WMO GRIB as a Data Model
Gil Ross, UK Met Office
2009-09-14
WMO GRIB
• Owned by WMO
– maintained by WMO CBS IPET-DR&C
• Grib Edition 0 – 1985
– based on WMO GRID and NOAA H-code
– Binary
– Table driven
• Grib Edition 1 - 1990
– single level, single parameter, single time
– Still used by aviation
• Grib Edition 2 - 1997
– Multi-dimensional, template driven
– Failed to converge with NetCDF and HDF in 1990s
• Used daily, intensively, by WMO, ICAO, IOC since 1994.
WMO GRIB as a Data Model
• GRIB is many things:
– Exchange container,
– Multidimensional data format,
– Reference tables, semantics, controlled vocab.
– Compression mechanism,
– Data model.
GRIB as a Container
• GRIB container has metadata:
– Container metadata
– Identification metadata
• describing the producer, producing mechanism,
• which registry - standard or local/private.
– Temporal metadata (potentially 3 temporal dimensions!)
– Projection grid metadata (potentially multiple
dimensional grids)
– Data description metadata
• Describing the coverage(s)
– Compression metadata
– Local metadata
GRIB as Multidimensional Format
• Data description metadata
– 2 D scalar, or 2 D vector (winds on 2 D grid)
– Tensor data possible on each grid point
– 3rd or more parametric dimension to define coverages
(at multiple values of 3rd dimensional grid)
– Multiple coverages possible in same container
– Missing value masks
• Registry table to define coverage (dependent
variable) is the same as table to define
parametric grid term (independent variable)
• This is a major change to ISO 19123 coverages
WMO GRIB: some semantics (1)
Code
Parameter
.
052 Relative humidity
053 Humidity mixing ratio
054 Precipitable water
055 Vapour pressure
056 Saturation deficit
057 Evaporation
058 Cloud Ice
059 Precipitation rate
060 Thunderstorm probability
061 Total precipitation
062 Large scale precipitation
063 Convective precipitation
064 Snowfall rate water equiv.
065 Water equiv. accum. Snow
066 Snow depth
067 Mixed layer depth
068 Transient thermocline depth
069 Main thermocline depth
070 Main thermocline anomaly
071 Total cloud cover
072 Convective cloud cover
073 Low cloud cover
074 Medium cloud cover
075 High cloud cover
076 Cloud water
.
.
Units
%
kg/kg
kg/m2
Pa
Pa
kg/m2
kg/m2
kg/m2/s
%
kg/m2
kg/m2
kg/m2
kg/m2s
kg/m2
m
m
m
m
m
%
%
%
%
%
kg/m2
Code
Vertical Coordinate
(Units)
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 (parcel lifted from surface)
06 Maximum wind speed level
07 Tropopause level
08 Nominal top of atmosphere
09 Sea bottom
10-99
Reserved
100 Isobaric level (hPa)
101 Layer between two isobaric levels (kPa)
102 Mean sea level
103 Fixed height above MSL (m)
104 Layer between two height levels above MSL
105 Fixed height above ground (m)
106 Layer between two height levels above ground
107 Sigma level, value in 1/10000
108 Layer between two sigma levels
109 Hybrid level (level number)
110 Layer between two hybrid levels
111 Depth below land surface (cm)
112 Layer between two depths below land surface (cm)
113 Isentropic (theta) level (Potential Temperature K)
114 Layer between two isentropic levels
.
.
WMO GRIB: some semantics (2)
Code Projection Grid
Some settings
0 Latitude/Longitude also called Equidistant Cylindrical or
Plate Carrée
1 Mercator
2 Gnomonic
3 Lambert Conformal, secant or tangent, conical or
bipolar (normal or oblique)
4 Gaussian Latitude/Longitude
5 Polar Stereographic
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.
13 Oblique Lambert conformal, secant or tangent,
conical or bipolar
50 Spherical Harmonic Coefficients
90 Space view perspective or orthographic
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
0 no bit map
1 bit map present, one bit per data point of grid
GRIB as Compression Mechanism
•
•
•
•
•
•
•
•
•
Gridded n-dimensional data
2D horizontal geospatial grid originally
CRS/projection defined functionally
Spherical harmonics option
Scaled n-bit integers, difference from minimum
Minimum may be for 2D field or row by row
Option for 2nd order differences
Several scanning patterns
JPEG compression added to GRID Ed 2
GRIB as a data model
•
Temporal metadata
– Start time of forecast
– Forecast lead time
– Start and end lead time of averaging period.
•
Projection grid metadata
– Succinct mathematical definition
– Defining projection and rectangular grid within that projection
– Spherical Harmonic grid of wavenumbers (space is Fourier transformed rather
than the data)
– Grid is referenced externally, as standard or irregular grid, defined by array of
lat/lon points (deprecated)
•
Compression Metadata
–
–
–
–
Either find min and max and defined bit-length to get steps for integer digits
Or define precision of steps, find minimum and derive bit length
Lossy mechanism, but it loses only irrelevant random floating point precision.
Properly used this gives good compression, which e.g. gzip may have difficulty
improving (because of the high floating point precision).
– Attempts at further compression abandoned as unnecessary, e.g. run-length
encoding, boustrophedonic wrapping.
GRIB as data model
• Same registry table for dependent and independent
variable
• On a rectangular lat-lon grid
• temperature = f(height) or height = f(temperature)
– At fixed (independent) heights we can have coverages of (dependent)
temperature
– OR
– At fixed (independent) temperatures we can have coverages of
(dependent) height values
• cloud amount = f(height) or height = f(cloud amount)
– Alternatively, the cloud amount at fixed height
– OR
– The height at which fixed amount (3/8ths) of cloud is reached
• This is not an artificial construct,
– it describes the legal height at which aircraft may fly under Visual Flying
Rules.
GRIB as an ISO model or OGC
O&M package
• GRIB is a WMO operational standard.
• WMO is a Standards body with cross-accreditation with ISO.
• WMO and ISO have agreed to cross-adopt each others’ appropriate
standards
• To be an OGC O&M package
– Need to decode metadata in an O&M wrapper with payload as GRIB
mime-type
– Need to publish WMO registries under ISO Registry mechanism
– GRIB to XML converter straightforward
• particularly metadata
• GRIB data to XML conversion not a normal use case
• More usually GRIB data to IEEE numbers
– Anything more?
• Free Source GRIB decoders available (e.g. NOAA NCEP and
ECMWF)
– However packages are NOT supported to the level of NetCDF
• WMO (or rather the members) needs to implement a GRIB to XML
metadata and to create registries