Processing the GRIB message

When DMGRID 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.

GRIB messages may be discarded based on the model generating the message, the model run time, the valid time of the forecast fields, the geographic location the message represents, the level the data represents, or the meteorological parameter. If disk space is a concern, you can save only those fields 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 file where this information is stored is NOGRIB.CFG. This file is read when the data monitor is started. If you change the values in NOGRIB.CFG, you must restart the data monitor. If the decoder cannot find NOGRIB.CFG, all messages are decoded. The file has eleven positions separated by the pipe character (| ). Below is a description of each position.

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 ~oper/mcidas/data/gbtbpds001.av1 file contains a complete list of the known values.
	Common values	Description
	10	Global Wind Wave Model
	39	Nested Grid Model
	64	Regional Optimal Interpolation
	80	GFS model, 62-Wave Spectral Model
	81	GFS model, Spectral Statistics, Aviation Run
	82	GFS model, Spectral Statistics, Final Run
	83	NAM model, 80 km
	84	NAM model, 40 km
	85	NAM model, 30 km
	86	MAPS (RUC) model
	89	NAM model, 48 km
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 ~oper/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 ~oper/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

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

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

39  |  -1  |  -1  |  -1  |  -1  |  -1  |  -1  |  500  |  700  |  52  | 52  |

This entry discards relative humidity fields (52) from 500 to 700 millibars for the Nested Grid Model (39).

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

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

Contact Us • Site Map • Search • Terms and Conditions • Privacy Policy • Participation Policy

Unidata is a member of the UCAR Office of Programs, is managed by the University Corporation for Atmospheric Research, and is sponsored by the National Science Foundation.

P.O. Box 3000 • Boulder, CO 80307-3000 USA • Tel: 303-497-8643 • Fax: 303-497-8690