WRF ARW User's Guide - MMM - UCAR

WPS
coordinate data), AFWA's AGRMET land surface model output, ECMWF, and other data
sets. Users can create their own Vtable for other model output using any of the Vtables as
a template; further details on the meaning of fields in a Vtable are provided in the section
on creating and editing Vtables.
Ungrib can write intermediate data files in any one of three user-selectable formats: WPS
– a new format containing additional information useful for the downstream programs; SI
– the previous intermediate format of the WRF system; and MM5 format, which is
included here so that ungrib can be used to provide GRIB2 input to the MM5 modeling
system. Any of these formats may be used by WPS to initialize WRF, although the WPS
format is recommended.
Program metgrid
The metgrid program horizontally interpolates the intermediate-format meteorological
data that are extracted by the ungrib program onto the simulation domains defined by the
geogrid program. The interpolated metgrid output can then be ingested by the WRF real
program. The range of dates that will be interpolated by metgrid are defined in the
“share” namelist record of the WPS namelist file, and date ranges must be specified
individually in the namelist for each simulation domain. Since the work of the metgrid
program, like that of the ungrib program, is time-dependent, metgrid is run every time a
new simulation is initialized.
Control over how each meteorological field is interpolated is provided by the
METGRID.TBL file. The METGRID.TBL file provides one section for each field, and
within a section, it is possible to specify options such as the interpolation methods to be
used for the field, the field that acts as the mask for masked interpolations, and the grid
staggering (e.g., U, V in ARW; H, V in NMM) to which a field is interpolated.
Output from metgrid is written in the WRF I/O API format, and thus, by selecting the
NetCDF I/O format, metgrid can be made to write its output in NetCDF for easy
visualization using external software packages, including the new version of RIP4.
Installing the WPS
The WRF Preprocessing System uses a build mechanism similar to that used by the WRF
model. External libraries for geogrid and metgrid are limited to those required by the
WRF model, since the WPS uses the WRF model's implementations of the WRF I/O
API; consequently, WRF must be compiled prior to installation of the WPS so that the I/O
API libraries in the WRF external directory will be available to WPS programs.
Additionally, the ungrib program requires three compression libraries for GRIB Edition 2
support; however, if support for GRIB2 data is not needed, ungrib can be compiled
without these compression libraries.
WRF-ARW V3: User’s Guide 3-4