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ii U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION NATIONAL WEATHER SERVICE NATIONAL METEOROLOGICAL CENTER G R I B Edition 1 (WMO Code FM 92-VIII Ext.) 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) March 13, 1992 REVISION HISTORY March 15, 1991 Original Publication of GRIB Edition 1 June 3, 1991 p.6: Correct originating center for Montreal to 53 p.7: Add generating process code numbers for SSI analysis p.9: Add grid no. 103 to list of NMC storage grids (Table B) p.13: Move MARD grid 206 so that the lat/lon point (35N,95W) falls exactly on grid point (30,16) p.14: Move MARD grid 209 so that the lat/lon point (35N,95W) falls exactly on grid point (59,31) p.15: Replace regional West CONUS grid with regional full CONUS grid, as no. 211 p.16: Replace regional East CONUS grid with regional full CONUS grid, double resolution, as no. 212 p.21: additional locally defined parameters pp.A.1 & A.2: Add optional characters to WMO header for multi-part messages. p.A.4: Northern Hemisphere P.S. grid added; Regional CONUS replaces east & west CONUS Appendix B: All new: Description of BLOK. October 11, 1991 p.6: added OPC Fog forecasting model as no.73 p.7&8: added ETA model p.9&10: added NMC "Super C" grids 104 & 105 p.21: added parameter 136 vertical speed shear p.35: corrected bit 2 scanning direction March 13, 1992 p.2: corrected footnote p.5: clarified "century" p.16 corrected pole and corner points p.21: added 137 visibility deleted 202 (use 57) evaporation supplied units for 207 & 213 p.30 corrected content of octets 24-27 Then totally reprinted with new pagination. 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<1>. Some additional tables not found in the WMO Manual are indicated by letters. 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 Section 0); 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. TABLES FOR THE PDS TABLE 0. ORIGINATING CENTER (PDS Octet 5) VALUE CENTER 07 US Weather Service - National Met. Center 34 Japanese Meteorological Agency - Tokyo 53 Canadian Meteorological Service - Montreal 58 US Navy - Fleet Numerical Oceanography Center 74 U.K. Met Office - Bracknell 98 European Center for Medium-Range Weather Forecasts - Reading TABLE A. Generating Process or Model (PDS Octet 6) VALUE MODEL 10 Significant 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 73 Fog Forecast model - Ocean Prod. Center 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 81 Spectral Statistical Interpolation (SSI) analysis from "Aviation" run. 82 Spectral Statistical Interpolation (SSI) analysis from "Final" run. 83 ETA mesoscale forecast model TABLE B. GRID IDENTIFICATION (PDS Octet 7) INTERNATIONAL EXCHANGE 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 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 TABLE B cont. NMC STORAGE GRIDS VALUE GRID GRID INCREMENT ----------------------------------------------------------------- 5 3021-point (53x57) N. Hemisphere polar 190.5 km stereographic grid oriented 105W; Pole at 60N at (27,49). 6 2385-point (53x45) N. Hemisphere polar 190.5 km stereographic grid oriented 105W; Pole at 60N at (27,49). 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). 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) 68 104 A 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) 69 105 A 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) 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. TABLE B, cont. 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 = 79 Nj = 71 La1 = 29.263S Lo1 = 129.470E Res.& Comp. flag = 1 0 0 0 0 0 0 0 La2 = 60.547N Lo2 = 111.096W 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) = 29.263S, 129.470E (1,71) = 60.547N, 129.470E (79,71) = 60.547N, 111.096W (79,1) = 29.263S, 111.096W The longitudinal grid spacing is 1.706 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 = 25 Nj = 25 La1 = 10.656N Lo1 = 193.781E = 166.219W Res.& Comp. flag = 1 0 0 0 0 0 0 0 La2 = 27.917N Lo2 = 147.844W 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) = 10.656N, 166.219W (1,25) = 27.917N, 166.219W (25,25) = 27.917N, 147.844W (25,1) = 10.656N, 147.844W The longitudinal grid spacing is 0.766 degrees ----------------------------------------------------------------- 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 80.000 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 037 038 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 077 078 079 080 081 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 m 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 shortwave radiation (surface) W/m2 112 Net longwave radiation (surface) W/m2 113 Net shortwave radiation (top of atmos.)W/m2 114 Net longwave 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 usage as follows... 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 201 Ice-free water surface % 207 Moisture availability % 208 Exchange coefficient (kg/m3)(m/s) 209 No. of mixed layers next to surface integer 213 Amount of non-convective cloud % 216 Temperature tendency by all radiation Deg.K/s 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 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 | |---------------------------+-----------------+----------------| | 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 | | | |---------------------------+-----------------+----------------| | 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 top, 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 isotherm level 05 adiabatic condensation level 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 -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, 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 - 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. 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, PV, and PL 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. TABLES FOR THE GDS TABLE 6. 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 Descriptions LATITUDE/LONGITUDE GRIDS (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 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 (west to east) (same units as Lo1) (if not given, all bits set = 1) 26 - 27 Regular Lat/Lon Grid: Dj - Latitudinal Direction Increment (south to north) (same units as La1) (if not given, all bits set = 1) 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 is m 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 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 data point is packed 12 - Variable, depending on octet 4; zero filled to an even number of octets. NOTE: 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 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. The same situation may occur if all of the spectral coefficients, other than the (0,0) coefficient, have the same value. In this case octet 16 is set to zero to accomplish the zero fill to an even number of octets. TABLES FOR THE BDS TABLE 11. FLAG (BDS Octet 4) Bit Value Meaning 1 0 Grid point data 1 Spherical Harmonic Coefficients 2 0 Simple packing 1 Second order 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 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. At present, the "extension" of Table 11 into octet 14 and the associated "advanced" features of GRIB is not employed in the National Weather Service. Thus binary data always begins in octet 12. 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 and for International Exchange or The character 'Y' 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 and International Exchange; "Y" 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) and International Exchange variables. FOS and international GRIB messages (with H as the initial character) draw upon the left hand columns only. AWIPS GRIB messages (with Y as the initial character) use letters from both columns. If both columns contain entries for the same designator, the T1 character (H or Y) indicates which entry to use. TABLE A.1 TYPE PARAMETERS - T2 (Header Octet 2) DESIGNATOR PARAMETER Usage FOS & International AWIPS A B Vertical Wind Shear C Vorticity D Thunderstorm probability E Total Precipitation F 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 AWIPS 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 202 - National CONUS J 203 - National Alaska K 204 - National Hawaii L 205 - National Puerto Rico M 206 - Regional MARD N 207 - Regional Alaska O 208 - Regional Hawaii P 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 Usage FOS & International AWIPS (additional) 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 0 hour (initialized) 0 (zero) 2 hour fcst 1 4 " 2 8 " 3 10 " 4 14 " 5 16 " 6 20 " 7 22 " 8 54 " 9 66 " TABLE A.4 LEVEL DESIGNATORS - ii (Header Octets 5 and 6) DESIGNATOR LEVEL or LAYER 99 1000 hPa 00 Entire Atmosphere 98 Air Properties at Surface of Earth 97 Level of the tropopause 96 Level of the maximum wind 94 Level of 0 deg C isotherm 93 Land/Water Properties at Surface of Earth/Ocean 92 Boundary Layer 91 Reduced to Sea Level 18 1828 m level (= 6000 ft) 27 2743 m level (= 9000 ft) 37 3658 m level (= 12000 ft) 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 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. _______________________________ <1> 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