ACCESS Output Data Specifications
This document provides an overview of the data specifications for data produced by ACCESS models. The initial draft of this specification targets ESM1.6 and will be expanded to other ACCESS models in time. This version of the specification is intended to be relatively lightweight as a first step towards bringing data produced in the ACCESS ecosystem towards a common data and metadata standard.
Included here are file and directory naming conventions, variable conventions, and variable and global attributes. Variable mappings from ACCESS output variables to CMIP6/7 variables can be found here.
More information on the ACCESS models can be found here. Please direct any issues, feedback or queries on the data specification to data.access.nri@anu.edu.au.
Directory and Filename
Directory Structure
The directory structure for ACCESS-ESM1.6 data output is still being finalised. The current draft has the following structure under the current working directory:
<run>/output<xxx>/<realm>/<filename.nc>
File naming
ACCESS-ESM1.6 filenames are also still under development
All information contained in filenames should be present in file metadata attributes.
File content
Output files should be NetCDF4 files wherever practical.
Data variables should be compressed using zlib with deflate level of at least 1 and shuffle enabled — if the compression level used is greater than 1 please consider the benefit of improved compression ratios against cost of increased compression/decompression times.
Where possible files should conform to the CF metadata conventions (version 1.11) and use the CF Convention Standard Name Table.
For ACCESS-ESM1.6 every file should contain a single data variable/field from a single simulation.
Time Dimensions
Time dimensions should use the proleptic Gregorian calendar with units of days since yyyy-mm-dd hh:mm.
Where possible time_bnds should be included as an additional coordinate variable.
Metadata Attributes
Global Attributes
Global attributes provide information on the context for the data such as the creation time, experiment it is part of, or science configurations used.
All the attributes in the table below are recommended but not all are required, see the Required column, and attributes not specified are permitted.
All these global attributes have type string.
Where possible sort the attributes alphabetically by name.
Note that for any given experiment run the combination of model and model_version should identify the code, and experiment_repo and run_id should identify a specific commit in the repository containing the configuration used.
| Title | Description | Examples | Rules | Required |
|---|---|---|---|---|
| base_configuration | Configuration modified for experiment, see configs repo: https://github.com/ACCESS-NRI/access-esm1.6-configs | release-preindustrial+concentrations-2.0 | Yes | |
| contact | Email address or other contact details for who to contact regarding this data. |
|
Yes | |
| Conventions | Convention(s) used with their versions, as a comma separated list | CF-1.11,ACDD-1.3 | Yes | |
| data_specification | Version of this data specification used to generate the data | ACCESS Output Data Specification v2-0-0 XX.XXXX/zenodo.XXXXX | Yes | |
| date_created | Date and time the file was created. Follow ISO 8601, i.e. 'YYYY-MM-DDTHH:MM:SSZ' | 2025-10-07T11:10:00Z | Must match regex: ^\d{4}-(1[012]|0[1-9])-(3[01]|[12][0-9]|0[1-9])T([01][0-9]|2[0-3]):[0-5][0-9]:[0-5][0-9]Z$ | Yes |
| date_metadata_modified | Date and time the metadata for this file was last modified. Note that this applied just to the metadata, not the data. Follow ISO 8601, i.e. 'YYYY-MM-DDTHH:MM:SSZ' | 2025-10-07T11:10:00Z | Must match regex: ^\d{4}-(1[012]|0[1-9])-(3[01]|[12][0-9]|0[1-9])T([01][0-9]|2[0-3]):[0-5][0-9]:[0-5][0-9]Z$ | No |
| date_modified | Date and time the data for this file was last modified. Note that this applied just to the data, not the metadata. Follow ISO 8601, i.e. 'YYYY-MM-DDTHH:MM:SSZ' | 2025-10-07T11:10:00Z | Must match regex: ^\d{4}-(1[012]|0[1-9])-(3[01]|[12][0-9]|0[1-9])T([01][0-9]|2[0-3]):[0-5][0-9]:[0-5][0-9]Z$ | No |
| experiment_repo | Git repository URL that describes the experiment | https://github.com/ACCESS-NRI/access-esm1.6-configs/ | No | |
| experiment_uuid | The experiment UUID generated by Payu. Note: this may be the same as id. | 698E600B-BECF-4CBA-994F-A663A22FCDDF | Yes | |
| frequency | Sampling frequency of the data |
|
Must match one of these regex:
|
Yes |
| grid | Brief description of output grid characteristics or reference to grid specification. Should be included if the grid is not defined by the dimensions in the file. | No | ||
| license | Information on the license for the data to ensure all users have access to the terms of use. Use SPDX license identifiers where possible. The default license for ACCESS-NRI is CC-BY-4.0, users should change as needed. | CC-BY-4.0 | Yes | |
| model | Name of the model used to create the data | ACCESS-ESM1.6 | Yes | |
| model_version | Version of the model used to create the data. Please note here if the model has been modified from an official release, ideally with links to the changes. |
|
Yes | |
| realm | Realm where the data variable is defined |
|
Must be one of the following:
|
Yes |
| run_id | Git hash for the commit associated with the experiment run |
|
Yes | |
| title | Name of the dataset. Typically following the Payu naming scheme detailed here - https://payu.readthedocs.io/en/stable/usage.html#experiment-names | my_expt-perturb-416af8c6 | Yes | |
| variable_id | A list of short variable names, separated by commas, for the data variable/s that appear in this file (e.g. tas for surface temperature but not time/latitude/longitude). These names should match the netCDF variable names. |
|
No |
Variable Attributes
Variable attributes provide information on the data variable such as the units used, standard_name, or cell_methods used to generate the data.
In netCDF files there can be multiple coordinate variables such as time, latitude, or time_bnds and the main output field variable of which there should be only one.
Where possible variables and their attributes should follow CF-v1.11 conventions.
| Title | Description | Type | Examples |
|---|---|---|---|
| cell_methods | A string comprising a list of space-separated pairs, "name:method", which indicate that for axis "name" the values representing the fields have been determined by "method". | string |
|
| long_name | A long descriptive name which may, for example, be used for labelling plots. | string |
|
| standard_name | Where posisble use the CF standard_name of the variable, otherwise use a unique short phrase separated by underscores to describe the variable. | string |
|
| units | The units of measurement for the variable | string |
|
Acknowledgements
We would like to acknowlege the specifications and conventions we have used and consulted to construct this specification and the tremendous work of their authors: