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 major 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

You can download the CRU data using the various CEDA data download tools as detailed below:

  1. Using the standard CEDA data download through the web interface, scripted interactions using the OPeNDAP service or ftp
  2. Using the CEDA Web Processing Service (WPS)
    1. Guide to downloading data from the WPS 
    2. Guide on data formats from the WPS CSV output

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

  1.  Go to the CEDA WPS site and click ‘Sign In’ at the upper right corner of the page. This will take you to a page presenting you with an option to sign in with your CEDA User Account.

  • Note: The first time you do this you may be asked to authorise the CEDA-WPS-UI application to access your account details. Click ‘Authorise’ in the lower right corner of the page. 

2.  Once signed in, your view will change slightly to show your username in the top right.  Next, click the ‘Processes’ tab at the top of the page.

  • The ‘Processes’ tab gives a list of available processes (i.e. a collection of tools around a common purpose – e.g. a dataset) within the CEDA WPS service.

3.  The ‘Processes’ page will look like this; showing a set of processes. Click the ‘Data Subsetter’ process to access the tools needed for the CRU Time Series.

  • Within the ‘Data Subsetter’, click ‘Subset CRU Time Series’ 

4.  Ensure that the Dataset is set to ‘Climate Research Unit (CRU) TS (time-series) datasets 4.04 (4.04 is the available version at the time of writing). Then select a variable from the drop-down list provided. Currently, the available variables are:

  • a.       Precipitation
  • b.       Near-surface temperature
  • c.       Near-surface temperature maximum
  • d.       Potential evapotranspiration
  • e.       Ground frost frequency
  • f.        Cloud cover
  • g.       Wet day frequency
  • h.       Diurnal temperature range
  • i.         Vapour pressure
  • j.         Near-surface temperature minimum

5.  Select a start date/time and an end date/time using the blue Time Period range bar. Drag each end of the bar to the required dates. 

6.  Select a boundary box by adding coordinates to the boxes provided. Alternatively, use the interactive map to draw a boundary box. NOTE this is optional and defaults as the globe.

7. Select an output format using the drop-down option. Options include:

  • NetCDF
  • csv

8. Press ‘Submit’ at the bottom of the page.

9. You will be taken to the ‘Job Monitor’ page. The list shows the status and progress of your jobs. Once a job has finished with success, you can see the results by clicking the ‘Details’ button.

10.  You can download and view the output by clicking the ‘Outputs’ tab. The ‘Inputs’ tab details the form selections made, whilst the ‘Job Log’ tab shows the log for the process as it ran, which may be helpful if there is an issue. Details can also be viewed in XML if desired. 

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:

  1. Install the NetCDF browser plugin by selecting "Manage and Install Plugins" under "Plugins" and searching NetCDF browser.
  2. Click on the NetCDF browser plugin
  3. Select your previously downloaded NetCDF file and then from the time selection drop-down select the timestep you require frames and click add selection.
  4. Select a coordinate system to display the data, if you are unsure a reasonable default choice would be WGS 84.
  5. A map of the data should then be loaded.

Known issues with gzipped files

It has been reported that some web browsers (eg. Chrome) have been unzipping the compressed files (e.g. those with ".dat.gz" or ".nc.gz" extensions) on download, but without renaming the files to remove the ".gz" file extension. Consequently, users may encounter issues with the downloaded file not being recognised by unzipping programmes. To avoid this issue users can manually rename the files to remove the ".gz" suffix.
Due to this for each variable in each version of the  CRU  TS data one unzipped NetCDF file has been provided. It is possible to interact with this file directly or via a script using the  CEDA OPeNDAP service. Simply find the file in the list that is unzipped (i.e. has extension ".nc") on the right hand side you will see a wheel with a button named "subset" click this button and then follow the instructions (or refer to the OPeNDAP documentation) to download a subset of the data that you require. You can download data in either NetCDF or ASCII (text) format.  (Note if you download CRU data through this OPeNDAP service no scaling of any variables have been applied, this is not the case if you download the gzipped text files ".dat.gz" in which case you should refer to the version specific file formats guide.) 

Further information

  • The Climate Research Unit homepage
  • Scientific paper associated with the CRU data
  • Contact CEDA via support@ceda.ac.uk
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