Processing the GRIB message

When DMGRID data monitor or the DMBIN data monitor successfully reads a complete message, it calls the McIDAS-XCD GRIB decoder. The decoder first decodes the Product Definition Section (PDS) to determine the type of data contained in the message. After processing this section of the message, the decoder has enough information to determine whether to continue processing. The administrators at your site can configure the decoder to process or discard messages based on various criteria.

GRIB1 and GRIB2 messages may be discarded based on several attributes: GRIB1 based on the model (specified as the value stored in byte 6 of the PDS), the model run time, the valid time of the forecast fields, the geographic location the message represents, the level the data represents, and the meteorological parameter; GRIB2 based on the model and domain (specified as the model prefix assigned in file grib2NavLookup.txt), the model run time, and the valid time of the forecast fields.

If disk space is a concern, you can use this discard feature to save only the models and data that you typically use: 500 and 1000 mb height and temperature fields, for example. Several models are sent in more than one projection. If you only need one of them, configure the decoder so the other projections are discarded.

The configuration files where this information is stored are NOGRIB.CFG (for GRIB1 messages) and NOGRIB2.CFG (for GRIB2 messages). These files are read when the data monitor is started. If you change the values in either file, you must restart the data monitor. If the decoder cannot find the files, all messages are decoded. The NOGRIB.CFG file has eleven positions separated by the pipe character (|). Below is a description of each position in NOGRIB.CFG. The format of NOGRIB2.CFG is the same but it has only five positions, so is not described below. See the NOGRIB2.CFG file for its format and an example entry.

NOGRIB.CFG format

Position#	Description

1	Model number to discard. If this value is -1, the model number is not used as selection criteria. This is the value stored in byte 6 of the PDS. The commonly used values are listed below. The ~mcidas/data/gbtbpds001.av1 file contains a complete list of the known values.
	Common values	Description
	10	Global Wind Wave Forecast Model
	80	GFS model, 62-Wave Spectral Model
	81	GFS model, Spectral Statistics, Aviation Run
	83	NAM model, 80 km
	84	NAM model, 12 km
	86	MAPS (RUC) model
	89	NAM model, 48 km
	120	Ice Concentration Analysis
2	Beginning of the model run time range to discard. If this value is -1, the model run time is not used as selection criteria. This value is stored in byte 16 of the PDS.
3	End of the model run time range to discard.
4	Beginning of the model valid time range to discard. If this value is -1, the valid time of the model is not used as selection criteria. This value is stored in bytes 19 and 20 of the PDS.
5	End of the model valid time range to discard.
6	Beginning of the geographic ID range to discard. If this value is -1, the geographic ID is not used as selection criteria. This value is stored in byte 7 of the PDS. The file ~mcidas/data/gbtbpds001.bv1 contains a list of the IDs and their corresponding geographic coverage and projections.
7	End of the geographic ID range to discard.
8	Beginning of the pressure level range to discard. If this value is -1, the pressure level is not used as selection criteria.
9	End of the pressure level range to discard.
10	Beginning of the parameter number range to discard. If this value is -1, the parameter number is not used as selection criteria. This value is stored in byte 9 of the PDS. The file ~mcidas/data/gbtbpds001.2v2 contains a complete list of the known values. Below is a table of commonly used values.
	Common values	Description
	1	Pressure
	2	Pressure reduced to MSL
	7	Geopotential Height
	11	Temperature
	33	u-component wind
	34	v-component wind
	52	Relative Humidity
11	End of the parameter number range to discard.

Examples of NOGRIB.CFG entries

The entry below discards all fields of the aviation run (77) from projections 37 through 44. These projections are associated with the high resolution "thinned" grid format.

77  |  -1  |  -1  |  -1  |  -1  |  37  |  44  |  -1  |  -1  |  -1  |  -1  |

The entry below discards relative humidity fields (52) from 500 to 700 millibars for the 12 km NAM (84).

84  |  -1  |  -1  |  -1  |  -1  |  -1  |  -1  |  500  |  700  |  52  | 52  |

The entry below discards any field from a 12 UTC model run with a valid time between 36 and 42 hours, inclusive.

-1  |  12  |  12  |  36  |  42  |  -1  |  -1  |  -1  |  -1  |  -1  |  -1  |

Recommended NOGRIB.CFG entries

SSEC recommends that the following entries be added to your NOGRIB.CFG file.

The entry below is for ICA grids known by Unidata to be incorrectly formatted.

120  |  -1  |  -1  |  -1  |  -1  |  218  |  218  |  -1  |  -1  |  -1  |  -1

The entry below is for ICA grids known by Unidata to be incorrectly formatted.

120  |  -1  |  -1  |  -1  |  -1  |  242  |  242 |  -1  |  -1  |  -1  |  -1

The entry below is for WWF grids with bad geographic data.

10  |  -1  |  -1  |  -1  |  -1  |  233  |  233 |  -1  |  -1  |  -1  |  -1

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