wgrib2: -new_grid
Introduction
The -new_grid option interpolates the fields into 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 vector
quanties are not in (U,V) order, the vector quantites will not be interpolated.
The option is not part of the default configuration. The interpolation code is written
in fortran and combining fortran and C code requires some work. For example, on my
linux box with gcc and g95, I had to add the g95 libraries to the makefile. On NCEP's AIX,
I had figure out the needed libraries. Using gfortran was easy but I had to search for the
required libraries and add it to my 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.
Installing -new_grid: unsupported
Did I make myself clear? To make Fortran and C code work together requires knowledge
of the specific computer system. Consult your local expert. Don't ask me to read the
documentation specific to your computer system. Don't make me guess which libraries
are needed and where they hid the required libraries.
Limitations
- only limited number of input grids are supported (can be expanded)
- only limited number of output grids are supported (can be expanded)
- only limited number of NCEP grids described (can be expanded)
- interpolation library only uses degrees to the nearest milli-degree
- Lambert conformal: LatD must follow grib1 conventions
- Polar Stereographic: LatD must be 60 latitude
Installation
1) set USE_IPOLATES to 1 in the makefile
2) environment variable $FC must be set to the fortran-90 compiler, ex. export FC=gfortran
3) If $FC is not gfortran, openf95, xlf_r, or then
modify the makefile for the new fortran compiler
modify New_grid.c for the new fortran compiler
4) now you are ready to make
Usage
-new_grid_winds W -new_grid A B C outfile
W = earth or grid
earth means that the U wind goes eastward
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 grib-2 interpolated records are written in outfile
Examples
wgrib2 IN.grb -new_grid_winds earth -new_grid latlon 100:10:1 30:20:1 OUT.grb
Interpolates from IN.grb to OUT.grb
Makes a 10x20, 1x1 degree lat-lon grid, lower left corner: 100E 30N
wgrib2 IN.grb -new_grid_winds earth -new_grid ncep grid 221 OUT.grb
Interpolates from IN.grb to OUT.grb
Interpolates to NCEP grid 221.
Grid description format
-new_grid ncep grid I outfile I = 2,3,98,128,173,221,230,249 (ncep grid defintions)
-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 length, degrees of longitude
dlat = grid length, degrees of latitude
-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 length, meters in x direction at lad
dy = grid length, meters in y direction at lad
note: if latin2 >= 0, the north pole is on proj plane
if latin2 < 0, the south pole is on proj plane
-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
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 length, meters in x direction at lad
dy = grid length, meters in y direction at lad
-new_grid gaussian lon0:nx:dlon lat0:ny global Gaussian grids
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 = grid length, degrees of longitude
-new_grid ncep grid NCEP_GRID_NO some of my favorite NCEP grids
NCEP_GRID_NO=2, 3, 98, 126, 128, 173, 221, 230, 249,
and non-standard t62, t126, t382
Want more ncep grids? Modify ncep_grids.c
(v1.9.7)
-new_grid mercator:lad lon0:nx:dx:lonn lat0:ny:dy:latn outfile 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 length, meters in x direction at lad
dy = grid length, 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
Type of Interpolation
The IPOLATES library supports a number of interpolation schemes including bilinear (default),
bicubic, neighbor and budget. The wgrib2 was tested with the spectral interpolation but
that option was removed because it greatly increased the code size for a minor option.
The interpolation 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 an IPOLATES grid.
You can use different interpolations for different variables. For example, a
bilinear interpolation of soil or vegetation type is meaningless.
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 90:1811:-1 OUT.grb
line 2: set default interpolation to bilinear
line 3: if VGTYP or SOTYP then set the interpolatin nearest neighbor
line 4: do the interpolation
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). 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.
Quilting tiles - Merging files
Yes, it can be done using -new_grid, -bin, -import_bin -rpn, and
the -grib_out options. No, I haven't done it.
Limitations
The IPOLATES library is used in operations at NCEP but has its limitations.
Grids used by NCEP get supported and others don't. The library has a grib-1 interface, so
new features of grib2 are not supported and the precision of the grid parameters
are limited to the grib-1 values. For example, latitude and longitude
values are limited to milli-degrees.
- only not all grids supported by IPOLATES
- only common grids are supported by the wgrib2 "wrapper" for IPOLATES
- latitude, longitude values are in milli-degrees (affects interpolation)
- only grib1 scan order are supported (i.e., WE:SN and WE:NS)
- NDFD/Glahn scan order are not supported (i.e., WE|EW:SN)
NDFD work arounds
NDFD files are written in an order which is not supported by the IPOLATES library.
The NDFD files come in (WE|EW):SN order. Which means the odd rows are in WE order and
the even rows are EW order. The rows go from south to north. The solution is to
convert the grid to WE:SN order is
- for data in input=WE|EW:SN scan order (wgrib2 IN -scan)
- read data, change order scan order of data, change flag table 3.4, save data
- wgrib2 IN.grb -rpn alt_x_scan -set table_3.4 64 -grib_out OUT.grb
A variation of the previous trick can be used to put data in to (WE|EW):SN packing.
See also:
-new_grid_interpolation,
-new_grid_winds,
-lola,
-bin,
-import_bin,
-rpn,
-grib_out,
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