ACCESS GRIB2 Data File Notes

GRIB2 is a WMO standard format for transmission of gridded meteorological data. ACCESS GRIB2 data files have been optimised to be as compatible with the WMO standard as possible and to reduce file-sizes.

To achieve improved compression two steps are performed:

Conservative rounding of fields (e.g. screen-level temperature is rounded to the nearest 2 decimal places).
Compression using lossless JPEG-2000 or PNG methods (both are part of the GRIB2 standard).

If you will be accessing the GRIB2 files multiple times, it is recommended to uncompress the files before using them. This will reduce your CPU load and execution times. This can be done with the ecCodes command "grib_set -s packingType=simple in.grb2 out.grb2".

Conventions and Metadata

Where possible the standard WMO encodings have been used. Local tables have only been used where there is no obvious WMO equivalent. The following GRIB2 references were consulted to arrive at a suitable metadata encoding:

Currently all GRIB2 fields can be uniquely identified by looking at a combination of the GRIB2 metadata fields: 'discipline', 'parameterCategory', 'parameterNumber', 'typeOfStatisticalProcessing', 'typeOfFirstFixedSurface', 'scaleFactorOfFirstFixedSurface', 'scaledValueOfFirstFixedSurface', 'scaleFactorOfLowerLimit', & 'scaledValueOfLowerLimit'. Note that some of these fields are not defined for all GRIB messages; for example 'scaleFactorOfLowerLimit', & 'scaledValueOfLowerLimit' relate only to probabilistic quantities.

ECMWF GRIB_API key	WGRIB2 key	GRIB2 section information	Relevant GRIB2 code table
discipline	discipline (code_table_0.0)	Section 0, Octet 7	Code Table 0
parameterCategory	parmcat	Section 4, Octet 10	Code Tables 4.1
parameterNumber	parm	Section 4, Octet 11	Code Tables 4.2
typeOfStatisticalProcessing	code_table_4.10	Section 4, Octet 47 †	Code Table 4.10
typeOfFirstFixedSurface	code_table_4.5a	Section 4, Octet 23	Code Table 4.5
scaleFactorOfFirstFixedSurface	"-lev option"	Section 4, Octet 24	N/A
scaledValueOfFirstFixedSurface	"-lev option"	Section 4, Octet 25-28 §	N/A
scaleFactorOfLowerLimit	"-prob option"	Section 4, Octet 38 ‡	N/A
scaledValueOfLowerLimit	"-prob option"	Section 4, Octet 39-42 ‡	N/A

† Where applicable. Most fields do not have this key defined. All ACCESS fields where this is relevant have "productDefinitionTemplateNumber = 8 and numberOfTimeRange = 1".
§ Where applicable. For certain "typeOfFirstFixedSurface" values these keys are not defined and hence missing.
‡ Where applicable. Most fields do not have this key defined. All ACCESS fields where this is relevant have "productDefinitionTemplateNumber = 9".

Note also that some software packages designed to query GRIB metadata return the "value" of a level (e.g. "valueOfFirstFixedSurface"). This value can be calculated from "scaledValueOfFirstFixedSurface" & "scaleFactorOfFirstFixedSurface" via the relation:

"valueOfFirstFixedSurface" = "scaledValueOfFirstFixedSurface"*pow(10, -"scaleFactorOfFirstFixedSurface").

The following two tables list relevant identifying metadata attribute values for each ACCESS field. Here "d.c.n." is an abbreviation of 'discipline'.'parameterCategory'.'parameterNumber'. "NC var" is the NetCDF variable name used for the corresponding field. Note also that parameters with a parameterNumber 192 to 254 exist only in BoM local GRIB2 tables. All other parameters are defined as existing fields in the WMO GRIB2 specification.

Single-level Field Identifying Metadata

G2 d.c.n	Statistical Process	Level 1 Type	Level 1 Value	Prob Threshold Value	NC var	Description (units)
0.0.0	0	sfc	1.5	N/A	av_temp_scrn	average screen level air temperature (K)
0.0.0	N/A	sfc	1.5	N/A	temp_scrn	screen level temperature (K)
0.0.0	N/A	sfc	0.0	N/A	sfc_temp	surface temperature (K)
0.0.4	2	sfc	1.5	N/A	tmax_scrn	screen level max temperature (K)
0.0.5	3	sfc	1.5	N/A	tmin_scrn	screen level min temperature (K)
0.0.6	N/A	sfc	1.5	N/A	dewpt_scrn	screen level dewpoint temperature (K)
0.0.10	0	sfc	0.0	N/A	av_lat_hflx	average surface latent heat flux (W m-2)
0.0.10	N/A	sfc	0.0	N/A	lat_hflx	surface latent heat flux (W m-2)
0.0.11	0	sfc	0.0	N/A	av_sens_hflx	average surface sensible heat flux (W m-2)
0.0.11	N/A	sfc	0.0	N/A	sens_hflx	surface sensible heat flux (W m-2)
0.1.0	0	sfc	1.5	N/A	av_qsair_scrn	average screen level specific humidity (kg kg-1)
0.1.0	N/A	sfc	1.5	N/A	qsair_scrn	screen level specific humidity (kg kg-1)
0.1.1	N/A	sfc	1.5	N/A	rh2m	screen level relative humidity w.r.t. water (%)
0.1.3	N/A	sfc	0.0	N/A	precwtr	precipitable water (kg m-2)
0.1.6	1	sfc	0.0	N/A	accum_evap	accumulated evaporation from land soil surface (kg m-2)
0.1.8	1	sfc	0.0	N/A	accum_prcp	accumulated precipitation (kg m-2)
0.1.14	1	sfc	0.0	N/A	accum_conv_snow	accumulated convective snowfall (kg m-2)
0.1.15	1	sfc	0.0	N/A	accum_ls_snow	accumulated large scale snowfall (kg m-2)
0.1.47	1	sfc	0.0	N/A	accum_ls_rain	accumulated large scale rainfall (kg m-2)
0.1.48	1	sfc	0.0	N/A	accum_conv_rain	accumulated convective rainfall (kg m-2)
0.1.49	1	sfc	0.0	N/A	accum_rain	accumulated rainfall (kg m-2)
0.1.50	1	sfc	0.0	N/A	accum_snow	accumulated snowfall (kg m-2)
0.1.206	0	sfc	0.0	N/A	av_evap_sea	average rate of evaporation over open sea (kg m-2 s-1)
0.1.207	0	sfc	0.0	N/A	av_sfc_mois_flx	average total surface moisture flux (kg m-2 s-1)
0.2.2	N/A	sfc	10.0	N/A	u10	10m wind u component (m s-1)
0.2.2	0	sfc	10.0	N/A	av10u	average zonal wind at 10m (m s-1)
0.2.2	N/A	sfc	50.0	N/A	u50r	zonal wind at the 50m rho level (m s-1)
0.2.3	N/A	sfc	10.0	N/A	v10	10m wind v component (m s-1)
0.2.3	0	sfc	10.0	N/A	av10v	average meridional wind at 10m (m s-1)
0.2.3	N/A	sfc	50.0	N/A	v50r	meridional wind at the 50m rho level (m s-1)
0.2.17	0	sfc	0.0	N/A	av_uwnd_strs	average surface zonal wind stress (N m-2)
0.2.17	N/A	sfc	0.0	N/A	uwnd_strs	surface zonal wind stress (N m-2)
0.2.18	0	sfc	0.0	N/A	av_vwnd_strs	average surface meridional wind stress (N m-2)
0.2.18	N/A	sfc	0.0	N/A	vwnd_strs	surface meridional wind stress (N m-2)
0.2.22	N/A	sfc	10.0	N/A	wndgust10m	10m wind gust (m s-1)
0.3.0	N/A	sfc	0.0	N/A	conv_cldbse_pres	convective cloud-base pressure (Pa)
0.3.0	N/A	sfc	0.0	N/A	conv_cldtop_pres	convective cloud-top pressure (Pa)
0.3.0	N/A	sfc	0.0	N/A	sfc_pres	surface pressure (Pa)
0.3.1	0	sfc	0.0	N/A	av_mslp	average mean sea level pressure (Pa)
0.3.1	N/A	sfc	0.0	N/A	mslp	mean sea level pressure (Pa)
0.3.5	N/A	sfc	0.0	N/A	topog	topography height above MSL (gpm)
0.3.18	N/A	sfc	0.0	N/A	abl_ht	planetary boundary layer height (m)
0.4.7	0	sfc	0.0	N/A	av_swsfcdown	average downwards shortwave radiation at the surface (W m-2)
0.4.7	0	sfc	0.0	N/A	av_swirrtop	average incoming shortwave radiation flux (W m-2)
0.4.9	0	sfc	0.0	N/A	av_netswsfc	average net shortwave radiation at surface (W m-2)
0.4.202	0	sfc	0.0	N/A	av_sfc_sw_dif	average surface shortwave diffuse radiation flux (W m-2)
0.4.203	0	sfc	0.0	N/A	av_sfc_sw_dir	average surface shortwave direct radiation flux (W m-2)
0.5.3	0	sfc	0.0	N/A	av_lwsfcdown	average downwards longwave radiation at the surface (W m-2)
0.5.4	0	sfc	0.0	N/A	av_olr	average outgoing longwave radiation (W m-2)
0.5.5	0	sfc	0.0	N/A	av_netlwsfc	average net longwave radiation at surface (W m-2)
0.5.7	N/A	N/A	N/A	N/A	syn_ir_tbb	synthetic infrared satellite brightness temperature (K)
0.5.7	N/A	N/A	N/A	N/A	syn_wv_tbb	synthetic water vapour satellite brightness temperature (K)
0.6.1	0	sfc	0.0	N/A	av_ttl_cld	average total cloud coverage (%)
0.6.1	N/A	sfc	0.0	N/A	ttl_cld	total cloud cover (%)
0.6.3	N/A	sfc	111.0	N/A	low_cld	low cloud cover (%)
0.6.4	N/A	sfc	1949.0	N/A	mid_cld	middle cloud cover (%)
0.6.5	N/A	sfc	5574.0	N/A	hi_cld	high cloud cover (%)
0.6.32	N/A	sfc	1.5	N/A	fog_fraction	fog fraction (Numeric)
0.7.15	2	sfc	2000.0	N/A	max_updraft_helicity	maximum updraft helicity (m2 s-2)
0.7.15	3	sfc	2000.0	N/A	min_updraft_helicity	minimum updraft helicity (m2 s-2)
0.16.4	2	sfc	1000.0	N/A	max_radar_refl_1km	maxmium derived radar reflectivity at 1km AGL (dB)
0.16.5	2	sfc	0.0	N/A	max_maxcol_refl	maximum derived composite radar reflectivity (dB)
0.19.0	N/A	sfc	1.5	N/A	vis_precip	horizontal visibility (incl precip) (m)
0.19.0	N/A	sfc	1.5	5000.0	prob_vis_5km_ppt	probability of horizontal visibility less than 5 km (incl precip) (1)
2.0.0	N/A	sfc	0.0	N/A	lnd_mask	land mask (1)
2.0.1	N/A	sfc	0.0	N/A	sfc_rough_len	surface roughness length for momentum (m)
2.0.3	N/A	sfc	0.0	N/A	soil_mois	soil moisture content in a layer (kg m-2)
2.0.3	N/A	sfc	0.1	N/A	soil_mois	soil moisture content layer 2 (kg m-2)
2.0.3	N/A	sfc	0.35	N/A	soil_mois	soil moisture content layer 3 (kg m-2)
2.0.3	N/A	sfc	1.0	N/A	soil_mois	soil moisture content layer 4 (kg m-2)
2.0.13	N/A	sfc	0.0	N/A	canopy_wtr_cont	canopy water content (kg m-2)
2.0.206	N/A	sfc	0.0	N/A	soil_mois_cont	soil moisture available for transpiration (kg m-2)
2.3.18	N/A	sfc	0.0	N/A	soil_temp	soil temperature in a layer (K)
2.3.18	N/A	sfc	0.1	N/A	soil_temp	soil temperature layer 2 (K)
2.3.18	N/A	sfc	0.35	N/A	soil_temp	soil temperature layer 3 (K)
2.3.18	N/A	sfc	1.0	N/A	soil_temp	soil temperature layer 4 (K)

Multi-level (Model and Pressure) Field Identifying Metadata

G2 d.c.n	Statistical Process	Level 1 Type	Level 1 Value	Prob Threshold Value	NC var	Description (units)
0.0.0	N/A	ml	1.0	N/A	air_temp	temperature (K)
0.0.6	N/A	ml	1.0	N/A	dewpt	dew point (K)
0.1.0	N/A	ml	1.0	N/A	spec_hum	specific humidity (kg kg-1)
0.1.1	N/A	ml	1.0	N/A	relhum	relative humidity w.r.t. water (%)
0.1.83	N/A	ml	1.0	N/A	clwc	cloud specific liquid water content (kg kg-1)
0.1.84	N/A	ml	1.0	N/A	ciwc	cloud specific ice water content (kg kg-1)
0.2.2	N/A	ml	1.0	N/A	zonal_wnd	zonal wind (m s-1)
0.2.3	N/A	ml	1.0	N/A	merid_wnd	meridional wind (m s-1)
0.2.9	N/A	ml	1.0	N/A	vertical_wnd	vertical velocity (m s-1)
0.3.0	N/A	ml	1.0	N/A	pressure	pressure (T/q or theta surface) (Pa)
0.3.5	N/A	ml	1.0	N/A	geop_ht	geopotential height above MSL (gpm)

GRIB2 Local Tables

Not all ACCESS fields maps to predefined WMO GRIB2 fields. In these cases a local table entry has been created.

The Bureau's local table is currently at version "1". It includes the following entries:

G2 d.c.n	Units	G2 tablesVersion	Description
0.1.206	kg m-2 s-1	9	Average rate of evaporation over open sea
0.1.207	kg m-2 s-1	9	Average total surface moisture flux
0.4.202	W m-2	9	Average surface shortwave diffuse radiation flux
0.4.203	W m-2	9	Average surface shortwave direct radiation flux
2.0.206	kg m-2	9	Soil moisture available for transpiration

Zonal and meridional winds at the 50m rho level have a locally defined surface type ("typeOfFirstFixedSurface"):

Fixed surface type	Description
201	Nominal height above ground (m)

In addition APS3 ACCESS data can take on the following "generatingProcessIdentifier" metadata values:

Model name	generatingProcessIdentifier (ID of model)
ACCESS-G	1
ACCESS-GE	2
ACCESS-VT	50
ACCESS-PH	54
ACCESS-BN	58
ACCESS-AD	62
ACCESS-SY	66
ACCESS-DN	70
ACCESS-TC01	80
ACCESS-TC02	81
ACCESS-TC03	82

Field changes introduced in APS3

The APS2 fields "accumulated convective precipitation (kg m-2)" (d.c.n: 0.1.10) and "accumulated large scale precipitation (kg m-2)" (d.c.n: 0.1.9) were labelled incorrectly. These fields actually represent accumulated rainfall (i.e. water precipitation) quantities and now have d.c.n codes of 0.1.48 and 0.1.47 respectively.
The field "summed evaporation over open sea" (d.c.n 0.1.206) from APS2 has been replaced by "average rate of evaporation over open sea (kg m-2 s-1)", assigned to the same d.c.n code but with a different "typeOfStatisticalProcessing" value (0 "Average" instead of "1" Accumulation).
In APS2 the ACCESS-R "roughness length" field (d.c.n 2.0.1) was found to represent a roughness length quantity valid only over the ocean and this variant is now deprecated, replaced with the ACCESS-G "surface roughness length for momentum (m)" valid over both land and sea. No field metadata changes were made however.
The new synthetic satellite fields with D.C.N 0.5.7 have "master" tablesVersion set to 16 (as opposed to tablesVersion of 9 for all other fields). These make use of the WMO "productDefinitionTemplate" number 32 introduced in this master tables version. This template is specifically for fields constituting an "analysis or forecast at a horizontal level or in a horizontal layer at a point in time for simulated (synthetic) satellite data".
Subsurface layer depth metadata values for soil temperature and soil moisture content fields were found to be incorrect in APS2 and have been corrected in APS3 as per the following:

Layer	APS2 listed depth	APS3 (corrected) depth
layer 2	10 to 25 cm	10 to 35 cm
layer 3	25 to 65 cm	35 to 100 cm
layer 4	65 to 200 cm	100 to 300 cm

Known Issues and Troubleshooting

Accumulated Field Metadata

There is a known issue with a GRIB2 metadata attribute that affects all ACCESS fields representing accumulated quantities. This attribute describes the start of the accumulation time interval and is set incorrectly. This can cause some software that parses GRIB2 to misrepresent the length of time over which a quantity was accumulated. Fortunately this piece of metadata in the GRIB2 header is redundant and other metadata attributes (which are set correctly) can be used to deduce the start of the accumulation time interval (and indeed many software packages that read GRIB will get the accumulation periods correct because of this, so not all software is affected by this issue).

The metadata in question is the so-called "forecastTime" key present in section 4 of the GRIB2 header. For accumulated quantities this metadata attribute should represent a "time delta" relative to the "reference time". A choice was made for ACCESS data to set the reference time to the nominal analysis date-time of the model (i.e. the base date/time of the model). As such the correct value of this "forecastTime" attribute should be -3 hours for ACCESS models, or -30 minutes for ACCESS-C models. However current GRIB software can't encode/represent negative values for this attribute (although the current GRIB2 standard does not explicitly disallow negative values in this case). As such "forecastTime" attributes have been set arbitrarily to "0". To calculate the length of the accumulation time period it is recommended that the "lengthOfTimeRange" attribute be used which is also present in the GRIB2 header (section 4) and is set correctly.

To check if you are affected you should keep in mind that all accumulated fields in ACCESS actually represent a quantity accumulated from the start of a so called "4DVAR assimilation window". For ACCESS models this window currently starts 3 hours before the nominal base-time/analysis-time of the model run, except for the ACCESS-C models whose assimilation window starts 30 minutes before the nominal base-time/analysis-time of the model run. Hence the +24 hour forecast of accumulated precipitation, for example, actually represents an accumulation of all analysed/forecast precipitation from -3 hours to +24 hours (where these hours are of course relative to the nominal base-time/analysis-time of the model run). If your GRIB software suggests such a field is accumulated from +0 to +24 hours then it is affected by this metadata issue.

Compressed Bitmap Messages

Currently bitmap messages are compressed using JPEG-2000 or PNG methods to reduce file sizes. Some GRIB software is not able to interpret these fields as it is not advised by the WMO to compress bitmap messages. If you have this problem, you can decompress the file, e.g. with the ecCodes grib_set utility: "grib_set -s packingType=grid_simple in.grb2 out.grb2".

Older GRIB Software

Currently ACCESS GRIB2 data is based on a GRIB2 "tablesVersion" ("master_table") version = 9 (except for the new synthetic satellite fields as noted above). These versions may be too recent for some GRIB2 parsing libraries with the effect that such software can not automatically interpret the meaning of GRIB2 metadata with these version numbers. For WGRIB2 a flag can be passed to attempt to interpret the data with an older master table version. For example the flag "-set master_table 8" can be passed in to allow the software to use master_table version 8 to interpret the metadata (e.g. "wgrib2 IDY25300.APS1.wind.10m.2013031618.024.surface.grb2 -d 1 -v -set master_table 8"). For ACCESS data, with some exceptions, version 9 can be regarded as backward compatible with 8 and possibly earlier revisions. For ecCodes (which replaces GRIB_API) it is best to download the latest stable version which can handle recent tablesVersion editions. Alternatively the ecCodes utility grib_set can be used to reset the tablesVersion to an earlier version, e.g. "grib_set -s tablesVersion=8 in.grb2 out.grb2". This may allow earlier versions of ecCodes/GRIB_API to display the meaning of various key code values with commands like "grib_dump -O out.grb2".

Other Notes

For native (hybrid-height) level data, a GRIB-2 message will include a list of vertical coordinate parameters ("PV") values corresponding to "A" and "B" values for either "rho" or "theta" levels. Level parameters 1..70 (for ACCESS-G) and 1..80 (for ACCESS-C) are the A(k) values and level parameters 71..140 (for ACCESS-G) and 81-160 (for ACCESS-C) are the B(k) values listed in the Hybrid Height Levels table (Appendix B) of the relevant product's documentation.