Encyclopedia of Graphics File Formats Companion/GFF_CD.ISO/formats/grib/spec/grib.txt

Text File | 1994-06-01 | 92KB | 2,896 lines

U.S. DEPARTMENT OF COMMERCE
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
NATIONAL WEATHER SERVICE
NATIONAL METEOROLOGICAL CENTER


Office Note 388


G  R  I  B

(Edition 1)




THE WMO FORMAT

FOR

THE STORAGE OF WEATHER PRODUCT INFORMATION

AND

THE EXCHANGE OF WEATHER PRODUCT MESSAGES

IN GRIDDED BINARY FORM




John D. Stackpole
Automation Division



Revised
(see overleaf)





August 31, 1993

This is an unreviewed manuscript,
primarily intended for informal exchange of information
among NMC staff members


REVISION HISTORY

since the last full revision/reprinting

dated March 3, 1993


August 31, 1993    p.6:    Added orig center for NWS/NMC/Reanal
    and corrected DeBilt number.
p.7:    Added gen code 70 for QLM hurricane
p.9:    Added grids 75,76,77 for QLM
p.13,14: Reoriented Grids 37-44 (WAFC/ICAO)
    such that left hand column is at
    330E (30W)
p.13,13.1:  Added table of number of points in
    lat circles of grids 37-44
p.15:    Added description of Grid 1
p.18:    Corrected long grid spacing in degrees
    and corrected location of Map 204
p.20    Adjusted location of map 208 to
    conform to change in map 204
p.28:    Added MSL variants 128 & 129; Added
    lat. lon. as parameters 176 & 177;
    added parameters 204, 205, 211, 212,
    218.
p.29:    Added Note 4
p.48:    Clarification of table 11.

Appendix A.  Revised to include "Z" as first
character in WMO header.  Used for "off-hour"
forecast hours.


GRIB Edition 1

INTRODUCTION

The World Meteorological Organization (WMO) Commission for
Basic Systems (CBS) Extraordinary Meeting Number VIII (1985)
approved a general purpose, bit-oriented data exchange format,
designated FM 92-VIII Ext. GRIB (GRIdded Binary).  It is an
efficient vehicle for transmitting large volumes of gridded data
to automated centers over high speed telecommunication lines using
modern protocols.  By packing information into the GRIB code,
messages can be made more compact than character oriented
bulletins, producing faster computer-to-computer transmissions.
GRIB can equally well serve as a data storage format, generating
the same efficiencies relative to information storage and
retrieval devices.

Changes and extensions to GRIB were approved at the regular
meeting of the WMO/CBS in February, 1988; additional changes were
introduced at the CBS/WGDM/Sub-Group on Data Representation
meetings in May 1989 and in October 1990.  The last set of changes
were of such structural magnitude as to require a new Edition of
GRIB, Edition 1, which this document describes.

Note: the Edition number is in the same location, relative to
the start of the GRIB message, for all Editions. Thus, decoding
programs can detect which Edition layout to expect in the
remaining portion of the message.  This is appropriate for
archives of messages encoded in earlier Editions or during
transition periods.

Each GRIB record intended for either transmission or storage
contains single value, or a matrix of values, at an array of grid
points, or a set of spectral coefficients, for parameters at a
single level (or layer) as a continuous bit stream.  Logical
divisions of the record are designated as "sections", each of
which provides control information and/or data.  A GRIB record
consists of six sections, two of which are optional:

        (0) Indicator Section
        (1) Product Definition Section (PDS)
        (2) Grid Description Section (GDS)  -  optional
        (3) Bit Map Section (BMS)  -  optional
        (4) Binary Data Section (BDS)
        (5) '7777' (ASCII Characters)

Most centers require bulletin headers to enable them to
receive, identify, and switch messages; NMC is no exception.  The
standard WMO abbreviated heading for GRIB is described in Appendix
A.

In this documentation, certain symbols are used to clarify
the contents of octets (groups of eight consecutive binary bits,
"bytes" in American usage).  If unadorned letters are used, they
are symbolic and their meanings are described in the text; a
decimal number is simply printed as is; a character or string of
characters is represented inside single quote marks.
International Alphabet No. 5, which is identical in its essential
elements to the U.S. National Standard 7-bit ASCII, is used for
character representation in the GRIB code.

Octets are numbered consecutively from the start of each
section; bits within an octet are also numbered from left (the
most significant bit) to right (the least significant bit).  Thus
an octet with bit 8 set to the value 1 would have the integer
value 1; bit 7 set to one would have a value of 2, etc.
The numbering of Tables in the following text corresponds to
the description of GRIB in the WMO Manual on Codes< >.  Some
additional tables not found in the WMO Manual are indicated by
letters.  These, generally, contain information unique to the NWS
or NOAA.

A final caveat: this document does not include all the
features in GRIB as described in the Manual on Codes.  This
document selects out those features which are of particular
interest to the National Weather Service at the present time.
Some of the "advanced" features are alluded to in this document;
please refer to the Manual for full details.

DATA PACKING METHOD.

The code form represents numeric data as a series of binary
digits (bits).  Such data representation is independent of any
particular machine representation; by convention data lengths are
measured in octets.  Data are coded as binary integers using the
minimum number of bits required for the desired precision.
Numeric values, with units as shown in Table 2, are first scaled
by a power of ten to achieve an appropriate decimal precision, a
reference value is subtracted from them to reduce redundancy and
eliminate negative values, and they are then further scaled by a
power of two to pack them into a pre-selected word length.
The representation of a single value is such that:

Y * 10D = R + (X * 2E)

where

Y = original or unpacked value; units as in Table 2;
D = decimal scale factor, to achieve desired precision
        (sign bit, followed by a 15-bit integer);
R = reference value (32 bits);
X = internal value (No. of bits varies for each record);
E = binary scale factor for word-length packing
        (sign bit, followed by a 15-bit integer).

The reference value (R) is the minimum value of the decimally
scaled data that is being encoded.  R is placed in the Binary Data
Section in four octets as a single precision floating-point
number:

sAAAAAAA BBBBBBBB BBBBBBBB BBBBBBBB

where s = sign bit, encoded as   0 => positive
                                 1 => negative
      A = 7-bit binary integer, the characteristic
      B = 24-bit binary integer, the mantissa.

The appropriate formula to recover the value of R is:
R = (-1)s * 2(-24) * B * 16(A-64)
This formula is the standard IBM representation for a single
precision (real) floating point number.  (Consideration is being
given to using the IEEE floating point representation in the
future, in a later Edition of GRIB.)
Both the decimal scaling factor and/or the binary scaling
factors may equal 0.  It is not necessary to use the binary
scaling to "fit" the numbers into a prespecified word size; an
alternative encoding practice is to change the word length (the
same for all the points, of course) to accommodate the largest
departure from the reference value.  A properly written GRIB
decoder should be able to decode either option with no change in
the logic as long as the decoder makes no prior assumptions about
word lengths.  All the necessary parameters are included in the
GRIB message, of course.

GRIB CODE FORM:

With the exception of the Indicator Section and the End
Section all octets contain binary values.  All sections contain an
even number of octets; the variable length sections are padded
with zero values as necessary.  These extra bits must be accounted
for in finding one's way through the sections; their content
should be ignored.

SECTION 0: THE INDICATOR SECTION (IS)

The indicator section serves to identify the start of the
record in a human readable form; to indicate the total length of
the message, and to indicate the Edition number of the message.
It is always eight octets long.

Octet no.             IS Content

    1-4    'GRIB'  (Coded CCITT-ITA No. 5) (ASCII);

    5-7    Total length, in octets, of GRIB message
            (including Sections 0 & 5);

    8    Edition number - currently  1

SECTION 1: THE PRODUCT DEFINITION SECTION (PDS).

The PDS contains indicators for the Parameter table Version,
the originating center, the numerical model (or "generating
process") that created the data, the geographical area covered by
the data, the parameter itself, and the actual values for the
appropriate vertical level or layer, the decimal scale factor, and
date/time information.  The PDS is at least 28 octets long but it
may be longer if an originating center chooses to make it so.
Users of GRIB messages are strongly urged to use the length-of-
section portion of the PDS to determine where the next section
begins.  Do not assume a fixed octet length.



Octet no.                PDS Content

1 - 3      Length in octets of the Product Definition Section
 4         Parameter Table Version number.
           Currently Version 1 for international exchange
       Parameter table version numbers 128-254 are
       reserved for local use.
 5         Identification of center (See Table 0)
 6         Generating process ID number
       (allocated by the originating center; See Table A)
 7         Grid Identification (geographical location and

            area; See Table B)
 8         Flag specifying the presence or absence of a GDS
                  or a BMS (See Table 1)
 9         Indicator of parameter and units (See Table 2)
 10        Indicator of type of level or layer
                    (See Tables 3 & 3a)
 11-12     Height, pressure, etc. of the level or layer
                    (See Table 3)
 13        Year of century     \  Initial (or Reference)
                               |  time of forecast - UTC
 14        Month of year       |
                               |          or
 15        Day of month         >
                               |  Start of time period
 16        Hour of day         |    for averaging or
                               |    accumulation of
 17        Minute of hour      /    analyses

 18        Forecast time unit (see Table 4)
 19        P1 - Period of time (Number of time units)
                (0 for analysis or initialized analysis.)
                Units of time given by content of octet 18.
 20        P2 - Period of time (Number of time units)
                    or
                Time interval between successive analyses,
                successive initialized analyses, or forecasts,
                undergoing averaging or accumulation.
                Units given by octet 18.
 21        Time range indicator (See Table 5)
 22-23     Number included in average, when octet 21
                (Table 5) indicates an average or
               accumulation; otherwise set to zero.
 24       Number Missing from averages or accumulations.
 25       Century of Initial (Reference) time
                 (=20 until Jan. 1, 2001)
 26       Reserved - set to 0
 27-28    The decimal scale factor D.  A negative value
    is indicated by setting the high order bit
    (bit No. 1) in octet 27 to 1 (on).
 29-40    Reserved (need not be present)
 41-...    Reserved for originating center use.


Note: Octet 8 may indicate the presence of the Grid
Description Section (GDS) even though octet 7 specifies a
predefined grid.  In this case the GDS must describe that grid -
this device serves as a mechanism for transmitting new
"predefined" grids to users prior to their formal publication in
this or the official WMO documentation.

TABLE 0.  TABLES FOR THE PDS ORIGINATING CENTER
(PDS Octet 5)

VALUE            CENTER

    07    US Weather Service - National Met. Center
    08    US NWS - NMC - Reanalysis project.
    34    Japanese Meteorological Agency - Tokyo
    54    Canadian Meteorological Service - Montreal
    58    US Navy  - Fleet Numerical Oceanography Center
    59    NOAA Forecast Systems Lab, Boulder CO
    74    U.K. Met Office - Bracknell
    97    European Space Agency (ESA)
    98    European Center for Medium-Range Weather
        Forecasts - Reading
    99    DeBilt

    150    ABRFC - Arkansas-Red River RFC, Tulsa OK
    151    Alaska RFC, Anchorage, AK
    152    CBRFC - Colorado Basin RFC, Salt Lake City, UT
    153    CNRFC - California-Nevada RFC, Sacramento, CA
    154    LMRFC - Lower Mississippi RFC, Slidel, LA
    155    MARFC - Middle Atlantic RFC, State College, PA
    156    MBRFC - Missouri Basin RFC, Kansas City, MO
    157    NCRFC - North Central RFC, Minneapolis, MN
    158    NERFC - Northeast RFC, Hartford, CT
    159    NWRFC - Northwest RFC, Portland, OR
    160    OHRFC - Ohio Basin RFC, Cincinnati, OH
    161    SERFC - Southeast RFC, Atlanta, GA
    162    WGRFC - West Gulf RFC, Fort Worth, TX

    170    OUN   - Norman OK WFO


TABLE A.        Generating Process or Model
(PDS Octet 6)

    VALUE        MODEL

    10    Global Wind-Wave Forecast Model
    19    Limited-area Fine Mesh (LFM) analysis
    25    Snow Cover Analysis
    39    Nested Grid forecast Model (NGM)
    42    Global Optimum Interpolation Analysis (GOI)
        from "Aviation" run
    43    Global Optimum Interpolation Analysis (GOI)
        from "Final" run
    44    Sea Surface Temperature Analysis
    53    LFM-Fourth Order Forecast Model
    64    Regional Optimum Interpolation Analysis (ROI)
    68    80 wave triangular, 18-layer Spectral model
        from "Aviation" run
    69    80 wave triangular, 18 layer Spectral model
        from "Medium Range Forecast" run
    70    Quasi-Lagrangian Hurricane Model (QLM)
    73    Fog Forecast model - Ocean Prod. Center
    74    Gulf of Mexico Wind/Wave
    75    Gulf of Alaska Wind/Wave
    76    Bias corrected Medium Range Forecast
    77    126 wave triangular, 18 layer Spectral model
        from "Aviation" run
    78    126 wave triangular, 18 layer Spectral model
        from "Medium Range Forecast" run
    79    Backup from the previous run
    80    62 wave triangular, 18 layer Spectral model
        from "Medium Range Forecast" run
    81    Spectral Statistical Interpolation (SSI)
        analysis from "Aviation" run.
    82    Spectral Statistical Interpolation (SSI)
        analysis from "Final" run.
    83    ETA Model - 80 km version
    84    ETA Model - 40 km version
    85    ETA Model - 30 km version
    86    MAPS Model, from Forecast Systems Lab
            (Isentropic; scale: 60km at 40N)
    150    NWS River Forecast System (NWSRFS)
    151    NWS Flash Flood Guidance System (NWSFFGS)
    152    WSR-88D Stage II Precipitation Analysis
    153    WSR-88D Stage III Precipitation Analysis


TABLE B.   GRID IDENTIFICATION
(PDS Octet 7)
 MASTER LIST OF NMC STORAGE GRIDS
 VALUE                GRID
                                                      INCREMENT

    1    1679-point (73x23) Mercator grid with    5 degs of
        (1,1) at (0W,48.09S), (73,23) at (0W,    longitude
        48.09N); I increasing eastward,
        Equator at J=12.

    2    10512-point (144x73) global longitude-    2.5 deg
        latitude grid.  (1,1) at 0E, 90N,
        matrix layout.  N.B.: prime meridian
        not duplicated.

    3    65160-point (360x181) global longitude-    1.0 deg
        latitude grid.  (1,1) at 0E, 90N,
        matrix layout.  N.B.: prime meridian
        not duplicated.

    5    3021-point (53x57) N. Hemisphere polar    190.5 km
        stereographic grid oriented 105W; Pole    at 60N
        at (27,49). (LFM analysis)

    6    2385-point (53x45) N. Hemisphere polar    190.5 km
        stereographic grid oriented 105W; Pole    at 60N
        at (27,49). (LFM Forecast)

    21-26    International Exchange and
        Family of Services (FOS) grids - see below

  27         4225-point (65x65) N. Hemisphere polar   381 km
             stereographic grid oriented 80W; Pole    at 60N
             at (33,33).

  28         4225-point (65x65) S. Hemisphere polar   381 km
             stereographic grid oriented 100E; Pole   at 60S
             at (33,33).

  29         5365-point (145x37) N. Hemisphere lon-   2.5 degs
             gitude/latitude grid for latitudes 0N
             to 90N; (1,1) at (0E,0N).

  30         5365-point (145x37) S. Hemisphere lon-   2.5 degs
             gitude/latitude grid for latitudes 90S
             to 0S; (1,1) at (0E,90S).



  33         8326-point (181x46) N. Hemisphere lon-   2 degs
             gitude/latitude grid for latitudes 0N
             to 90N; (1,1) at (0E,0N).

    34    8326-point (181x46) S. Hemisphere lon-    2 degs
        gitude/latitude grid for latitudes 90S
        to 0S; (1,1) at (0E,90S).

    37 - 44    Eight lat-long 1.25x1.25 "thinned" grids,
        covering the globe by octants of 3447 points.
        Full GRIB specifications below.
        For WAFC, ICAO, Family of Services (FOS),
        and International exchange.

    50    Family of Services "regional grid" - see below.

    55    6177-point (87x71) N. Hemisphere polar    254 km
        stereographic grid oriented 105W; Pole    at 60N
        at (44,38). (2/3 bedient NH sfc anl)

    56    6177-point (87x71) N. Hemisphere polar    127 km
        stereographic grid oriented 105W; Pole    at 60N
        at (40,73). (1/3 bedient NA sfc anl)

    61-64    International Exchange & FOS grids - see below.

    75    12321-point (111x111) N. Hemisphere    40 km at
        Lambert Conformal grid.  No fixed    30&60 deg N
        location; used by QLM Hurricane model.

    76    12321-point (111x111) S. Hemisphere    40 km at
        Lambert Conformal grid.  No fixed    30&60 deg S
        location; used by QLM Hurricane model.

    77    12321-point (111x111) N. Hemisphere    40 km at
        Mercator grid.  No fixed location;    22.5 deg N&S
        used by QLM Hurricane model.

    85    32400-point (360x90) N. Hemisphere    1 deg
        longitude/latitude grid;
        longitudes: 0.5E to 359.5E (0.5W);
        latitudes: 0.5N to 89.5N;
        origin (1,1) at (0.5E,0.5N)

    86    32400-point (360x90) S. Hemisphere    1 deg
        longitude/latitude grid;
        longitudes: 0.5E to 359.5E (0.5W);
        latitudes: 89.5S to 0.5S;
        origin (1,1) at (0.5E,89.5S)

    87    5022 point (81x62) N. Hemisphere  polar    68.153 km
        stereographic grid oriented at 105W.    at 60N
        Pole at (31.91, 112.53) Used for RUC.
        (60 km at 40N).  See below for GRIB
        specification.

    90    12902 point (92x141 semi-staggered)    lat.14/26 deg
        lat. long., rotated such that center   lon.15/26 deg
        located at 52.0N, 111.0W; LL at 37.5W, 35S
        Unfilled E grid for 80 km ETA model

    91    25803 point (183x141)                  lat.14/26 deg
        lat. long., rotated such that center   lon.15/26 deg
        located at 52.0N, 111.0W; LL at 37.5W,35S
        Filled E grid for 80 km ETA model

    92    2 4162 point (127x191 semi-staggered)  lat. 5/57 deg
        lat. long., rotated such that center   lon. 5/18 deg
        located at 41.0N, 97.0W; LL at 35W,25S
        Unfilled E grid for 40 km ETA model

    93    48323 point (253x191)lat. long.,       lat.15/57 deg
        rotated such that center located       lon.5/18 deg
        at 41.0N, 97.0W; LL at 35W ,25S
        Filled E grid for 40 km ETA model

    98    Global Gaussian T62 grid.  See GRIB
        specifications below

    100    6889-point (83x83) N. Hemisphere polar   91.452 km
               stereographic grid oriented 105W; Pole   at 60N
               at (40.5,88.5). (NGM Original C-Grid)

    101    10283-point (113x91) N. Hemisphere    91.452 km
        polar stereographic grid oriented 105W;    at 60N
        Pole at (58.5,92.5). (NGM "Big C-Grid")

    103    3640-point (65x56) N. Hemisphere polar    91.452 km
        stereographic grid oriented 105W; Pole    at 60N
        at (25.5,84.5) (used by ARL)

    104    16170-point (147x110) N. Hemisphere    90.75464 km
        polar stereographic grid oriented 105W;    at 60N
        pole at (75.5,109.5). (NGM Super C grid)

    105    6889-point (83x83) N. Hemisphere polar    90.75464 km
        stereographic grid oriented 105W;    at 60N
        pole at  (40.5,88.5).  (U.S. area
        subset of NGM Super C grid, used
        by ETA model)

    106    19305 point (165x117) N. Hemisphere    45.37732 km
        stereographic grid oriented 105W;    at 60N
        pole at (80,176)
        Hi res. ETA (2 x resolution of Super C)

    107    11040 point (120x92) N. Hemisphere    45.37732 km
        stereographic grid oriented 105W;    at 60N
        pole at (46,167)
        subset of Hi res. ETA; for ETA & MAPS/RUC

    126    Global Gaussian T126 grid.  See GRIB
        specifications below

    201-nnn    AWIPS grids. See specifications below.

    255    (non-standard grid - defined in the GDS)

NOTE ON NMC STORAGE GRIDS:

On the polar stereographic grids, the vector wind is resolved
into u and v components with respect to the grid coordinates,
i.e., u represents motion in the direction of increasing x (i)
coordinate, v in the direction of increasing y (j).  On the
latitude-longitude grids, u and v are true eastward and northward
components, respectively.  However, take note of Table 7, below,
which allows for the specification of other possibilities.

INTERNATIONAL EXCHANGE AND FAMILY OF SERVICES (FOS) GRIDS
 VALUE  RESOLUTION      AREA              GRID          GRID
        (degrees)     COVERAGE            SHAPE        POINTS
        lon x lat     (degrees)        cols  rows

 21    5.0 x 2.5   0-180E, 0-90N         37   36 + pole  1333
 22    5.0 x 2.5   180W-0, 0-90N         37   36 + pole  1333
 23    5.0 x 2.5   0-180E, 90S-0  pole + 37   36         1333
 24    5.0 x 2.5   180W-0, 90S-0  pole + 37   36         1333
 25    5.0 x 5.0   0-355E, 0-90N         72   18 + pole  1297
 26    5.0 x 5.0   0-355E, 90S-0  pole + 72   18         1297
 50    2.5 x 1.25             (see note iv)               964
 61    2.0 x 2.0   0-180E, 0-90N         91   45 + pole  4096
 62    2.0 x 2.0   180W-0, 0-90N         91   45 + pole  4096
 63    2.0 x 2.0   0-180E, 90S-0  pole + 91   45         4096
 64    2.0 x 2.0   180W-0, 90S-0  pole + 91   45         4096

255         (non-standard grid - defined in the GDS)

NOTES ON INTERNATIONAL EXCHANGE/FOS GRIDS:

(i)    The grid points are laid out in a linear array such
that the longitude index (the columns) is the most rapidly
varying.  For the northern hemisphere grids the first point in the
record is at the intersection of the western-most meridian and
southern-most circle of latitude; the last point is the single
polar value (see note iii, below).  For the southern hemisphere
grids the first point in the record is the single polar value (see
note iii, below); the last point is at the intersection of the
eastern-most meridian and northern-most circle of latitude.  For
those familiar with FORTRAN subscripting conventions, longitude is
the first subscript, latitude the second.
 (ii)   In grids 21 through 26, and 61 through 64, the values
on the shared boundaries are included in each area.
 (iii)  The datum for the pole point is given only once in
each grid.  The user must expand, if desired, the single pole
point value to all the pole "points" at the pole row of a
latitude-longitude grid.  Scalar quantity values are the same for
all pole points on a the grid.  Wind components at the poles are
given by the formulae:

u = -speed * sin(dd)   &   v = -speed * cos(dd)

where dd is the direction of the wind as reported according to the
specification of wind direction at the poles (refer to WMO Manual
on Codes <1>, code table 878).

The WMO convention can be given this operational definition:
At the North Pole, face into the wind and report the value of the
west longitude meridian along which the wind is coming at you; at
the South Pole do likewise but report the east longitude meridian
value. This is equivalent to placing the origin of a right-handed
Cartesian coordinate system on the North Pole with the y-axis
pointing to the prime (0 degree) meridian and the x-axis pointing
to the 90 degrees west meridian, and then resolving any vector
wind at the pole point into components along those axes.  At the
South Pole the coordinate axes are oriented such that the y-axis
points toward 180 degrees west.  Those components are the u- and
v-values given as the single pair of pole point winds in the GRIB
format.

In terms of a longitude/latitude grid these are the wind
components for the pole point at the 180 degree meridian.  For
example, on a 2.5x2.5 degree northern hemisphere grid (145x37
points), with the abscissa along the equator and the ordinate
along the prime meridian, the transmitted north pole wind
components are those that belong at the gridpoint (73,37).  The
wind components at the other grid points along the pole row may be
obtained through suitable rotation of the coordinate system.  All
the components at the pole row are, of course, simply
representations of the same vector wind viewed from differing
(rotated) coordinate systems.  In the southern hemisphere the
analogous situation holds; the single set of transmitted pole
point wind components belong at the gridpoint (73,1).
(iv)   Grid 50 is a set of points over the contiguous United
States and environs on a grid extending from 20N (row No. 1) to
60N (row  No. 33) in 1.25 degree intervals.  The grid increases in
longitudinal extent from south to north in the following manner:

ROWS   NO. POINTS   LONGITUDINAL EXTENT

 1-4      22          122.5W  - 70.0W
 5-8      24          125.0W  - 67.5W
 9-12     26          127.5W  - 65.0W
13-16     28          130.0W  - 62.5W
17-20     30          132.5W  - 60.0W
21-24     32          135.0W  - 57.5W
25-28     34          137.5W  - 55.0W
29-33     36          140.0W  - 52.5W


WAFC/ICAO/INTERNATIONAL EXCHANGE/FOS GRIDS

(Grids 37 - 44)

Global Coverage of Grids

Octants of the Globe

In the figure the boxes indicate the location of the octants
of the globe, the numbers are the corresponding grid
identification numbers (PDS Octet 7), and the letters are the grid
identification used in the WMO heading (see Appendix A).
The left and right meridional columns of each octant/grid are
shared with the neighbors.

The basic grid point separation is 1.25x1.25 deg. on a
latitude / longitude array, but the grid is "thinned" by reducing
the number of points in each row as one goes northward (or
southward) away from the equator.  The latitudinal increment is
always 1.25 deg.; this results in 73 rows where the pole is
included as a "row", not a single gridpoint.
The longitudinal spacing at the equator is also 1.25 deg.;
there will be 73 gridpoints there in each octant.
The number of points on each latitudinal row, other than the
equator, is given by this formula (using FORTRAN notation):

NPOINTS = IFIX(2.0 + (90.0/1.25) * COS(LATITUDE))

Thus at the pole there will be two gridpoints, one each at the
meridians that delineate the edges of the octant.  The formula was
worked out so that there will be (approximately) equal geographic
separation between the grid points uniformly across the globe.

Because of variations in precision and roundoff error in
different computers, the value of NPOINTS may vary by 1 at
"critical" latitudes when calculated on various hardware
platforms.  Here is a table of the exact values of NPOINTS as a
function of latitude as used in the internationally exchanged
grids.  These numbers will, of course, be found in the Grid
Description Section of each GRIB bulletin.


Latitude Range    NPOINTS
inclusive
(north or south)

     0.00 -  8.75    73
    10.00 - 12.50    72
    13.75 - 16.25    71
    17.50 - 18.75    70
    20.00 - 21.25    69
    22.50    68
    23.75 - 25.00    65
    30.00    64
    31.25    63
    32.50    62
    33.75    61
    35.00 - 36.25    60
    37.50    59
    38.75    58
    40.00    57
    41.25    56
    42.50    55
    43.75    54
    45.00    52
    46.25    51
    47.50    50
    48.75    49
    50.00    48
    51.25    47
    52.50    45
    53.75    44



Latitude Range    NPOINTS
inclusive
(north or south)

    55.00    43
    56.25    42
    57.50    40
    58.75    39
    60.00    38
    61.25    36
    62.50    35
    63.75    33
    65.00    32
    66.25    30
    67.50    29
    68.75    28
    70.00    26
    71.25    25
    72.50    23
    73.75    22
    75.00    20
    76.25    19
    77.50    17
    78.75    16
    80.00    14
    81.25    12
    82.50    11
    83.75     9
    85.00     8
    86.25     6
    87.50     5
    88.75     3
    90.00     2




When all this is put together the result is that there are
3447 points of data actually transmitted in any individual GRIB
bulletin containing one octant of the globe.

In the GRIB bulletins all of this information will be
included in the Grid Description Section (GDS); the GDS must be
included in order to describe the thinned or "quasi-regular" grid
structure. See Section 2 and Table C for the general description
of the GDS; what follows are the specific values of the variables
in the GDS that describe these eight grids.

GDS Contents

    Octets    Value or variable

    1-3    178 (length of GDS)
    4    0 (or 255, either indicating no PV)
    5    33 (pointer to start of PL list)
    6    0
    7-32    Grid description - see below
    33-178    number of points in each of 73 rows
        (2 octets per point)

Details of Octets 7-32 - Grid Description
    Octets    Variable & Value

    7-8    Ni = 73
    9-10    Nj = 73



GRID:
37
38
39
40
41
42
43
44

11-13
La1 =
0
0
0
0
90S
90S
90S
90S

14-16
Lo1 =
330
60
150
240
330
60
150
240


    17    Resolution & Component Flag = [10000000] (binary)


GRID:
37
38
39
40
41
42
43
44

18-20
La2 =
90N
90N
90N
90N
0
0
0
0

21-23
Lo2 =
60
150
240
330
60
150
240
330


    24-25    Di = 1.25 deg
    26-27    Dj = 1.25 deg
    28    Scan Mode = [01000000] (binary)
    29-32    Set to 0 (unused)

Note that the scanning direction is from the bottom (south
edge) to the top of the octant grids, regardless of the
hemisphere.  Thus in the northern hemisphere the first 73 data
points (in the BDS) will be the equatorial values and the last two
will be the polar values.  The PL counts in the GDS octets 33-178
will, of course, indicate contain these numbers.
In the southern hemisphere, the first two data points will be
the south pole values, and the last 73 points will be the
equatorial values.  Octets 33-34 in the GDS will contain "2",
octets 35-36 will contain a "3", and so on to octet 177-178 which
will contain "73".


SELECTED NMC GRIDS DEFINED USING GRIB SPECIFICATIONS
(See Table C for definition of symbols)

VALUE           GRID DESCRIPTIONS

1    Tropical Strip
    (Mercator)

    Ni =    73
    Nj =    23
    La1 =     48.09S
    Lo1 =      0.0E
    Res. & Comp. flag =    1 0 0 0 0 0 0 0
    La2 =     48.09N
    Lo2 =      0.0W
    Latin =      22.5
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Di  = Dj =    513.669 km

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     48.09S, 0.00E
    (1,23)  =    48.09N, 0.00E
    (73,23) =     48.09N, 0.00W
    (73,1)  =    48.09S, 0.00W

The longitudinal grid spacing is 5.00 degrees.


87     U.S. Area; used in MAPS/RUC
    (60km at 40N)
     (N. Hem. polar stereographic)

    Nx =    81
    Ny =    62
    La1 =    22.8756N
    Lo1 =    239.5089E = 120.4911W
    Res. & Comp. flag = 0 0 0 0 1 0 0 0
    Lov =     255.000E = 105.000W
    Dx  = Dy =    68.153 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0


For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     22.8756N, 120.4911W
    (1,62)  =    52,4887N, 136.5458W
    (81,62) =     46.0172N,  60.8284W
    (81,1)  =    20.1284N,  81.2432W

The pole point is at     (I,J) =     (31.91,112.53)


 98           Global Gaussian Latitude/Longitude T62 Resolution

    Ni =    192
    Nj =    94
    La1 =     88.542N
    Lo1 =      0.0E
    Res. & Comp. flag =    1 0 0 0 0 0 0 0
    La2 =     88.542S
    Lo2 =    358.125E = 1.875W
    Di =     1.875 degrees
    N  =    47 (number of lat. circles, pole
            to equator)
    Scanning Mode = 00000000(NB:matrix style)

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     88.542N, 0.0E  (upper left)
    (1,190)  =    88.542S, 0.0E
    (384,190) =     88.542S, 359.0625E
    (384,1)  =    88.542N, 359.0625E

-----------------------------------------------------------------

 126           Global Gaussian Latitude/Longitude T126 Resolution

    Ni =    384
    Nj =    190
    La1 =     89.277N
    Lo1 =      0.0E
    Res. & Comp. flag =    1 0 0 0 0 0 0 0
    La2 =     89.277S
    Lo2 =    359.0625E = 0.9375W
    Di =     0.9375 degrees
    N  =    95 (# of lat circles pole
            to equator)
    Scanning Mode = 00000000(NB:matrix style)

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     89.277N, 0.0E  (upper left)
    (1,190)  =    89.277S, 0.0E
    (384,190) =     89.277S, 359.0625E
    (384,1)  =    89.277N, 359.0625E


AWIPS-90 STORAGE AND TRANSMISSION GRIDS

Note: The following grids are intended for use in the U.S.
Weather Service's Advanced Weather Information Processing System
for the 1990s (AWIPS-90).  Their definition is subject to change
as the AWIPS-90 requirements are further refined.

VALUE                   AWIPS GRID DESCRIPTIONS
            (See Table C for definition of symbols)

201       Hemispheric
     (polar stereographic)

    Nx =    65
    Ny =    65
    La1 =    -20.826N = 20.826S
    Lo1 =    210.000E = 150.000W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     255.000E = 105.000W
    Dx  = Dy =    381.000 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

The pole point is at     (I,J) =     (33,33)

Map 201 is the same as NMC storage grid 27, except it is rotated
to 105 deg. orientation.

-----------------------------------------------------------------

202    National - CONUS
    (polar stereographic)

    Nx =    65
    Ny =    43
    La1 =      7.838N
    Lo1 =    218.972E = 141.028W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     255.000E = 105.000W
    Dx  = Dy =    190.500 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =      7.838N, 141.028W
    (1,43)  =    35.616N, 168.577E
    (65,43) =     35.617N,  18.576W
    (65,1)  =     7.838N,  68.973W

The pole point is at     (I,J) =     (33,45)


203    National - Alaska
    (polar stereographic)

    Nx =    45
    Ny =    39
    La1 =     19.132N
    Lo1 =    174.163E = 185.837W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     210.000E = 150.000W
    Dx  = Dy =    190.500 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     19.132N, 174.163E
    (1,39)  =    44.646N, 115.601E
    (45,39) =     57.634N,  53.660W
    (45,1)  =    24.361N, 123.434W

The pole point is at     (I,J) =     (27,37)

-----------------------------------------------------------------

204    National - Hawaii
    (Mercator)

    Ni =    93
    Nj =    68
    La1 =     25.000S
    Lo1 =    110.000E
    Res. & Comp. flag =    1 0 0 0 0 0 0 0
    La2 =     60.644N
    Lo2 =    109.129W
    Latin =      20.000
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Di  = Dj =    160.000 km

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     25.000S, 110.000E
    (1,68)  =    60.644N, 110.000E
    (93,68) =     60.644N, 109.129W
    (93,1)  =    25.000S, 109.129W

The longitudinal grid spacing is 1.531 degrees.


205    National - Puerto Rico
    (polar stereographic)

    Nx =     45
    Ny =     39
    La1 =      0.616N
    Lo1 =    275.096E = 84.904W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     300.000E = 60.000W
    Dx  = Dy =    190.500 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:

    (1,1)   =      0.616N,  84.904W
    (1,39)  =    36.257N, 115.304W
    (45,39) =     45.620N,  15.000W
    (45,1)  =     3.389N,  42.181W

The pole point is at     (I,J) =     (27,57)

-----------------------------------------------------------------

206    Regional - Central US MARD
    (Lambert Conformal)

    Nx =    51
    Ny =    41
    La1 =     22.289N
    Lo1 =    242.009E = 117.991W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     265.000E = 95.000W
    Dx = Dy =     81.2705 km
    Projection Flag = 0  (not bipolar)
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Latin 1 =      25.000N
    Latin 2 =    25.000N   (tangent cone)

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     22.289N, 117.991W
    (1,41)  =    50.081N, 124.898W
    (51,41) =     50.072N,  73.182W
    (51,1)  =    23.142N,  78.275W

The Dx, Dy grid increment (at 25 deg north) was selected so that
the grid spacing would be exactly 80.000 km at 35 deg north; the
intersection of 35N & 95W falls on point (30,16).


207    Regional - Alaska
    (polar stereographic)

    Nx =     49
    Ny =     35
    La1 =     42.085N
    Lo1 =    184.359E = 175.641W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     210.000E = 150.000W
    Dx  = Dy =     95.250 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =     42.085N, 175.641W
    (1,35)  =    63.976N, 153.689E
    (49,35) =     63.976N,  93.689W
    (49,1)  =    42.085N, 124.359W

The pole point is at     (I,J) =     (25,51)

-----------------------------------------------------------------

208    Regional - Hawaii
    (Mercator)

    Ni =    29
    Nj =    27
    La1 =      9.343N
    Lo1 =    192.685E = 167.315W
    Res. & Comp. flag =    1 0 0 0 0 0 0 0
    La2 =     28.092N
    Lo2 =    145.878W
    Latin =      20.000
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Di  = Dj =     80.000 km

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =      9.343N, 167.315W
    (1,27)  =    28.092N, 167.315W
    (29,27) =     28.092N, 145.878W
    (29,1)  =     9.343N, 145.878W

The longitudinal grid spacing is 0.766 degrees.  The grid is
positioned such that the odd-numbered rows and columns coincide
with the National grid, No. 204;  the lower left corner of the
regional grid is located at National (204) grid-point (55,24) and
the upper right corner is located at (69,37).


209    Regional - Central US MARD - Double Res.
    (Lambert Conformal)

    Nx =    101
    Ny =    81
    La1 =     22.289N
    Lo1 =    242.00962E = 117.991W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     265.000E =  95.000W
    Dx = Dy =     40.63525 km
    Projection Flag =     0 (not bipolar)
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Latin 1 =      25.000N
    Latin 2 =    25.000N   (tangent cone)

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)    =     22.289N, 117.991W
    (1,81)   =    50.081N, 124.898W
    (101,81) =     50.072N,  73.182W
    (101,1)  =    23.142N,  78.275W

The Dx, Dy grid increment (at 25 deg north) was selected so that
the grid spacing would be exactly 40.000 km at 35 deg north; the
intersection of 35N & 95W falls on point (59,31).

-----------------------------------------------------------------

210    Regional - Puerto Rico
    (Mercator)

    Ni =    25
    Nj =    25
    La1 =      9.000N
    Lo1 =    283.000E = 77.000W
    Res. & Comp. flag =    1 0 0 0 0 0 0 0
    La2 =     26.422N
    Lo2 =     58.625W
    Latin =      20.000
    Di  = Dj =     80.000 km
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)   =      9.000N, 77.000W
    (1,25)  =    26.422N, 77.000W
    (25,25) =     26.422N, 58.625W
    (25,1)  =     9.000N, 58.626W

The longitudinal grid spacing is 0.766 degrees


211    Regional - CONUS
    (Lambert Conformal)

    Nx =    93
    Ny =    65
    La1 =     12.190N
    Lo1 =    226.541E = 133.459W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     265.000E = 95.000W
    Dx = Dy =     81.2705 km
    Projection Flag = 0  (not bipolar)
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Latin 1 =      25.000N
    Latin 2 =    25.000N   (tangent cone)

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)    =     12.190N, 133.459W
    (1,65)   =    54.536N, 152.856W
    (93,65)  =     57.290N,  49.385W
    (93,1)   =    14.335N,  65.091W

The Dx, Dy grid increment (at 25 deg north) was selected so that
the grid spacing would be exactly 80.000 km at 35 deg north; the
intersection of 35N & 95W falls on point (53,25).

-----------------------------------------------------------------

212                   Regional  - CONUS - double resolution
(Lambert Conformal)

    Nx =    185
    Ny =    129
    La1 =     12.190N
    Lo1 =    226.514E = 133.459W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     265.000E =  95.000W
    Dx = Dy =     40.63525 km
    Projection Flag = 0  (not bipolar)
    Scanning Mode (Bits 1 2 3) = 0 1 0
    Latin 1 =      25.000N
    Latin 2 =    25.000N   (tangent cone)

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)     =     12.190N, 133.459W
    (1,129)   =       54.536N, 122.856W
    (185,129) =     57.290N,  49.385W
    (185,1)   =        14.335N,  65.091W

The Dx, Dy grid increment (at 25 deg north) was selected so that
the grid spacing would be exactly 40.000 km at 35 deg north; the
intersection of 35N & 95W falls on point (105,49).


213    National - CONUS - Double Resolution
    (polar stereographic)

    Nx =    129
    Ny =    85
    La1 =      7.838N
    Lo1 =    218.972E = 141.028W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     255.000E = 105.000W
    Dx  = Dy =     95.250 km
    Projection Flag (Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:
    (1,1)    =     7.838N, 141.028W
    (1,85)   =    35.617N, 168.577E
    (129,85) =    35.617N,  18.577W
    (129,1)  =     7.838N,  68.973W

The pole point is at     (I,J) =     (65,89)

-----------------------------------------------------------------

214    Regional - Alaska - Double Resolution
    (polar stereographic)

    Nx =     97
    Ny =     69
    La1 =     42.085N
    Lo1 =    184.359E = 175.641W
    Res. & Comp. flag =    0 0 0 0 1 0 0 0
    Lov =     210.000E = 150.000W
    Dx  = Dy =     47.625 km
    Projection Flag(Bit 1) = 0
    Scanning Mode (Bits 1 2 3) = 0 1 0

For reference here are the lat/lon values of the corners of the
grid:
    (1,1) =     42.085N, 175.641W


    (1,69) =    63.975N, 153.690E
    (97,69) =     63.975N,  93.689W
    (97,1) =    42.085N, 124.358W

The pole point is at     (I,J) =     (49,101)


TABLE 1.  FLAG FOR GDS OR BMS

(PDS Octet 8)

The bit flag indicates the omission or inclusion of the Grid
Description and/or Bit Map Sections.
    BIT    VALUE    MEANING
    1      0    GDS Omitted
          1    GDS Included

    2      0    BMS Omitted
          1    BMS Included

    3-8      0    reserved


TABLE 2.  PARAMETERS & UNITS
Version 1
(PDS Octet 9)

VALUE               PARAMETER                 UNITS


      000     Reserved

      001     Pressure                               Pa
      002     Pressure reduced to MSL                Pa
      003     Pressure tendency                      Pa/s
      004
      005
      006     Geopotential                          m2/s2
      007     Geopotential height                    gpm
      008     Geometric height                       m
      009
      010

      011     Temperature                            deg. K
      012     Virtual temperature                    deg. K
      013     Potential temperature                  deg. K
      014     Pseudo-adiabatic potential temperature deg. K
      015     Maximum temperature                    deg. K
      016     Minimum temperature                    deg. K
      017     Dew point temperature                  deg. K
      018     Dew point depression (or deficit)      deg. K
      019     Lapse rate                           deg. K/m
      020

      021     Radar Spectra (1)                    -
      022     Radar Spectra (2)                    -
      023     Radar Spectra (3)                    -
      024
      025     Temperature anomaly                    deg. K
      026        Pressure anomaly                     Pa
      027        Geopotential height anomaly             gpm
      028     Wave Spectra (1)                      -
      029     Wave Spectra (2)                      -
      030     Wave Spectra (3)                      -

      031     Wind direction                     deg. true
      032     Wind speed                             m/s
      033     u-component of wind                    m/s
      034     v-component of wind                    m/s
      035     Stream function                        m2/s
      036     Velocity potential                     m2/s
      037     Montgomery stream function             m2/s2
      038     Sigma coord. vertical velocity          /s
      039     Pressure Vertical velocity            Pa/s
      040     Geometric Vertical velocity            m/s

      041     Absolute vorticity                     /s
      042     Absolute divergence                    /s
      043     Relative vorticity                     /s
      044     Relative divergence                    /s
      045     Vertical u-component shear             /s
      046     Vertical v-component shear             /s
      047     Direction of current                deg. true
      048     Speed of current                       m/s
      049     u-component of current                 m/s
      050     v-component of current                 m/s

      051     Specific humidity                      kg/kg
      052     Relative humidity                      %
      053     Humidity mixing ratio                  kg/kg
      054     Precipitable water                     kg/m2
      055     Vapor pressure                         Pa
      056     Saturation deficit                     Pa
      057     Evaporation                            kg/m2
      058
      059        Precipitation rate                   kg/m2/s
      060     Thunderstorm probability               %

      061     Total precipitation                   kg/m2
      062     Large scale precipitation             kg/m2
      063     Convective precipitation              kg/m2
      064     Snowfall rate water equivalent        kg/m2s
      065     Water equiv. of accum. snow depth     kg/m2
      066     Snow depth                             m
      067     Mixed layer depth                      m
      068     Transient thermocline depth            m
      069     Main thermocline depth                 m
      070     Main thermocline anomaly               m

      071     Total cloud cover                      %
      072     Convective cloud cover                 %
      073     Low cloud cover                        %
      074     Medium cloud cover                     %
      075     High cloud cover                       %
      076     Cloud water                            kg/m2
      077     Condensation pressure of parcel        Pa
            lifted from indicated surface
      078
      079
      080     Water Temperature                  deg K
      081     Land-sea mask (1=land; 0=sea)         1/0
      082     Deviation of sea level from mean       m
      083     Surface roughness                      m
      084     Albedo                                 %
      085     Soil temperature                      deg. K
      086     Soil moisture content                 kg/m2
      087     Vegetation                             %
      088     Salinity                              kg/kg
      089     Density                               kg/m3
      090

      091     Ice concentration (ice=1; no ice=0)    1/0
      092     Ice thickness                          m
      093     Direction of ice drift              deg. true
      094     Speed of ice drift                     m/s
      095     u-component of ice drift               m/s
      096     v-component of ice drift               m/s
      097     Ice growth rate                        m/s
      098     Ice divergence                         /s
      099
      100     Significant height of combined wind    m
              waves and swell

      101     Direction of wind waves             deg. true
      102     Significant height of wind waves       m
      103     Mean period of wind waves              s
      104     Direction of swell waves            deg. true
      105     Significant height of swell waves      m
      106     Mean period of swell waves             s
      107     Primary wave direction              deg. true
      108     Primary wave mean period               s
      109     Secondary wave direction            deg. true
      110     Secondary wave mean period             s

      111     Net short-wave radiation (surface)      W/m2
      112     Net long wave radiation (surface)       W/m2
      113     Net short-wave radiation (top of atmos.)W/m2
      114     Net long wave radiation (top of atmos.) W/m2
      115     Long wave radiation                    W/m2
      116     Short wave radiation                   W/m2
      117     Global radiation                       W/m2
      118
      119
      120

      121     Latent heat flux                       W/m2
      122     Sensible heat flux                     W/m2
      123     Boundary layer dissipation             W/m2
      124
      125
      126
      127        Image data




    (128 - 254  Reserved for use by originating center)

                NWS/NMC usage as follows...

    128    Mean Sea Level Pressure (MSLSA)    Pa
        (Standard Atmosphere Reduction)
    129    Mean Sea Level Pressure (MSLMA)    Pa
        (MAPS System Reduction)
    130    Mean Sea Level Pressure  (MSLET)    Pa
        (ETA Model Reduction)
    131    Surface lifted index    Deg. K
    132    Best (4 layer) lifted index    Deg. K
    133    K index    Deg. K
    134    Sweat index    Deg. K
    135    Horizontal moisture divergence    kg/kg/s
    136    Vertical speed shear    /s
    137    Visibility    m

    150    Covariance between meridional    m2/s2
        and zonal components of the wind.
        Defined as [uv]-[u][v], where
        "[]" indicates the mean over the
        indicated time span.

    151    Covariance between temperature    K*m/s
        and zonal component of the wind.
        Defined as [uT]-[u][T], where
        "[]" indicates the mean over the
        indicated time span.

    152    Covariance between temperature    K*m/s
        and meridional component of the
        wind.  Defined as [vT]-[v][T],
        where "[]" indicates the mean
        over the indicated time span.

    157    Convective Available Potential Energy    J/kg
    158    Turbulent Kinetic Energy    J/kg

    176    latitude (-90 to +90) (NLAT)    deg
    177    east longitude (0-360)(ELON)    deg

    201    Ice-free water surface    %
    204    downward short wave rad. flux (DSWRF)    W/m**2
    205    downward long wave rad. flux (DLWRF)    W/m**2
    207    Moisture availability    %
    208    Exchange coefficient                (kg/m3)(m/s)
    209    No. of mixed layers next to surface    integer
    211    upward short wave rad. flux (USWRF)    W/m**2
    212    upward long wave rad. flux (ULWRF)    W/m**2
    213    Amount of non-convective cloud    %
    216    Temperature tendency by all radiation    Deg.K/s
    218    precip.index(0.0-1.00)(see note)(PREIX) fraction
    220    Natural log of surface pressure    ln(kPa)
    222    5-wave geopotential height    gpm

    255    Missing


Notes:

1)    By convention, downward fluxes of radiation or
other quantities are assigned negative values;
upward fluxes of radiation or other quantities
are assigned positive values.
2)    The u and v components of vector quantities are
defined with reference to GDS Octet 17 and
Table 7.
3)    provision is made for three types of spectra:

1)    Direction and Frequency
2)    Direction and radial number
3)    Radial number and radial number
4) Precipitation index (#218) defined as the fraction
of satellite observed pixels with temperatures
<235K over 1.0x1.0 box, centered at the
gridpoint.




TABLE 3.  TYPE AND VALUE OF LEVEL
(PDS Octets 10, 11, & 12)

       Octet Number 10            Number 11        Number 12

  VALUE       MEANING                      CONTENTS


|---------------------------+-----------------+----------------|
| 0 - 99  special codes,    |        0        |       0        |
|         see Table 3a      |                 |                |
|---------------------------+-----------------+----------------|
| 100     isobaric level    |  pressure in hectoPascals (hPa)  |
|                           |            (2 octets)            |
|---------------------------+-----------------+----------------|
| 101     layer between two |  pressure of    |  pressure of   |
|         isobaric levels   |  top (kPa)      |  bottom (kPa)  |
|---------------------------+-----------------+----------------|
| 102     mean sea level    |        0        |       0        |
|---------------------------+-----------------+----------------|
| 103     fixed height level|   height above mean sea level    |
|                           |        (MSL) in meters           |
|---------------------------+-----------------+----------------|
| 104     layer between two |  height of top  |  height of     |
|           height levels   |     (hm)        |  bottom (hm)   |
|            above msl      |                 |                |
|---------------------------+-----------------+----------------|
| 105     fixed height above|         height in meters         |
|           ground          |            (2 octets)            |
|---------------------------+-----------------+----------------|
| 106     layer between two |  height of top  |  height of     |
|           height levels   |      (hm)       |  bottom (hm)   |
|           above ground    |                 |                |
|---------------------------+-----------------+----------------|
| 107     sigma level       |     sigma value in 1/10000       |
|                           |            (2 octets)            |
|---------------------------+-----------------+----------------|
| 108     layer between two |  sigma value at |  sigma value   |
|           sigma levels    |       top       |  at bottom     |
|                           |     in 1/100    |   in 1/100     |
|---------------------------+-----------------+----------------|
| 109     Hybrid level      |           level number           |
|                           |            (2 octets)            |
|---------------------------+-----------------+----------------|
| 110     layer between two | level number of | level number   |
|           hybrid levels   |       top       |  of bottom     |
|---------------------------+-----------------+----------------|
| 111     depth below       |            centimeters           |
|         land surface      |             (2 octets)           |
|---------------------------+-----------------+----------------|
| 112     layer between two |  depth of upper | depth of lower |
|         depths below      |    surface (cm) |   surface (cm) |
|         land surface      |                 |                |
|---------------------------+-----------------+----------------|


|---------------------------+-----------------+----------------|
| 113     isentropic        |     Potential Temp. degrees K    |
|         (theta) level     |             (2 octets)           |
|---------------------------+-----------------+----------------|
| 114     layer between two |    475K minus   |  475K minus    |
|         isentropic        |   theta of top  |theta of bottom |
|         levels            |    in Deg. K    |   in deg. K    |
|---------------------------+-----------------+----------------|
| 121     layer between two |  1100 hPa minus | 1100 hPa minus |
|         isobaric surfaces |  pressure of    | pressure of    |
|         (high precision)  |  top, in hPa    | bottom, in hPa |
|---------------------------+-----------------+----------------|
| 128     layer between two |  1.1 minus sigma| 1.1 minus sigma|
|         sigma levels      |  of top, in     | of bottom, in  |
|         (high precision)  |  1/1000 of sigma| 1/1000 of sigma|
|---------------------------+-----------------+----------------|
| 141     layer between two |  pressure of    | 1100hPa minus  |
|         isobaric surfaces |  top, in kPa    | pressure of    |
|         (mixed precision) |                 | bottom, in hPa |
|---------------------------+-----------------+----------------|
| 160     depth below       |               meters             |
|         sea level         |             (2 octets)           |
|---------------------------+-----------------+----------------|

Note: The numbering allows for additions within this framework:

100-119    normal precision
120-139    high precision
140-159    mixed precision



TABLE 3a.   SPECIAL LEVELS
(PDS Octet 10)

VALUE               LEVEL

01                surface (of the Earth, which
            includes sea surface)
02                cloud base level
03                cloud top level
04                0 deg (C) isotherm level
05                adiabatic condensation level
              (lifted from boundary layer)
06                maximum wind speed level
07                tropopause level
08-99             reserved


TABLE 4.  FORECAST TIME UNIT
(PDS Octet 18)


    VALUE            TIME UNIT

    0            minute
    1            hour
    2            day
    3            month
    4            year
    5            decade
    6            normal (30 years)
    7            century
    8-253            reserved
    254            second

TABLE 5.
TIME RANGE INDICATOR
(PDS Octet 21)

         VALUE            MEANING



    0    Forecast product valid
        at reference time + P1 (P1>0),
        or
        Uninitialized analysis product
        for reference time (P1=0).
        or
        Image product
        for reference time (P1=0)

    1    Initialized analysis product
        for reference time (P1=0).

    2    Product with a valid time ranging
            between reference time + P1 and
        reference time + P2

    3    Average
        (reference time + P1 to
        reference time + P2)

    4    Accumulation
        (reference time + P1 to
        reference time + P2)
        product considered valid at
        reference time + P2

    5    Difference
        (reference time + P2 minus
        reference time + P1)
        product considered valid at
        reference time + P2

    6-9    reserved

    10    P1 occupies octets 19 and 20;
        product valid at reference time + P1

    11-112    reserved

    113    Average of N forecasts (or initialized
        analyses); each product has forecast
        period of P1 (P1=0 for initialized
        analyses); products have reference
        times at intervals of P2, beginning at
        the given reference time.

    114    Accumulation of N forecasts (or
        initialized analyses); each product
        has forecast period of P1 (P1=0 for
        initialized    analyses); products have
        reference times at intervals of P2,
        beginning at the given reference time.

    115    Average of N forecasts, all with the
        same reference time; the first has a
        forecast period of P1, the remaining
        forecasts follow at intervals of P2.

    116    Accumulation of N forecasts, all with
        the same reference time; the first has
        a    forecast period of P1, the remaining
        follow at intervals of P2.

    117    Average of N forecasts, the first has
        a period of P1, the subsequent ones
        have forecast periods reduced from the
        previous one by an interval of P2; the
        reference time for the first is given
        in octets 13-17, the subsequent ones
        have reference times increased from
        the previous one by an interval of P2.
        Thus all the forecasts have the same
        valid time, given by the initial
        reference time + P1.

    118    Temporal variance, or covariance, of N
        initialized analyses; each product has
        forecast period P1=0; products have
        reference times at intervals of P2,
        beginning at the given reference time.

    119 -122    reserved

    123    Average of N uninitialized    analyses,
        starting at the reference time,
        at intervals of P2.

    124    Accumulation of N uninitialized
        analyses, starting at the reference
        time, at intervals of P2.


    125-254    reserved


NOTES:

1)    For analysis products, or the first of a series of analysis
products, the reference time (octets 13 to 17) indicates the
valid time.
2)    For forecast products, or the first of a series of forecast
products, the reference time indicates the valid time of the
analysis upon which the (first) forecast is based.
3)    Initialized analysis products are allocated numbers distinct
from those allocated to uninitialized analysis products.
4)    A value of 10 allows the period of a forecast to be extended
over two octets; this is to assist with extended range
forecasts.
5)    Where products or a series of products are averaged or
accumulated, the number involved is to be represented in
octets 22-23 of Section 1, while any number missing is to be
represented in octet 24.
6)    Forecasts of the accumulation or difference of some quantity
(e.g. quantitative precipitation forecasts), indicated by
values of 4 or 5 in octet 21, have a product valid time given
by the reference time + P2; the period of accumulation, or
difference, can be calculated as P2 - P1.


A few examples may help to clarify the use of Table 5:
For analysis products P1 is zero and the time range indicator
is also zero; for initialized products (sometimes called "zero hour
forecasts") P1 is zero, but octet 21 is set to 1.
For forecasts, typically, P1 contains the number of hours of
the forecast (the unit indicator given in octet 18 would be 1) and
octet 21 contains a zero.
Value 115 would be used, typically, for multiple day mean
forecasts, all derived from the same initial conditions.
Value 117 would be used, typically, for Monte Carlo type
calculations: many forecasts valid at the same time from different
initial (reference) times.

Averages, accumulations, and differences get a somewhat
specialized treatment.  If octet 21 (Table 5) has a value between 2
and 5 (inclusive) then the reference time + P1 is the initial
date/time and the reference time + P2 is the final date/time of the
period over which averaging or accumulation takes place.  If,
however, octet 21 has a value of 113, 114, 115, 116, 117, 118, 123,
or 124 then P2 specifies the time interval between each of the
fields (or the forecast initial times) that have been averaged or
accumulated.  These latter values of octet 21 require the
quantities averaged to be equally separated in time; the former
values, 3 and 4 in particular, allow for irregular or unspecified
intervals of time between the fields that are averaged or
accumulated.


SECTION 2: GRID DESCRIPTION SECTION (GDS)

The purpose of the (optional) GDS is to provide a grid
description for grids not defined by number in Table 3.

Octet no.                    GDS Content

 1 - 3  Length in octets of the Grid Description Section
 4    NV, the number of vertical coordinate parameters
 5    PV, the location (octet number) of the list of
    vertical coordinate parameters, if present
    or
    PL, the location (octet number) of the list of
    numbers of points in each row (when no vertical
    parameters are present), if present
    or
    255 (all bits set to 1) if neither are present
 6    Data representation type (See Table 6)
 7 - 32 Grid description, according to data
    representation type, except Lambert or Mercator.
or
 7 - 42    Grid description for Lambert or Mercator grid
PV    List of vertical coordinate parameters
    (length = NV x 4 octets);
    if present, then PL = 4 x NV + PV
PL    List of numbers of points in each row, used for
    quasi-regular grids
    (length = NROWS x 2 octets, where NROWS is the
    total number of rows defined within the grid
    description)

Note:     NV and PV relate to "advanced" features of GRIB not,
at present, in use in the National Weather Service.
See the WMO Manual on Codes<1> for the descriptions
of those features.

PL is used for "quasi-regular" or "thinned" grids;
e.g., a lat/lon grid where the number of points in
each row is reduced as one moves poleward from the
equator.  The reduction usually follows some
mathematical formula involving the cosine of the
latitude, to generate an (approximately) equally
spaced grid array.  The association of the numbers
in octet PL (and following) with the particular row
follows the scanning mode specification in Table 8.


TABLE 6.  TABLES FOR THE GDS DATA REPRESENTATION TYPE
(GDS Octet 6)

     VALUE    MEANING

    0    Latitude/Longitude Grid
        also called Equidistant Cylindrical
        or Plate Carree projection grid
    1    Mercator Projection Grid
    2    Gnomonic Projection Grid
    3    Lambert Conformal, secant or
        tangent, conical or bipolar
        (normal or oblique) Projection Grid
    4    Gaussian Latitude/Longitude Grid
    5    Polar Stereographic Projection Grid

    6 - 12    (reserved - see Manual on Codes)

    13    Oblique Lambert conformal, secant or
        tangent, conical or bipolar,
        projection

    14 - 49    (reserved - see Manual on Codes)

    50    Spherical Harmonic Coefficients

    51 - 89    (reserved - see Manual on Codes)

    90    Space view perspective or
        orthographic grid

    91 - 254    (reserved - see Manual on Codes)





TABLE C. Sundry Grid Definitions




LATITUDE/LONGITUDE GRIDS
INCLUDING GAUSSIAN
(GDS Octets 7 - 32)

    OCTET NO.    CONTENT & MEANING

 7 -  8    Ni - No. of points along a latitude circle

    9 - 10    Nj - No. of points along a longitude meridian

    11 - 13    La1 - latitude of first grid point
        units: millidegrees (degrees x 1000)
        values limited to range 0 - 90,000
        bit 1 (leftmost) set to 1 for south latitude

    14 - 16    Lo1 - longitude of first grid point
        units: millidegrees (degrees x 1000)
        values limited to range 0 - 360,000
        bit 1 (leftmost) set to 1 for west longitude

    17    Resolution and component flags (Table 7)

    18 - 20    La2 - Latitude of last grid point
        (same units, value range, and bit 1 as La1)

    21 - 23    Lo2 - Longitude of last grid point
        (same units, value range, and bit 1 as Lo1)

    24 - 25    Di - Longitudinal Direction Increment
        (same units as Lo1)
        (if not given, all bits set = 1)

     26 - 27    Regular Lat/Lon Grid:
            Dj - Latitudinal Direction Increment
            (same units as La1)
            (if not given, all bits set = 1)
            or
            Gaussian Grid:
            N - number of latitude circles between
                a pole and the equator
            Mandatory if Gaussian Grid specified

     28    Scanning mode flags  (See Table 8)

     29 - 32    Reserved (set to zero)

Note: The latitude and longitude of the first and last grid points
should always be given, for regular grids.



POLAR STEREOGRAPHIC GRIDS
(GDS Octets 7 - 32)

    OCTET NO.    CONTENT & MEANING

     7 -  8    Nx - Number of points along x-axis
     9 - 10    Ny - Number of points along y-axis
    11 - 13    La1 - Latitude of first grid point
    14 - 16    Lo1 - Longitude of first grid point
         17    Resolution and component flags (see Table 7)
    18 - 20    Lov - The orientation of the grid;
        i.e., the east longitude value of the
        meridian which is parallel to the
        y-axis (or columns of the grid) along
        which latitude increases as the
        y-coordinate increases. (Note: The
        orientation longitude may, or may not,
        appear within a particular grid.)
    21 - 23    Dx - the X-direction grid length
              (see Note 2)
    24 - 26    Dy - the Y-direction grid length
              (see note 2)
         27    Projection center flag  (see note 5)
         28    Scanning mode           (see Table 8)
    29 - 32    Set to 0 (reserved)

NOTES:
1.    Latitude and longitude are in millidegrees (thousandths)
2.    Grid lengths are in units of meters, at the 60 degree
latitude circle nearest to the pole in the projection plane.
3.    Latitude values are limited to the range 0 - 90,000.  Bit 1
is set to 1 to indicate south latitude.
4.    Longitude values are limited to the range 0 - 360,000.  Bit
one is set to 1 to indicate west longitude.
5.    Octet 27:
Bit 1 set to 0 if the North pole is on the projection plane.
Bit 1 set to 1 if the South pole is on the projection plane.
6.    The first and last grid points may not necessarily be the
same as the first and last data points if the bit map section
(BMS) is used.
7.    The resolution flag (bit 1 of Table 7) is not applicable.



 LAMBERT CONFORMAL SECANT OR TANGENT CONE GRIDS
(GDS Octets 7 - 42)

    OCTET NO.    CONTENT & MEANING

     7 -  8    Nx - Number of points along x-axis
     9 - 10    Ny - Number of points along y-axis
    11 - 13    La1 - Latitude of first grid point
    14 - 16    Lo1 - Longitude of first grid point
         17    Resolution and component flags (see Table 7)
    18 - 20    Lov - The orientation of the grid;
        i.e., the east longitude value of the
        meridian which is parallel to the
        y-axis (or columns of the grid) along
        which latitude increases as the
        y-coordinate increases. (Note: The
        orientation longitude may, or may not,
        appear within a particular grid.)
    21 - 23    Dx - the X-direction grid length
              (see note 2)
    24 - 26    Dy - the Y-direction grid length
              (see Note 2)
         27    Projection center flag   (see note 5)
         28    Scanning mode            (see Table 8)
    29 - 31    Latin 1 - The first latitude from the pole
        at which the secant cone cuts the spherical
        earth.       (See Note 8)
    32 - 34    Latin 2 - The second latitude from the pole
        at which the secant cone cuts the spherical
        earth.       (See Note 8)
    35 - 37    Latitude of southern pole (millidegrees)
    38 - 40    Longitude of southern pole (millidegrees)
    41 - 42    Reserved (set to 0)

NOTES:
1.    Latitude and longitude are in millidegrees (thousandths)
2.    Grid lengths are in units of meters, at the intersection
latitude circle nearest to the pole in the projection plane.
3.    Latitude values are limited to the range 0 - 90,000.  Bit 1
is set to 1 to indicate south latitude.
4.    Longitude values are limited to the range 0 - 360,000.  Bit
one is set to 1 to indicate west longitude.
5.    Octet 27:
Bit 1 set to 0 if the North pole is on the projection plane.
Bit 1 set to 1 if the South pole is on the projection plane.
Bit 2 set to 0 if only one projection center used
Bit 2 set to 1 if projection is bipolar and symmetric
6.    The first and last grid points may not necessarily be the
same as the first and last data points if the bit map section
(BMS) is used.
7.    The resolution flag (bit 1 of Table 7) is not applicable.
8.    If Latin 1 = Latin 2 then the projection is on a tangent
cone.

MERCATOR GRIDS
(GDS Octets 7 - 42)

    OCTET NO.    CONTENT & MEANING

     7 -  8    Ni - Number of points along a
              latitude circle
     9 - 10    Nj - Number of points along a
              longitude meridian
    11 - 13    La1 - Latitude of first grid point
    14 - 16    Lo1 - Longitude of first grid point
         17    Resolution and component flags (see Table 7)
    18 - 20    La2 - latitude of last grid point
    21 - 23    Lo2 - longitude of last grid point
    24 - 26    Latin - The latitude(s) at which the
                Mercator projection cylinder
                intersects the earth.
         27    Reserved (set to 0)
         28    Scanning mode (see Table 8)
    29 - 31    Di - the longitudinal direction increment
              (see Note 2)
    32 - 34    Dj - the latitudinal direction increment
              (see note 2)
    35 - 42    Reserved (set to 0)

NOTES:
1.    Latitude and longitude are in millidegrees (thousandths)
2.    Grid lengths are in units of meters, at the circle of
latitude specified by Latin.
3.    Latitude values are limited to the range 0 - 90,000.  Bit 1
is set to 1 to indicate south latitude.
4.    Longitude values are limited to the range 0 - 360,000.  Bit
one is set to 1 to indicate west longitude.
5.    The latitude and longitude of the last grid point should
always be given.
6.    The first and last grid points may not necessarily be the
same as the first and last data points if the bit map section
(BMS) is used.



SPHERICAL HARMONIC COEFFICIENTS
(GDS Octets 7 - 32)

    OCTET NO.    CONTENT & MEANING

     7 -  8    J - Pentagonal Resolution Parameter

     9 - 10    K - Pentagonal Resolution Parameter

    11 - 12    M - Pentagonal Resolution Parameter

    13    Representation Type (See Table 9)

    14    Coefficient Storage Mode (See Table 10)

    15 - 32    Set to zero (reserved)




TABLE 7 - RESOLUTION AND COMPONENT FLAGS
(GDS Octet 17)

    Bit    Value    Meaning

    1    0    Direction increments not given
        1    Direction increments given

    2    0    Earth assumed spherical with
            radius = 6367.47 km
        1    Earth assumed oblate spheroid with size
            as determined by IAU in 1965:
            6378.160 km, 6356.775 km, f = 1/297.0

    3-4        reserved (set to 0)

    5    0    u- and v-components of vector quantities
            resolved relative to easterly and
            northerly directions
        1    u and v components of vector quantities
            resolved relative to the defined grid in
            the direction of increasing x and y
            (or i and j) coordinates    respectively

    6-8        reserved (set to 0)




TABLE 8.  SCANNING MODE FLAG
(GDS Octet 28)

    BIT    VALUE            MEANING

    1    0    Points scan in +i direction
        1    Points scan in -i direction

    2    0    Points scan in -j direction
        1    Points scan in +j direction

    3    0    Adjacent points in i direction are consecutive
              (FORTRAN: (I,J))
        1       Adjacent points in j direction are consecutive
              (FORTRAN: (J,I))

Note:     i direction is defined as west to east along a parallel
of latitude, or left to right along an x axis.

j direction is defined as south to north along a
meridian of longitude, or bottom to top along a y axis.


TABLE 9. SPECTRAL REPRESENTATION TYPE
(GDS Octet 13)

        VALUE           MEANING

    1    Associated Legendre Polynomials
        of the First Kind with normalization
        such that the integral equals 1



TABLE 10. COEFFICIENT STORAGE MODE
(GDS Octet 14)

        VALUE             MEANING

          1     The complex coefficients Xnm are stored for
        m > 0 as pairs of real numbers Re(Xnm),
        Im(Xnm) ordered with n increasing from m to
        N(m), first for m = 0 and then for m = 1, 2,
        3,...M.  The real part of the (0,0)
        coefficient is stored in octets 12-15 of the
        BDS, as a floating point number in the same
        manner as the packing reference value, with
        units as in Table 2.  The remaining
        coefficients, starting with the imaginary part
        of the (0,0) coefficient, are packed according
        to the GRIB packing algorithm, with units as
        given in Table 5, in octets 16 and onward in
        the BDS.


NOTES ON SPECTRAL TRUNCATION:

Using the associated Legendre Polynomials of the First Kind,
Pnm, as typical expansion functions, any variable x(*,*), which is
a function of longitude, * , and sin(latitude), * , can be
represented by

In the summations, M is the maximum zonal wave number that is
to be included, and K & J together define the maximum meridional
total wave number N(m), which, it should be noted, is a function
of m.  A sketch shows the relationships:


In this figure, the ordinate is n the zonal wave number, the
abscissa, m, is the total meridional wave number, the vertical
line at m = M is the zonal truncation, and the diagonal passing
through (0,0) is the line n = m.  The Legendre Polynomials are
defined only on or above this line, that is for n > m.  On the n-
axis, the horizontal line at n = K indicates the upper limit to n
values, and the diagonal that intersects the n-axis at n = J
indicates the upper limit of the area in which the Polynomials are
defined.  The shaded irregular pentagon defined by the n-axis, the
diagonal from n = J, the horizontal n = K, the vertical m = M, and
the other diagonal n = m surrounds the region of the (n x m) plane
containing the Legendre Polynomials used in the expansion.


This general pentagonal truncation reduces to some familiar
common truncations as special cases:

Triangular:     K = J = M and N(m) = J
Rhomboidal:     K = J + M and N(m) = J + m
Trapezoidal:    K = J, K > M and N(m) = J

In all of the above m can take on negative values to
represent the imaginary part of the spectral coefficients.

SECTION 3: BIT MAP SECTION (BMS).

The purpose of the (optional) BMS is to provide either a bit
map or a reference to a bit map pre-defined by the center.  The
bit map consists of contiguous bits with a bit-to-data-point
correspondence as defined in the grid description.  A bit set
equal to 1 implies the presence of a datum for that grid point in
the BDS; a value of zero implies the absence of such.

Octet no.

 1 - 3       Length in octets of Bit Map Section
 4           Number of unused bits at end of Section 3.
 5 - 6       Numeric:
             Numeric = 0: a bit map follows;
             otherwise  : the numeric refers to a
             predefined bit map provided
         by the center
 7 - nnn     Bit map, zero filled to an even number of octets


SECTION 4: BINARY DATA SECTION (BDS).

The BDS contains the packed data and the binary scaling
information needed to reconstruct the original data from the
packed data.  The required decimal scale factor is found in the
PDS, above.  The data stream is zero filled to an even number of
octets.

Octet no.

 1 - 3    Length in octets of binary data section
 4    Bits 1 through 4: Flag - See Table 11
    Bits 5 through 8: Number of unused bits at end
    of Section 4.

 5 - 6  The binary scale factor (E).  A negative value
    is indicated by setting the high order bit
    (bit No. 1) in octet 5 to 1 (on).
 7 - 10 Reference value (minimum value); floating
    point representation of the number.
 11    Number of bits into which a datum point is
    packed
 12 -nnn     Variable, depending on octet 4;
    zero filled to an even number of octets.
 14    Optionally, may contain an extension of the
    flags in octet 4.  See Table 11.

Here are some of the various forms the binary data can take;
the flag table in BDS octet 4, possibly extended into octet 14,
identifies which variant is in use.

Grid-point data - Simple packing

Here the data simply begin in octet 12 and continue, packed
according to the algorithm described above, without any particular
regard for computer "word" boundaries, until there is no more
data.  There may be some "zero-fill" bits at the end.

If all the data in a grid point field happen to have the same
value, then all of the deviations from the reference value are set
to zero.  Since a zero value requires no bits for packing, octet
11 is set to zero, thus indicating a field of constant data, the
value of which is given by the reference value.  Under these
circumstances, octet 12 is set to zero (the required "zero fill to
an even number of octets") and bits 5-8 of octet 4 contain an 8.
The number of data points in the field is implied by the grid
identification given in the PDS and/or the GDS and BMS.

Spherical Harmonic Coefficients - Simple packing

Octets 12-15 contain the real part of the (0.0) coefficient
in the same floating point format as the reference value in octets
7-10.  The imaginary part of the (0.0) coefficient,
mathematically, is always equal zero.  Octets 16 to the end
contain the remaining coefficients packed up as binary data with
the same sort of scaling, reference value, and the like, as with
grid-point numbers.  Excluding the (0,0) coefficient, which is
usually much larger than the others, from the packing operation
means that the remaining coefficients can be packed to a given
precision more efficiently (fewer bits per word) than would be the
case otherwise.

Grid-Point Data - Complex Packing

[This section to be completed at a later date.]


 TABLE 11. TABLES FOR THE BDS FLAG
(BDS Octet 4 and, optionally, 14)

    Bit     Value       Meaning


        1    0    Grid point data
        1    Spherical Harmonic Coefficients

    2    0    Simple packing
        1    Second order ("Complex") Packing

     3    0    Original data were floating point values
        1    Original data were integer values

    4    0    No additional flags at octet 14
        1    Octet 14 contains flag bits 5 - 12

The following gives the meaning of the bits in octet 14 ONLY if
bit 4 is set to 1.  Otherwise octet 14 contains regular binary
data.

    5        Reserved (set to 0)
    6    0    Single datum at each grid point
        1    Matrix of values at each grid point

    7    0    No secondary bit maps
        1    Secondary bit maps present

    8    0    Second order values have constant width
        1    Second order values have different widths

    9-12        Reserved (set to 0)

Notes:
(1)    Bit 4 is set to 1 to indicate that bits 5 to 12 are
contained in octet 14 of the data section.
(2)    Bit 3 is set to 1 to indicate that the original data was
integers; when this is the case any non-zero reference
values should be rounded to an integer value.
(3)    When secondary bit maps are present in the data (used in
association with second order packing and, optionally,
with a matrix of values at each point) this is indicated
by setting bit 7 to 1.
(4)    When octet 14 contains the extended flag information
octets 12 and 13 will also contain "special"
information; the actual data will begin in a subsequent
octet.  See above.
At present, the "extension" of Table 11 into octet 14 and the
associated "advanced" features of GRIB are limited to spherical
harmonics and second order("complex") packing in the National
Weather Service.  Additional variations are included in the WMO
Documentation.

SECTION 5: END SECTION

The end section serves a human readable indication of the end
of a GRIB record.  It can also be used for computer verification
that a complete GRIB record is available for data extraction.  It
should not be used as a search target since a '7777' bit
combination could exist anywhere in the binary data stream.

Octet no.

 1-4          '7777' (Coded CCITT-ITA No. 5)












APPENDIX A






OUTLINE OF WMO BULLETIN HEADERS




USED WITH



G  R  I  B


 WMO BULLETIN HEADER

The WMO abbreviated heading is used to identify the NMC GRIB
messages; however, it is not a complete description of their
content.  The user is cautioned against using the header as the
sole determiner of the record content; she should, of course, rely
on the Product Definition Section for that purpose.

Note: In the following, a hexadecimal number is enclosed in
parentheses followed by the designation "hex".
The information needed to identify the NMC product is
contained in 21 octets.  The characters are encoded using the
CCITT-ITA No. 5, also known (in the US) as ASCII characters, and
are defined as follows:

 Octet no.                Header Content

 1      The character 'H' for GRIB bulletins sent to the NWS
    Family of Services, used for the WAFS program, and for
    general International Exchange
    or
    The characters 'Y' or 'Z' for GRIB bulletins intended
    for the NWS AWIPS program.
 2      A letter character specifying the type parameter as
    shown in Table A.1.
 3      A letter character specifying the grid area as defined
    in Table A.2.
 4      A letter or numeric character indicating the time
    difference between the reference time and valid time of
    the data as listed in Table A.3, i.e., the forecast
    length.
 5-6    Numeric characters as defined in Table A.4.  Usually the
    pressure level, sometimes just a sequence number.  Some
    values have special level or layer meanings.
 7      Blank (20)hex
 8-11   Four characters identifying the originating center.
    These are always 'KWBC' for NMC-produced messages.
 12     Blank (20)hex
 13-14  Two numeric characters providing the reference day of
    the month (01-31) of the data.
 15-18  Four numeric characters providing the reference hour and
    minute of the data.
 19-22  Four OPTIONAL characters: one blank (20)hex, then 'Pxx',
    where xx=01-99.  Used to indicate sequential parts of
    very long messages that have been divided using the BLOK
    feature. See Appendix B.
     19-21 OR 23-25  Two ASCII carriage returns and a line feed,
    (0D0D0A)hex.

The first six characters are commonly referred to as
T1 T2 A1 A2 ii

In summary...

Generic Meaning of   T1  T2  A1  A2  ii:

T1:    Type of bulletin:      "H" for GRIB messages for
                Family of Services, WAFS, and
                International Exchange;
                "Y" or "Z" for AWIPS GRIB messages
T2:    Type of data/parameter
A1:    Grid
A2:    Analysis or forecast hour

  ii:    Numeric.  Usually the pressure level, sometimes just a
sequence number.  Some values have special level or
layer meanings.

In the following tables, the columns headed AWIPS are
augmentations to the common Family of Services (FOS),National, and
International Exchange variables.  FOS, National and International
GRIB messages (with H as the initial character) draw upon the left
hand columns only.  AWIPS GRIB messages (with Y or Z as the
initial character) use letters from both columns.  If both columns
contain entries for the same designator, the T1 character (H, Y,
or Z) indicates which entry to use.


TABLE A.1   TYPE PARAMETERS - T2
(Header Octet 2)

 DESIGNATOR                 PARAMETER
                     Usage

       FOS & International (H)            AWIPS (Y or Z)

A
B    Vertical Wind Shear
C    Vorticity
D    Thunderstorm probability
E       Total Precipitation
F       Long Wave Radiation    Precipitable Water
G    Convective Precip.
H       Height (geopotential)
I
J
K       Primary Wave Period
L       Primary Wave Direction
M       Secondary Wave Period
N       Secondary Wave Direction
O       Vertical velocity
P       Pressure
Q    Stability Index
    (Best 4-layer index)
R       Relative humidity
S       Snow
T       Temperature
U       u wind component
V       v wind component
W
X       Surface Lifted index
Y
Z




TABLE A.2   GRID DESIGNATOR - A1
(Header Octet 3)

      DESIGNATOR            GRID Number
               (See Table B)

      FOS & International (H)         AWIPS (Y or Z)

    A    21    201 - Northern Hemisphere
    B    22
    C    23
    D    24
    E    25
    F    26
    G    50
    H        213 - National CONUS with
                  Double Resolution
    I    37    202 - National CONUS
    J    38    203 - National Alaska
    K    39    204 - National Hawaii
    L    40    205 - National Puerto Rico
    M    41    206 - Regional MARD
    N    42    207 - Regional Alaska
    O    43    208 - Regional Hawaii
    P    44    210 - Regional Puerto Rico
    Q        211 - Regional CONUS
    R        212 - Regional CONUS with
                    Double Resolution
    S        209 - Regional MARD with
                    Double Resolution
    T    61    214 - Regional Alaska with
                    Double Resolution
    U    62
    V    63
    W    64
    X        (Used for experimental transmissions)
    Y
    Z



TABLE A.3   FORECAST HOUR DESIGNATOR - A2
(Header Octet 4)

DESIGNATOR       HOUR

           FOS & International (H)

    A       00 hour analysis
    B       06 hour fcst
    C       12     "
    D       18     "
    E       24     "
    F       30     "
    G       36     "
    H       42     "
    I       48     "
    J       60     "
    K       72     "
    L       84     "
    M       96     "
    N      108     "
    O      120     "
    P      132     "
    Q      144     "
    R      156     "
    S      168     "
    T      180     "
    U      192     "
    V      204     "
    W      216     "
    X      228     "
    Y      240     "
    Z      Reserved for special purposes

Note: The following designators are used for AWIPS only, with "Z"
as the first character in the header.

DESIGNATOR    HOUR

    A    2 hour fcst
    B    3    "
    C    4    "
    D    8    "
    E    9    "
    F    10    "
    G    14    "
    H    15    "
    I    16    "
    J    20    "
    K    21    "
    M    54    "
    N    66    "


TABLE A.4   LEVEL DESIGNATORS - ii
(Header Octets 5 and 6)
(H, Y, or Z)

DESIGNATOR        LEVEL or LAYER

00    Entire Atmosphere
99    1000 hPa
98    Air Properties at Surface of Earth
97    Level of the tropopause
96    Level of the maximum wind
95    950 hPa
94    Level of 0 deg C isotherm
93    Land/Water Properties at Surface of
    Earth/Ocean
92    Boundary Layer
91    Any parameter reduced to Sea Level

Note: The following levels are used to indicate
geometric height for aviation flight levels, not
pressure levels

81    810 hPa ~ 1828 m = 6000 ft FL
73    730 hPa ~ 2743 m = 9000 ft FL
64    640 hPa ~ 3658 m = 12000 ft FL


Otherwise, the designator given is the hundreds and tens digits of
the hPa level in the atmosphere, e.g. 70=700hPa; 03=30hPa, etc..


SPECIAL NOTE

The following version of Table A.4 contains changes recently
approved by the WMO.  They are NOT in effect as yet but are
scheduled to go into effect on November 3, 1993.  The table is
presented here for your edification, so you can make proper
preparations, so I won't have to print a new copy of this document
after Nov. 3, and so I can say "I told you so!" when all your
codes fail on Nov. 4, 1993

TABLE A.4   LEVEL DESIGNATORS - ii
(Header Octets 5 and 6)
(H, Y, or Z)

 DESIGNATOR        LEVEL or LAYER

00    Entire Atmosphere
99    1000 hPa
98    Air Properties at Surface of Earth
97    Level of the tropopause
96    Level of the maximum wind
95    950 hPa
94    Level of 0 deg. C isotherm
93    Not assigned
92    925 hPa
91    Not assigned
90    900 hPa
89    Any parameter reduced to Sea Level
88    Land/Water Properties at Surface of
    Earth/Ocean
87    Not assigned
86    Boundary Layer

Note: The following levels are used to indicate
geometric height for aviation flight levels, not
pressure levels

81    810 hPa ~ 1828 m = 6000 ft FL
73    730 hPa ~ 2743 m = 9000 ft FL
64    640 hPa ~ 3658 m = 12000 ft FL


Otherwise, the designator given is the hundreds and tens
digits of the hPa level in the atmosphere, e.g. 70=700hPa;
03=30hPa, etc.












APPENDIX B






The BLOK feature

for subdividing large GRIB (and/or BUFR) bulletins


 BLOK
The BLOK feature is used to subdivide large GRIB bulletins to
allow them to fit into existing (limited) communications or data
storage facilities.  Once divided, the individual subdivisions
(BLOKs) can not stand alone - they must be reassembled back into
the single bulletin whence they came for any processing.  However,
from a communications or storage standpoint, each individual BLOK
is considered as a distinct bulletin or record and will be
uniquely identified.

The application of the BLOK feature is quite straightforward:
a given GRIB record - all the bits from "GRIB" through "7777" -
may be partitioned at any point (without any regard for the
contents) or any number of points.  The only rule is that the
separate parts must contain an even number of octets.  The
separate parts are then encased in a BLOK envelope (details
follow), and that's it.  It is, of course, up to the receiver of a
collection of BLOKs to strip off the BLOK envelopes and put the
GRIB message back together again.  Just get all the king's horses
and all the king's men to help do so.
Here is the structure and content of the BLOK envelope.

    Section 0 - Indicator section

    Octet No.    Contents

    1-4    'BLOK' - 4 ASCII characters
    5-7    Total length of this BLOK, in octets
     8    BLOK Edition Number - currently 0
    9-11    Total length of original GRIB message,
        in octets
    12    Flag - see Table B.1 - indicates presence
        or absence of Section 1
    13    Total number of related BLOKs
    14    Sequence number of this BLOK
    15-16    reserved, set to 0


    Section 1 - Identification section - optional

If present, this section contains the complete Product
Definition Section (PDS) from the original GRIB message.  If
this section is included in one BLOK, it must be included in
all related BLOKs.


    Section 2 - Data section

This section contains an arbitrary part of the original GRIB
message - the only restriction is that this section contain
an even number of octets.


    Section 3 - End section

    Octet No.    Contents

    1-4    '7777' - 4 ASCII characters


Table B.1

Section 1 present Flag

    Bit    Value    Meaning

    1    0    Optional Section 1 omitted
        1    Optional Section 1 present

    2-8        Reserved (set to 0)

Keep in mind that each subdivision of the original bulletin
will be completely surrounded by a BLOK envelope, from 'BLOK'
through '7777'.

Note that the last BLOK of a set for a given bulletin will
have two '7777's in a row; the first is the one that marks the end
of the original bulletin, the second marks the end of the (last)
BLOK.

When BLOKs are sent out as individual messages with
identifying WMO headers, the 'Pxx' characters that are an optional
part of the header shall be included in the header.  See Appendix
A.  In this way the complete WMO header shall be unique for each
message even though the TTAAii portion of the headers will be the
same, reflecting that each BLOK, for a single GRIB message,
contains the same information.

< >    World Meteorological Organization publication No. 306, Manual
on Codes, Vol. 1, Part B, Secretariat of the WMO, Geneva,
Switzerland, 1988, plus Supplements No. 1, 2, & 3