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A demonstration of what a GRIB file looke like


Preamble

To most of us, GRIB messages are just something that a clever bit of software translates into a chart of vector winds, isobars, rain areas etc. This is for those with minds enquiring enough to wonder what is there and how to find out.


Getting GRIBs

As I have shown on other pages,there are many ways to get GRIB files using a browser, File Transfer Protocal, email and iPhone or iPad Apps. Each has its pros and cons. An early, if not the earliest, method was developed by Jim Corenman of Saildocs. Hims GRIB viewer, viewfax has crude topography but is commendably easy to use. An example is given here.

ENLARGE

This has one facility that I have not found in other viewers; Use File-Save As to save in text form ie as a .txt file. That will give the message format in normal text.

What a GRIB message text looks like

In the example shown, I used the message -

send gfs:45N,50N,10W,0E|0.5,0.5|0,12,...,72|

That asks for winds and pressures between the latitude/longitudes shown, at a spacing of 0.5 degrees and 12 hour intervaks from T=0 to T=72 hours.

Using the Save As a text file gives a document with blocks of numbers each preceded by a string of numbers. There are data for 3 parameters (west/east and south/north wind components and pressure) at 7 times making a total of 21 blocks looking like this -

[Record 0]
Center=7
Process=81
GridID=255
Parameter=U GRD
Layer=105/10
Date=2011/12/04 00:00
Valid Time=0
LatPts=11
LonPts=21
Lat=4500'N,5000'N
dLat=0.50
Lon=35000'E,00000'E
dLon=0.50
Flags=11

Data Values:

 10.2,  10.3,  10.5,  10.7,  10.9,  11.2,  11.5,  11.8,  11.9,  12.1,  12.1,  11.9,  11.8,  11.6,  11.3,  10.9,  11.2,   9.6,   4.0,   2.5,   3.2
11.4, 11.5, 11.6, 11.7, 11.8, 11.8, 11.6, 11.6, 11.6, 11.6, 11.5, 11.3, 11.1, 10.9, 10.6, 10.3, 10.7, 10.5, 6.4, 3.5, 3.6
12.0, 11.9, 11.9, 11.9, 12.1, 12.1, 11.8, 11.6, 11.4, 11.3, 11.0, 10.7, 10.4, 10.2, 9.9, 9.8, 10.2, 10.0, 6.5, 3.8, 3.8
11.7, 11.7, 11.8, 11.8, 11.9, 11.8, 11.6, 11.5, 11.4, 11.1, 10.7, 10.4, 10.2, 9.9, 10.0, 10.2, 8.9, 5.7, 4.0, 3.9, 4.2
11.3, 11.2, 11.3, 11.3, 11.3, 11.0, 10.7, 10.5, 10.4, 10.3, 10.0, 9.8, 9.8, 9.9, 10.4, 9.6, 5.9, 4.0, 4.1, 4.4, 4.3
10.8, 10.9, 10.9, 10.8, 10.6, 10.4, 10.3, 10.1, 9.8, 9.5, 9.3, 9.3, 9.0, 8.3, 8.2, 6.9, 4.7, 4.2, 4.4, 4.4, 4.2
10.7, 10.8, 10.9, 10.8, 10.7, 10.6, 10.4, 10.2, 10.0, 9.7, 9.2, 6.8, 4.1, 2.9, 3.1, 3.7, 3.9, 4.1, 4.4, 4.4, 4.0
10.8, 11.0, 11.1, 11.2, 11.2, 11.1, 10.8, 10.6, 10.4, 10.2, 9.7, 7.4, 4.9, 4.0, 4.7, 5.9, 5.6, 5.0, 4.8, 4.6, 4.1
10.3, 10.4, 10.6, 10.9, 11.1, 11.1, 10.9, 10.6, 10.3, 10.0, 9.9, 9.8, 9.9, 10.1, 10.8, 12.1, 11.9, 8.3, 6.2, 5.6, 5.1
9.1, 9.4, 9.9, 10.4, 10.8, 10.9, 10.7, 10.4, 9.9, 9.6, 9.5, 9.8, 10.5, 11.5, 12.2, 12.8, 12.3, 9.7, 10.5, 11.4, 10.2
9.1, 9.4, 9.9, 10.4, 10.8, 10.7, 10.2, 9.6, 9.2, 8.7, 8.5, 9.0, 10.0, 11.1, 12.0, 12.7, 13.1, 13.1, 12.9, 12.6, 11.8

In this case the block of numbers comprises 11 rows and 21 columns.

What all this means

In this case, Record goes from 0 to 20, ie a total of 21 blocks of data.

Parameter is U GRD for Records 0 to 6. These are the west to east components of wind. For Records 7 to 13, Parameter is V GRD. That is the south to north component of the wind. For Records 14 to 20 Parameter is PRMSL, the surface pressure
NOTE winds are in metres per second.

Date is the T=0, ie the DTG of the data upon which to forecast is based.

Valid time is the time to which the following data block refers.

LatPts and LongPts are the number of rows and columns in th following data block.

Lat gives the range of latitudes and dLat gives the spacing betseen latitude rows.
NOTE Longitude is counted eastwards from the Greenwich meridian so that 10 W is 35000'E

Lon and dLon does the same for longitude.
NOTE With Saildocs it is possible to have a different spacing for rows and columns. It is also possible to have a variable ime interval

The other headings are of no interest to the casual user

The full message is shown on another page


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