GEMPAK Decoders: Packing Files

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17.3 GEMPAK Decoders: Packing Files

Decoding Data into GEMPAK Format

GEMPAK Packing Files

When creating surface and upperair files, the variables which will be stored in the file must be specified. GEMPAK uses the variables SFPRMF and SNPRMF for surface and upperair files respectively to specify the the parameters that will be stored. Parameters may either be listed on the variable declaration as a list delimited by semicolons, or a packing file may be used. Packing files define the parameter names, ranges, and resolution to be stored in the associated file.

The standard surface packing file is hrly.pack and looks like:

PMSL       950.       1049.       .1
ALTI        24.        34.       .01
TMPF      -100.        150.      1.
DWPF      -100.        150.      1.
SKNT         0.        126.      1.
DRCT         0.        360.     10.
GUST         0.        250.      1.
WNUM         0.     512000.      1.
CHC1         0.      18009.      1.
CHC2         0.       8009.      1.
CHC3         0.       8009.      1.
VSBY         0.         51.       .1
P03D         0.         8999.    1.
P03I         0.         12.99    .01  
P06I         0.         12.99    .01  
HSUN         0.        999.      1.
CSYL         0.         9.       1.
CSYM        10.         19.      1.
CSYH        20.         29.      1.
SNOW         0.        1000.0    1.
WEQS         0.         100.0    .1
T06X      -100.        150.     .1                
T12X      -100.        150.     .1                
T24X      -100.        150.      1.
TMDX      -100.        150.      1.
T06N      -100.        150.     .1                
T12N      -100.        150.     .1                
T24N      -100.        150.      1.
TMDN      -100.        150.      1.
P24I         0.         40.      .01

The first column of the file is a four character identifier, the next three columns are the minimum, maximum, and resolution of the data respectively. Other packing files that are provided in $GEMTBL are snmerg.pack for sounding data, sfsyn.pack for synoptic data, sfship.pack for ship and buoy data, profiler.pack for profiler data, and nldn.pack for lightning data. Moreover, special packing files are available for particular data sets such as MOS bulletins, tropical store reports, watch box reports, and severe storm reports.

GRIB Files

Model gridpoint data is typically transmitted in a format known as GRIB. Unidata sites which receive HDS/HRS data through the IDD obtain GRIB bulletins which are delineated for transmission using product headers, just as other FOS bulletins. Additionally, some sites obtain GRIB data directly from NCEP. Since the data available on NCEP is not transmitted over FOS, it does not contain product headers.

The ldmbridge program dcgrib is designed to handle GRIB products with or without product headers. Dcgrib will handle all of the models that are transmitted over the IDD, and most that are available from NCEP (except rotated and ensemble grids). The GEMPAK program nagrib is designed to be be used with GRIB bulletins without headers only. Nagrib will decode the new native staggered and ensemble grids available from NCEP.

Due to the large size and numbers of model grids, GRIB formatted data employs an efficient method for compressing the file size. DCGRIB supports a command line option PACK to allow decoded GEMPAK grid data to be written in its own compressed format. All the GEMPAK grid programs support the compressed format, so this option should be used if disk space is a consideration.

Another inovation in GRIB data is the quasi-thinned grids. Currently the global AVN (Spectral) model is transmitted using thinned grids. Thinned grids take advantage of the convergence of longitude lines as they approach the poles by eliminating certain gridpoints as the grid separation narrows. DCGRIB supports decoding of thinned grids, and provides a command line option for "thickening" the grids.

Ensemble grids may posess grid fields for a given variable in multiple instances for successive perturbations. NAGRIB will identify ensemble fields and annotate them accordingly.

Lightning Data Files

Lightning data from the NLDN network is stored in files identical to ship format surface data files. Ship format files are used since just as ships are not fixed to a given location, lightning strikes have position information included with other parameters.

Since NLDN data is being stored in surface file format, we are able to use the surface programs SFLIST and SFMAP to view the data.

A Word of Note.......

Lightning data files tend to be much larger than surface data files, in particular during active storms since each lightning stroke is treated as a separate station. Since lightning flashes are recorded down to the nanosecond, typical decoder actions bin the data into more manageable time ranges (generally 5 minute groupings) that work well with other types of data (such as NIDS products).

Lets try it...

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