CMIP6 Data Request
The WGCM Infrastructure Panel (WIP) directly oversees the CMIP6 Data Request, which specifies the variables that should be archived from each of the over 300 CMIP6 experiments (and defines the metadata associated with each variable). All CMIP6 output has been written to NetCDF files, with one variable stored per file. This data conforms to CF Metadata conventions. These conventions define metadata which provide a definitive description of what the data in each variable represents, and the spatial and temporal properties of the data. A paper entitled “Requirements for a global data infrastructure in support of CMIP6” (Balaji et al. 2018) summarises the WIP’s activities, findings and recommendations.
Controlled vocabularies (CVs) play a key role in ensuring uniformity in the description of data sets across all models. A controlled vocabulary is an agreed list of terms definitions used to provide a unique label to a concept. CMIP6 controlled vocabularies cover many different aspects: experiments, variables, realms, models, sub-projects, frequency, resolution and grid labels. These definition and labels for variables, frequency and realms are often adopted by other climate data producers. To see the CVs for CMIP6, see here.
In files containing CMIP model-simulation output, global attributes are used to describe the source of the data, the imposed experiment conditions, the contents of the file, licensing restrictions, and other information useful to those analysing the data. In this file the global attributes are defined that should appear in CMIP6 files (some are required, others optional), along with the so-called CMIP6 “data reference syntax” (DRS).
The CMIP6 data request is defined in Juckes et al. (2020). A web interface has also been developed here. This defines the variables requested from each experiment and the time intervals for which they should be reported. This includes the variables of interest needed for most CMIP6 experiments, as well as more specialised variables only required for a specific MIP. The data request repository provides key documents and pages including:
- Specifications for file names, directory structures, and CMIP6 Data Reference Syntax (DRS)
- Specifications for output file content, structure, and metadata are available here.
- Guidance on grid requirements
- Information on pressure levels requested
- Guidance on time-averaging (with masking)
- Search for variables: here you can search for a variable by the variable name, standard name, long name, units, description, or UID. To view the information about all CMIP6 requested variables, you can search the CMIP6 MIP table here.
- Search for experiments: here you can search through the requested experiments for CMIP6 by their experiment ID, title, MIP or number of total years for the experiment (Ntot). Once an experiment is selected, a list of the MIPs which have requested that experiment (and the requested variables) can be found at the bottom of the page.
- Time and masked area averages in the data request: this page gives additional information on the use of cell methods, and masked means.
Experiments requested in CMIP6 may be organised into two tiers. All experiments in Tier 1 should be completed if a model has participated in a MIP. Tier 2 experiments are optional. Further detail on experiment tiers can be found in Table 2 of Juckes et al. (2020).
Each CMIP6 netCDF file contains an attribute called “further_info_URL” which is the URL of a landing page from which all of the relevant documentation relevant to the data may be accessed. This landing page will describe the ensemble for which the simulation was run and will contain links to documentation for the ensemble’s experiment, the simulation itself and any other simulations that the ensemble may contain.
Earth Science-Documentation (or ES-DOC) is a service which hosts detailed documentation on CMIP data. It provides documentation on CMIP requested experiments, models and MIPs.
To search through the information on ES-DOC click this link. From there you can search by document type:
- MIP: Searching ES-DOC by MIP will bring up a list of all CMIP6 MIPs (including CMIP itself). Selecting the relevant MIP will bring up the an overview of the MIP, a list of its requested experiments, details on key literature for the MIP, and contact details for the responsible points of contact.
- Experiment: Searching ES-DOC by experiment will bring up a list of all requested experiments from across CMIP6. Selecting the relevant experiment will bring up an overview of the experiment (including MIPs which have requested the experiment). Next you will find the relationships of the experiment. These are the experiment’s parent/children experiments and details on any control experiments linked to this one. Further technical information on the ensemble requirements, model configurations and experiment forcing constraints can also be find. Finally you will find details on key literature for the experiment, and contact details for the responsible points of contact.
- Model: Searching ES-DOC by model will bring up a list of some registered CMIP6 models. Selecting the relevant model will bring up detailed technical information on the model configuration. Please note, not all CMIP6 models are currently documented on ES-DOC, and some model documentation is incomplete. If you have additional model specific questions, please contact the modelling centre directly. The contact details for most modelling centres can be found by finding their page here, or within a data file itself.
For practical information on creating CMIP6 documentation, see the ES-DOC how-to-guide.
For further queries, the ES-DOC CMIP6 website contains a wealth of useful information on CMIP6 documentation. If you cannot find the answer you are looking for, we also recommend you check out our CMIP FAQs. If you still have a question please submit it to our FAQs by filling in this form.
A table of all models participating in CMIP6 can be found below (or open in a new tab by clicking here) alongside their associated modelling centre, MIPs to which they intend to and have actually submitted data to, and information about the atmospheric, ocean, land and sea ice models with their respective resolutions. For more detailed information on each model, please check ES-DOC. The contact details for most modelling centres can be found by finding their page here, or within a data file itself.
A table of all experiment IDs from CMIP6 can be found below organised either by MIP or parent activity ID.
CMIP6 experiments grouped by MIPs
Alternatively you can open the table in a new tab by (click here).
CMIP6 experiments grouped by parent activity ID
Alternatively you can open the table in a new tab (click here).
Different climate models use a variety of different grids. Knowledge of the underlying grid a simulation has been calculated on is essential for performing analysis of the dataset. Each CMIP6 NetCDF file will contain the global attribute “grid”. This is used to describe the horizontal grid and regridding procedure. Modelling centres can choose to either provide output on the model’s native grid or regrid to one (or more) of the target grids. To distinguish between output reported on different grids, the global attribute “grid_label” is used. For more details on this, see Note 11 in the CMIP6 global attributes document here. A full list of grid_label options can be found here. The global attribute “nominal_resolution” might also be useful. This provides an indication of the approximate output grid resolution. Appendix 2 of the global attributes document provides more detail on this attribute.
Different MIPs also different requirements for vertical grid reporting. Output can be defined either on the native model levels, or it can be remapped to pressure levels. If data is remapped to pressure levels, there are a number of different pressure axes defined as part of level harmonization in the data request. A table of the different axes can be found here.
See here the CMIP6 Output Grid Guidance provided in the data request.
As with grids, models report data on a variety of time steps. Even across different experiments, the same model might use a range of time steps. The MIP table defines the frequency with which requested variables in an experiment should be reported. Definitions of the frequency options can be found here.
Many variables in CMIP6 are defined as masked means, defined as the mean of a quantity over a portion of the grid cell defined by an area type. For more information on this, see this webpage.