The GrADS data descriptor file defines a grid structure into which the data will fit -- it gives "shape" to the data by identifying its spatial dimensions, the number of time steps, and the number of variables.
If the data is in binary format, its structure has already been
prescribed. If the data is in GRIB format, no consistent
relationship exists between the data and the grid structure defined in the data
descriptor file. Hence, the need for the gribmap
utility which "maps"
between the GRIB data and the GrADS data description.
As gribmap
reads each field in the GRIB data file, the
parameters for that field (e.g. variable name, vertical level, time)
are compared to the information in the data descriptor file until
a match is found. The process continues until all the GRIB elements have been
"mapped" to their location within the GrADS grid structure.
The syntax for the gribmap
command is as follows:
gribmap [-i fname] [-v] [-t0] [-0] [-fhr] [-sxxx] [-hxxx]
The options are as follows:
fname
gribmap
will
prompt the user.
-v
-t0
gribmap
will only match those grib records whose base
time is the same as the initial time in the data descriptor file.
This is used to pull out a forecast sequence (0, 12, 24, ... , 72
hours) starting a specific time.
-0
gribmap
will ignore the forecast time when setting up a
match. This is useful for reanalysis data sets in which some
diagnostic fields are "valid" at slightly different forecast time
even though the share the same starting time.
-fhr
gribmap
will only match those grib records whose forecast time
is hr
hours. This is used to isolate a sequence of
forecasts. For example, if you wanted to sample all the 120-hour
forecasts from the MRF ensemble runs, you would use gribmap -f120
.
-sxxx
gribmap
will scan over no more than xxx
bytes
when searching for the character string "GRIB" in each header. The default is 1000.
-hxxx
gribmap
will skip over xxx
bytes
before starting the scan process. If reading a special NMC format of
GRIB data, use -hnmc
.
Another feature was added to map by the GRIB "time-range-indicator" as specified
in the .ctl
file. This was put in for handling NMC reanalysis data
where the time-range-indicator distinguishes between monthly mean variances
and means.