IDY25401: Pressure (500) "pop-flds" fields

A number of raw and post-processed NWP products are available as outputs from the Australian Community Climate Earth-System Simulator (ACCESS) suite of Numerical Weather Prediction models which are run routinely by the Bureau of Meteorology's National Meteorological & Oceanographic Centre (NMOC). This document describes the content of the ACCESS-C SYDNEY (version APS0) IDY25401.pop-flds.500.YYYYMMDDHH.HHH.pressure series of products.

An overview of the scientific and technical details of the ACCESS models can be found in NMOC's Operations Bulletin 83

If you require further information or assistance about ACCESS NWP products contact the Bureau's Registered User Services section.

Model summary

ACCESS-C SYDNEY covers the Sydney domain, has a resolution of about 5 km and is run twice per day (00Z and 12Z basetimes) out to a forecast hour of +36. This model is nested in the ACCESS-R model and consists of only a forecast component.

NWP data filename convention

All NWP files in this product series have names that conform to the following convention:

IDY25401.pop-flds.500.YYYYMMDDHH.HHH.pressure.grb or


YYYYMMDDHHBase-date of model run in UTC time. YYYY = year, MM = month, DD = day, HH = hour
HHHForecast hour. A FHR of 000 is the analysis time-step.

Horizontal grid geometries

[Lon x Lat]
Domain limits
[W to E, N to S (Lon x Lat grid points)]
GRIB parameters
0.050 by 0.050 147.000 to 154.950, -30.050 to -38.000 (160 x 160) All

Please note that

  • Latitude values in GRIB and NetCDF files are ordered North to South while longitude values are ordered West to East.
  • Longitudes always lie in the range 0.0 to 360.0 degrees. Longitudes can be transformed to lie between -180 and +180 degrees by subtracting 360 degrees from any longitude values greater than 180.

Vertical levels

Pressure-levels (hPa):

Time steps

Forecast hours of multi-level fields:
0 to 36 by 1


Multi-level fields

Par NetCDF var Steps Units Description
017.228 dewpt All K Dew point
156.128 geop_ht All m Geopotential height
132.128 merid_wnd All m s-1 Meridional wind
130.128 air_temp All K Temperature
134.228 vertical_wnd All m s-1 Vertical velocity
131.128 zonal_wnd All m s-1 Zonal wind

Explanation of steps column terms

AllParameter available at all time-steps
fhr...Parameter available from fhr onward

Appendix A - Notes on specific NWP file-formats


ACCESS data in GRIB-1 format is defined relative to the Bureau's local GRIB tables. A copy of these tables in plain-text format is available for download from the ACCESS NWP web data interface.

Local extensions are also present in the GRIB-1 message headers. The following tables summarise these.

Section 1 octet Value Remarks
29-30 Stash ID 16-bit UM STASH ID.
31 4V offset Number of hours from start of 4V assimilation window to nominal analysis time.
19 P1 period of time Be aware that currently this is sometimes being set incorrectly for averaged, accumulated and max/min quantities (at certain timesteps only).
APS0 Model name GeneratingProcessIdentifier (ID of model)


  • ACCESS NetCDF files aim to be broadly compliant with CF 1.4 conventions (
  • NetCDF data is of the "classic format" variant (compatible with the unidata NetCDF library version 3.6.x+)
  • Please ignore the accum_value metadata value. This parameter is not always set correctly for non-instantaneous fields.
  • Longitudes in the NetCDF files lie in the range [0.0 degrees, 360.0 degrees]. The NetCDF files somewhat misleadingly describe the valid range attribute for this dimension as -180 to +360.

Appendix B - Extended Field Descriptions

Extended description of multi-level fields

Par NetCDF var Detailed description
017.228 dewpt Dewpoint temperature. Calculated by (iteratively) finding the temperature at which the partial pressure due to water vapour equals the saturated water vapour partial pressure where the saturation partial pressure of water vapour represents the saturation vapour pressure over liquid water for temperatures above the triple point, over ice for temperatures 20 degrees below the triple point, and a linear combination of the two for temperatures in-between. [K]
156.128 geop_ht Geopotential height of level (m above the geoid). [m]
132.128 merid_wnd Meridional (V) component of the wind velocity. [m s-1]
130.128 air_temp Air temperature. [K]
134.228 vertical_wnd Vertical (W) component of the wind velocity. [m s-1]
131.128 zonal_wnd Zonal (U) component of the wind velocity. [m s-1]

Please note:

  • Fields in the above tables are listed in the same order as shown in the Parameters section of this document.
  • The nature of fields can not always be deduced easily from their NetCDF variable names. It is always safest to refer to the detailed description of a parameter.
  • The geoid surface is at approximately mean sea level.