CRU data user guide
About the CRU data
The University of East Anglia (UEA) archive the Climatic Research Unit (CRU) data with CEDA. The CRU data contains two different products the gridded time-series (TS) data and Year-by-Year Variation of Selected Climate Variables by Country (CY) data.
In 2017 a new version of the CRU data was released, version 4 of the CRU data has an updated methodology compared with version 3. Complimentary versions of version 3 CRU data were released alongside the first two versions of the new version 4 data for comparison. Therefore there exists CRU TS/CY 3.24.01 and 4.00 that both cover the same time period but differ in methodology and similarly CRU TS/CY 3.25 and 4.01. CRU TS and CRU CY 3.25 are the final versions of the CRU version 3 data, no further version 3 data will be released and users should move to using version 4 of the CRU data.
- Downloading the CRU data
- Missing data values
- A step by step guide to downloading data from the WPS
- Geographic Information Systems (GIS)
- Known issues with gzipped files
- Further information
Downloading the CRU data
You can download the CRU data using the various CEDA data download tools as detailed below:
- Using the standard CEDA data download through the web interface, scripted interactions using the OPeNDAP service or ftp.
- Using the CEDA Web Processing Service (WPS)
If you are having difficulty please see the Known Issues section
Missing data values
The CRU TS gridded data provides data over land only, missing values are given over the oceans and seas.
- The ".dat" data files have a missing value of -999
- The ".csv" files downloaded through the web processing service (WPS) have a missing value of -9999.999
- NetCDF files have a missing value of 9.9692e+36
Since there can be many data values per file and that more than half the values are over the oceans or seas then it may appear that the file has no real data, however by locating the geographic region that you are interested in you will be able to find the real data values. If you are using a text file (".dat") or a ".csv" file you may be able to zoom out at which point you will see that the file contains values other than the missing value
Note that a global data file is ordered from -90S to 90N i.e. the data begins over Antarctica, no data are available there so the first rows will only contain missing values.
A step by step guide to downloading data from the WPS
- Go to the CEDA Web Processing Service (WPS)
- Ensure the Climate Research Unit (CRU) TS (time-series) dataset is selected as the dataset (version at the time of writing is CRU TS 4.00).
- Select a variable from the list provided, currently available variables are:
- Near-surface temperature
- Near-surface temperature maximum
- Potential evapotranspiration
- Ground frost frequency
- Cloud cover
- Wet day frequency
- Diurnal temperature range
- Vapour pressure
- Near-surface temperature minimum
- Select a start date time and end date time as YYYY-MM-DDThh:mm:ss format
- Select a boundary box. NOTE this is optional and defaults as the globe
- Select an output format:
- CSV: comma separated value
- NC: NetCDF (documentation)
- Select a yearly or monthly time chunk to separate the files into.
- Press "Submit"
- If required login
- Press "Submit" again
How to read CSV files from the web processing service
The CEDA web processing service (WPS) will have provided a comma separated value (CSV) file in a NASA Ames format, all NASA-Ames files have a header and then the data.
The header section
The header has lines that describe the data in the file (metadata):
Here an example of CRU precipitation data has been downloaded from the CEDA WPS and the header information is as shown below:
(You may wish to expand the second and third columns to see all the metadata.)
The header information can be decoded by using the following as a guide:
So for the example above this would be:
The data section
After the header, the data is displayed as shown in the image below. Each repeating data section starts with the time stamp (usually the number of days since 1900-01-01) and then a large section of latitudes and longitudes. Where a large geographic region (or global region) has been selected the data may look similar to:
where initially it may look like a file of missing values as described in the header:
However, if a month of the yearly files were selected and zoomed out, then the geographic region selected can be seen more clearly:
For the CRU data each value represents one grid box (part of the globe) and a subsection is shown above.
Geographic Information Systems (GIS)
The CRU data are available in the NetCDF format. NetCDF files of the CRU data can be downloaded through the data browser for a given dataset selecting the ".nc" file or by using the Web Processing Service (WPS) and selecting WPS as the output format. The CRU NetCDF files can then be loaded into various GIS platforms such as ArcGIS or QGIS.
Here an example shows how to open a NetCDF file in QGIS, it is assumed that you have a NetCDF file ready for use in QGIS:
- Install the NetCDF browser plugin by selecting "Manage and Install Plugins" under "Plugins" and searching NetCDF browser.
- Click on the NetCDF browser plugin
- Select your previously downloaded NetCDF file and then from the time selection drop-down select the timestep you require frames and click add selection.
- Select a coordinate system to display the data, if you are unsure a reasonable default choice would be WGS 84.
- A map of the data should then be loaded.