rows
  2. nx*(ny-1): length of the row is nx, there are ny-1 rows
  3. trim-x*ny: length of the row is either nx or nx-1, there are ny rows
  4. trim-x*(ny-1): length of the is either nx or nx-1, there are ny-1 rows

Usage

-grid
See also: -nxny, -nlons

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Page last modified: May 15, 2005, updated Nov 29, 2018, 5-2021,
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