Guide to WMO Table Driven Code Forms

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Preface This guide has been prepared to assist experts who wish to use the WMO Table Driven Data Representation Form FM 92 GRIB Edition 2 (GRIB2). This guide is designed in three layers to accommodate users who require different levels of understanding. Layer 1 is a general description designed for those who need to become familiar with the table driven code form but do not need a detailed understanding. Layer 2 focuses on the functionality and application of GRIB2, and is intended for those who must use software that encodes and/or decodes GRIB2, but will not actually write the software. Layer 3 is intended for those who must actually write GRIB2 encoding and/or decoding software, although those wishing to study the code form in depth, will find it equally useful. The WMO gratefully acknowledges the contributions of the experts who developed this guidance material. The Guide was prepared by Dr. Clifford H. Dey of the U. S. A. National Centre for Environmental Prediction. Contributions were also received in particular from Charles Sanders - Australia, Jean Clochard - Meteo-France, John Hennessy - ECMWF and Simon Elliott - EUMETSAT.
Layer 1: Basic Aspects of GRIB2 Contents 1.1 Overview ....................................................................................................................................1 1.2 General Description ...................................................................................................................3 1.2.1 Code Structure ............................................................................................................................................. 3 1.2.1.1 Code Sections................................................................................................................................... 3 1.2.1.2 Iterating the Sections ...................................................................................................................... 4 1.2.2 Code Templates .......................................................................................................................................... 6 1.2.3 Tables ............................................................................................................................................................. 6 1.2.3.1 Template Tables............................................................................................................................... 6 1.2.3.2 Code and Flag Tables .................................................................................................................... 7 1.2.4 Identification of Parameters ..................................................................................................................... 7 1.3 Updating the Code Form.................................................................................................................................. 9 1.3.1 General Procedures ................................................................................................................................... 9 1.3.2 Changes Requiring Software Modifications.......................................................................................10 1.3.2.1 Example of a Change to the Code Structure..........................................................................10 1.3.3 Changes Not Requiring Software Modifications...............................................................................11 1.3.3.1 Example of Adding a New Parameter ......................................................................................11 1.3.4 Validation of Updates................................................................................................................................12 Layer 2: Structure and Use of GRIB2 Messages 2.1 Sections of a GRIB2 Message ......................................................................................................................13 2.1.1 Section 0: Indicator Section ....................................................................................................................13 2.1.2 Section 1: Identification Section ............................................................................................................14 2.1.3 Section 2: Local Use Section..................................................................................................................17 2.1.4 Section 3: Grid Definition Section .........................................................................................................17 2.1.5 Section 4: Product Definition Section...................................................................................................21 2.1.6 Section 5: Data representation Section...............................................................................................23 2.1.7 Section 6: Bit-Map Section ......................................................................................................................24 2.1.8 Section 7: Data Section............................................................................................................................24 2.1.9 Section 8: End Section .............................................................................................................................25 2.2 GRIB2 Templates ..............................................................................................................................................25 2.2.1 Grid Definition Templates ........................................................................................................................25 2.2.2 Product Definition Templates .................................................................................................................29 2.2.3 Data Representation Data Templates .................................................................................................33 2.2.4 Data Templates...........................................................................................................................................34 2.3 GRIB2 Compression Schemes .....................................................................................................................35 2.3.1 Grid point data - Simple Packing...........................................................................................................36 2.3.2 Grid point data - Complex Packing .......................................................................................................39 2.3.3 Grid point data - Complex Packing and Spatial Differencing .......................................................43 2.3.4 Spectral Truncation....................................................................................................................................45 2.3.5 Spectral Data – Simple Packing ............................................................................................................46 2.3.6 Spectral Data – Complex Packing ........................................................................................................47 I PAGE
Page 1: Guide to the WMO Table Driven Code
Page 5 and 6: 1.1 Overview Layer 1: Basic Aspects
Page 7 and 8: 1.2 General Description 1.2.1 Code
Page 9 and 10: Section 0: Indicator Section Sectio
Page 11 and 12: 1.2.3.2 Code and Flag Tables A code
Page 13 and 14: 14 Minimum dew point depression K 1
Page 15 and 16: necessary flexibility is available
Page 17 and 18: Layer 2: Structure and Use of GRIB2
Page 19 and 20: 15 Month | 16 Day | Reference time
Page 21 and 22: Finally, octets 20 and 21 give addi
Page 23 and 24: Code Table 3.0: Source of Grid Defi
Page 25 and 26: 2.1.5 Section 4: Product Definition
Page 27 and 28: 2.1.6 Section 5: Data Representatio
Page 29 and 30: The various data compression scheme
Page 31 and 32: 4. Bit 2 of the projection flag is
Page 33 and 34: within the Product Definition Templ
Page 35 and 36: Code Table 4.2 Parameter number by
Page 37 and 38: Code table 4.5: Fixed surface types
Page 39 and 40: 5 Number of Section (“7”) 6-nn
Page 41 and 42: Octet No. Contents 1-4 Length of se
Page 43 and 44: “word” bit-length method. The c
Page 45 and 46: The sub-section set of numbers is t
Page 47 and 48: (increments) themselves are then st
Page 49 and 50: Bit Number Value Meaning 1 0 Points
Page 51 and 52: complex numbers F are stored in the
Page 53 and 54:
2.4 The Structure of GRIB2 Messages
Page 55 and 56:
22-25 59-62 all 1’s There is no s
Page 57 and 58:
2.4.2 Structure of A GRIB2 Message
Page 59 and 60:
Page 61 and 62:
24 227 0 The scale factor of first
Page 63 and 64:
11 417 5 The parameter number is 5
Page 66 and 67:
Currently Defined Product Definitio
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0 Vegetation/Biomass 1 Agri-aquacul
Page 70:
3.1 Quasi-regular Grids Layer 3: De
Page 73 and 74:
Now, let us consider the changes th
Page 75 and 76:
68-71 105-108 10000000 The j direct
Page 77 and 78:
generalized as follows: Ph = ah •
Page 79 and 80:
3.3 Data Compression 3.3.1 Complex
Page 81 and 82:
The Grid Point Data - Complex Packi
Page 83 and 84:
Octet No. Contents 1-4 Length of se
Page 85 and 86:
Octet No. Contents 1-4 Length of se
Page 87 and 88:
54600 The order of the spatial diff
Page 89:
3.4 Structure of a GRIB2 Message wi
Page 92 and 93:
Page 94 and 95:
31-34 234-237 36 There are 36 point
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3.4.3 Example of a GRIB2 Message wi
Page 98 and 99:
12 128 0 Since there is no optional
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DIFFERENCE FORMULATION” (Coded ac
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1-4 1574-1577 1000 This Section is
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