This lesson is being piloted (Beta version)

CMORization: adding new datasets to ESMValTool


Teaching: 15 min
Exercises: 45 min
  • CMORization: what is it and why do we need it?

  • How to use the existing CMORizer scripts shipped with ESMValTool?

  • How to add support for new (observational) datasets?

  • Understand what CMORization is and why it is necessary.

  • Use existing scripts to CMORize your data.

  • Write a new CMORizer script to support additional data.

Data flow with ESMValTool


This episode deals with “CMORization”. ESMValTool is designed to work with data that follow the CMOR standards. Unfortunately, not all datasets follow these standards. In order to use such datasets in ESMValTool we first need to reformat the data. This process is called “CMORization”.

What are the CMOR standards?

The name “CMOR” originates from a tool: the Climate Model Output Rewriter. This tool is used to create “CF-Compliant netCDF files for use in the CMIP projects”. So CMOR extends the CF-standard with additional requirements for the Coupled Model Intercomparison Projects (see e.g. here).

Concretely, the CMOR standards dictate e.g. the variable names and units, coordinate information, how the data should be structured (e.g. 1 variable per file), additional metadata requirements, but also file naming conventions a.k.a. the data reference syntax (DRS). All this information is stored in so-called CMOR tables. As an example, the CMOR tables for the CMIP6 project can be found here.

ESMValTool offers two ways to CMORize data:

  1. A reformatting script can be used to create a CMOR-compliant copy. CMORizer scripts for several popular datasets are included in ESMValTool, and ESMValTool also provides a convenient way to execute them.
  2. ESMValCore can execute CMOR fixes ‘on the fly’. The advantage is that you don’t need to store an additional, reformatted copy of the data. The disadvantage is that these fixes should be implemented inside ESMValCore, which is beyond the scope of this tutorial.

In this lesson, we will re-implement a CMORizer script for the FLUXCOM dataset that contains observations of the Gross Primary Production (GPP), a variable that is important for calculating components of the global carbon cycle. See the next section on how to obtain data.

As in the previous episode (Development and Contribution episode), we will be using the development installation of ESMValTool.

Obtaining the data

The data for this episode is available via the FluxCom Data Portal. First you’ll need to register. After registration, in the dropdown boxes, select FLUXCOM as the data choice and click download. Three files will be displayed. Click the download button on the “FLUXCOM (RS+METEO) Global Land Carbon Fluxes using CRUNCEP climate data”. You’ll receive an email with the FTP address to access the server. Connect to the server, follow the path in your email, and look for the file raw/monthly/ Download that file and save it in a folder called ~/data/RAWOBS/Tier3/FLUXCOM.

Note: you’ll need a user-friendly ftp client. On Linux, ncftp works okay.

What is the deal with those “tiers”?

Many datasets come with access restrictions. In this way the data providers can keep track of how their data is used. In many cases “restricted access” just means that one has to register with an email address and accept the terms of use, which typically ask that you acknowledge the data providers.

There are also datasets available that do not need a registration. The “obs4MIPs” or “ana4MIPs” datasets, for example, are specifically produced to facilitate comparisons with model simulations.

To reflect these different levels of access restriction, the ESMValTool team has created a tier-system. The definition of the different tiers are as follows:

  • Tier1: obs4MIPs and ana4MIPS datasets (can be used directly with the ESMValTool)
  • Tier2: other freely available datasets (most of them will need some kind of cmorization)
  • Tier3: datasets with access restrictions (most of these datasets will also need some kind of cmorization)

These access restrictions are also the reason why the ESMValTool developers cannot distribute copies or automate downloading of all observations and reanalysis data used in the recipes. As a compromise we provide the CMORization scripts so that each user can CMORize their own copy of the access restricted datasets if they need them.

Run the existing CMORizer script

Before we develop our own CMORizer script, let’s first see what happens when we run the existing one. There is a specific command available in the ESMValTool to run the CMORizer scripts:

cmorize_obs -c <config-user.yml> -o <dataset-name>

The config-user.yml is the file in which we define the different data paths, see the episode on Configuration. In the rootpath of your config-user.yml, make sure to add the right directory for “RAWOBS” data in which you downloaded the FLUXCOM dataset:


This enables ESMValTool to find the raw observational datasets stored in the “RAWOBS” folder. The dataset-name needs to be identical to the folder name that was created to store the raw observation data files, i.e. RAWOBS/TierX/dataset-name. In our case this would be “FLUXCOM”.

If everything is okay, the output should look something like this:

... Starting the CMORization Tool at time: 2021-02-26 14:02:16 UTC
... ----------------------------------------------------------------------
... input_dir  = /home/peter/data/RAWOBS
... output_dir = /home/peter/esmvaltool_output/cmorize_obs_20210226_140216
... ----------------------------------------------------------------------
... Running the CMORization scripts.
... Using cmorizer scripts repository: /home/peter/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/esmvaltool/cmorizers/obs
... Processing datasets {'Tier3': ['FLUXCOM']}
... Input data from: /home/peter/data/RAWOBS/Tier3/FLUXCOM
... Output will be written to: /home/peter/esmvaltool_output/cmorize_obs_20210226_140216/Tier3/FLUXCOM
... Reformat script: /home/peter/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/esmvaltool/cmorizers/obs/cmorize_obs_fluxcom
... CMORizing dataset FLUXCOM using Python script /home/peter/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/esmvaltool/cmorizers/obs/
... Found input file '/home/peter/data/RAWOBS/Tier3/FLUXCOM/GPP.ANN.CRUNCEPv6.monthly.*.nc'
... CMORizing variable 'gpp'
... Lmon
... Var is gpp
... ... UserWarning: Ignoring netCDF variable 'GPP' invalid units 'gC m-2 day-1'
... Fixing time...
... Fixing latitude...
... Fixing longitude...
... Flipping dimensional coordinate latitude...
... Saving file
... Converting data type of data from 'float64' to 'float32'
... Saving: /home/peter/esmvaltool_output/cmorize_obs_20210226_140216/Tier3/FLUXCOM/
... Cube has lazy data [lazy is preferred]
... Ending the CMORization Tool at time: 2021-02-26 14:02:16 UTC
... Time for running the CMORization scripts was: 0:00:00.605970

So you can see that several fixes are applied, and the CMORized file is written to the ESMValTool output directory, i.e. ~/esmvaltool_output/cmorize_obs_YYYYMMDD_HHMMSS/TierX/dataset-name/ In order to use it, we’ll have to copy it from the output directory to a folder called ~/data/OBS/Tier3/FLUXCOM and make sure the path to OBS is set correctly in our config-user file:

  OBS: ~/data/OBS

You can also see the path where ESMValTool stores the reformatting script: ~/ESMValTool/esmvaltool/cmorizers/obs/ You may have a look at this file if you want. The script also uses a configuration file: ~/ESMValTool/esmvaltool/cmorizers/obs/cmor_config/FLUXCOM.yml.

Make a test recipe

To verify that the data is correctly CMORized, we will make a simple test recipe. As illustrated in the figure at the top of this episode, one of the steps that ESMValTool executes is a CMOR-check. If the data is not correctly CMORized, ESMValTool will give a warning or error.

Create a test recipe

Create a simple recipe called recipe_check_fluxcom.yml that loads the FLUXCOM data. It should include a datasets section with a single entry for the “FLUXCOM” dataset with the correct dataset keys, and a diagnostics section with two variables: gpp. We don’t need any preprocessors or scripts (set scripts: null), but we have to add a documentation section with a description, authors and maintainer, otherwise the recipe will fail.

Use the following dataset keys:

  • project: OBS
  • dataset: FLUXCOM
  • type: reanaly
  • version: ANN-v1
  • mip: Lmon
  • start_year: 2000
  • end_year: 2000
  • tier: 3

Some of these dataset keys are further explained in the callout boxes in this episode.


Here’s an example recipe


  description: Test recipe for FLUXCOM data
   title: This is a test recipe for the FLUXCOM data.

    - kalverla_peter

    - kalverla_peter

  - {project: OBS, dataset: FLUXCOM, mip: Lmon, tier: 3, start_year: 2000, end_year: 2000, type: reanaly, version: ANN-v1}

    description: Check that ESMValTool can load the cmorized fluxnet data without errors.
    scripts: null

To learn more about writing a recipe, please refer to Writing your own recipe.

Try to run the example recipe with

esmvaltool run recipe_check_fluxcom.yml --log_level debug

If everything is okay, the recipe should run without problems.

Starting from scratch

Now that you’ve seen how to use an existing CMORizer script, let’s think about adding a new one. We will remove the existing CMORizer script, and re-implement it from scratch. This exercise allows us to point out all the details of what’s going on. We’ll also remove the CMORized data that we’ve just created, so our test recipe will not be able to use it anymore.

rm ~/data/OBS/Tier3/FLUXCOM/
rm ~/ESMValTool/esmvaltool/cmorizers/obs/
rm ~/ESMValTool/esmvaltool/cmorizers/obs/cmor_config/FLUXCOM.yml

If you now run the test recipe again it should fail, and somewhere in the output you should find something like:

No input files found for ...
Looking for files matching ['OBS_FLUXCOM_reanaly_ANN-v1_Lmon_gpp[_.]*nc'] in ['/home/peter/data/OBS/Tier3/FLUXCOM']

From this we can see that the first thing our CMORizer should do is to rename the file so that it follows the CMOR filename conventions.

Create a new CMORizer script and a corresponding config file

The first step now is to create a new file in the right folder that will contain our new CMORizer instructions. Create a file called

nano ~/ESMValTool/esmvaltool/cmorizers/obs/

and fill it with the following boilerplate code:

"""ESMValTool CMORizer for FLUXCOM GPP data.

<We will add some useful info here later>
import logging
from . import utilities as utils

logger = logging.getLogger(__name__)

def cmorization(in_dir, out_dir, cfg, _):
    """Cmorize the dataset."""

    # This is where you'll add the cmorization code
    # 1. find the input data
    # 2. apply the necessary fixes
    # 3. store the data with the correct filename

Here, in_dir corresponds to the input directory of the raw files, out_dir to the output directory of final reformatted data set and cfg to a configuration dictionary given by a configuration file that we will get to shortly. When you type the command cmorize_obs in the terminal, ESMValTool will call this function with the settings found in your configuration files.

The ESMValTool CMORizer also needs a dataset configuration file. Create a file called ~/ESMValTool/esmvaltool/cmorizers/obs/cmor_config/FLUXCOM.yml and fill it with the following boilerplate:

# filename: ???

  project_id: OBS6
#   dataset_id: ???
#   version: ???
#   tier: ???
#   modeling_realm: ???
#   source: ???
#   reference: ???
#   comment: ???

# variables:
#   ???:
#     mip: ???

Note: the name of this file must be identical to dataset-name.

As you can see, the configuration file contains information about the original filename of the dataset, and some additional metadata that you might recognize from the CMOR filename structure. It also contains a list of variables that’s available for this dataset. We’ll add this information step by step in the following sections.


In the configuration above we’ve already filled in the project_id. ESMValTool uses these project IDs to find the data on your hard drive, and also to find more information about the data. The RAWOBS and OBS projects refer to external data before and after CMORization, respectively. Historically, most external data were observations, hence the naming.

In going from CMIP5 to CMIP6, the CMOR standards changed a bit. For example, some variables were renamed, which posed a dilemma: should CMORization reformat to the CMIP5 or CMIP6 definition? To solve this, the OBS6 project was created. So OBS6 data follow the CMIP6 standards, and that’s what we’ll use for the new CMORizer.

You can try running the CMORizer at this point, and it should work without errors. However, it doesn’t produce any output yet:

cmorize_obs -c <config-user.yml> -o FLUXCOM

1. Find the input data

First we’ll get the CMORizer script to locate our FLUXCOM data. We can use the information from the in_dir and cfg variables. Add the following snippet to your CMORizer script:

# 1. find the input data"in_dir: '%s'", in_dir)"cfg: '%s'", cfg)

If you run the CMORizer again, it will print out the content of these variables.

Load the data

Try to locate the input data inside the CMORizer script and load it (we’ll use iris because ESMValTool includes helper utilities for iris cubes). Confirm that you’ve loaded the data by logging the correct path and (part of the) file content.


There are many ways to do it. In any case, you should have added the original filename to the configuration file (and un-commented this line): filename: 'GPP.ANN.CRUNCEPv6.monthly.*.nc'. Note the *: this is a useful shorthand to find multiple files for different years. In a similar way we can also look for multiple variables, etc.

Here’s an example solution (inserted directly under the original comment):

# 1. find the input data
filename_pattern = cfg['filename']
matches = Path(in_dir).glob(filename_pattern)

for match in matches:
    input_file = str(match)"found: %s", input_file)
    cube = iris.load_cube(input_file)"content: %s", cube)

To make this work we’ve added import iris and from pathlib import Path at the top of the file. Note that we’ve started a loop, since we may find multiple files if there’s more than one year of data available.

2. Save the data with the correct filename

Before we start adding fixes, we’ll first make sure that our CMORizer can also write output files with the correct name. This will enable us to use the test recipe for the CMOR compatibility check.

We can use the save function from the utils that we imported at the top. The call signature looks like this: utils.save_variables(cube, var, outdir, attrs, **kwargs).

We already have the cube and the outdir. The variable short name (var) and attributes (attrs) are set through the configuration file. So we need to find out what the correct short name and attributes are.

The standard attributes for CMIP variables are defined in the CMIP tables. These tables are differentiated according to the “MIP” they belong to. The tables are a copy of the PCMDI guidelines.

Find the variable “gpp” in a CMOR table

Check the available CMOR tables to find the variable “gpp” with the following characteristics:

  • standard_name: gross_primary_productivity_of_biomass_expressed_as_carbon
  • frequency: mon
  • modeling_realm: land


The variable “gpp” belongs to the land variables. The temporal resolution that we are looking for is “monthly”. This information points to the “Lmon” CMIP table. And indeed, the variable “gpp” can be found in the file here.

If the variable you are interested in is not available in the standard CMOR tables, you could write a custom CMOR table entry for the variable. This, however, is beyond the scope of this tutorial.

Fill the configuration file

Uncomment the following entries in your configuration file and fill them with appropriate values:

  • dataset_id
  • version
  • tier
  • modeling_realm
  • short_name (the ??? immediately under variables)
  • mip


The configuration file now look something like this:

filename: 'GPP.ANN.CRUNCEPv6.monthly.*.nc'

  project_id: OBS6
  dataset_id: FLUXCOM
  version: 'ANN-v1'
  tier: 3
  modeling_realm: reanaly
  source: ''
  reference: ''
  comment: ''

    mip: Lmon

Now that we have set this information correctly in the config file, we can call the save function. Add the following python code to your CMORizer script:

# 3. store the data with the correct filename
attributes = cfg['attributes']
variables = cfg['variables']

for short_name, variable_info in variables.items():
    all_attributes = {**attributes, **variable_info}  # add the mip to the other attributes
    utils.save_variable(cube=cube, var=short_name, outdir=out_dir, attrs=all_attributes)

Since we only have one variable (gpp), the loop is not strictly necessary. However, this makes it possible to add more variables later on.

Was the CMORization successful so far?

If you run the CMORizer again, you should see that it creates an output file named The “xxxx” and “yyyy” represent the start and end year of the data.

Great! So we have produced a NetCDF file with the CMORizer that follows the naming convention for ESMValTool datasets. Let’s have a look at the NetCDF file as it was written with the very basic CMORizer from above.

ncdump -h
netcdf OBS6_FLUXCOM_reanaly_ANN-v1_Lmon_gpp_200001-200012 {
        time = 12 ;
        lat = 360 ;
        lon = 720 ;
        float GPP(time, lat, lon) ;
                GPP:_FillValue = 1.e+20f ;
                GPP:long_name = "GPP" ;
        double time(time) ;
                time:axis = "T" ;
                time:units = "days since 1582-10-15 00:00:00" ;
                time:standard_name = "time" ;
                time:calendar = "gregorian" ;
        double lat(lat) ;
        double lon(lon) ;

// global attributes:
                :_NCProperties = "version=2,netcdf=4.7.4,hdf5=1.10.6" ;
                :created_by = "Fabian Gans [], Ulrich Weber []" ;
                :flux = "GPP" ;
                :forcing = "CRUNCEPv6" ;
                :institution = "MPI-BGC-BGI" ;
                :invalid_units = "gC m-2 day-1" ;
                :method = "Artificial Neural Networks" ;
                :provided_by = "Martin Jung [] on behalf of FLUXCOM team" ;
                :reference = "Jung et al. 2016, Nature; Tramontana et al. 2016, Biogeosciences" ;
                :temporal_resolution = "monthly" ;
                :title = "GPP based on FLUXCOM RS+METEO with CRUNCEPv6 climate " ;
                :version = "v1" ;
                :Conventions = "CF-1.7" ;

The file contains a variable named “GPP” that contains three dimensions: “time”, “lat”, “lon”. Notice the strange time units, and the invalid_units in the global attributes section. Also it seems that there is not information available about the lat and lon coordinates. These are just some of the things we’ll address in the next section.

3. Implementing additional fixes

Copy the output of the CMORizer to your folder ~/data/OBS6/Tier3/ and change the test recipe to look for OBS6 data instead of OBS (note: we’re upgrading the CMORizer to newer standards here!). Make sure the path to OBS6 is set correctly in our config-user file:

  OBS6: ~/data/OBS6

If we now run the test recipe on our newly ‘CMORized’ data,

esmvaltool run recipe_check_fluxcom.yml --log_level debug

it should be able to find the correct file, but it does not succeed yet. The first thing that the ESMValTool CMOR checker brings up is:

iris.exceptions.UnitConversionError: Cannot convert from unknown units. The
"units" attribute may be set directly.

If you look closely at the error messages, you can see that this error concerns the units of the coordinates. ESMValTool tries to fix them automatically, but since no units are defined on the coordinates, this fails.

The cmorizer utilities also include a function called fix_coords, but before we can use it, we’ll also need to make sure the coordinates have the correct standard name. Add the following code to your cmorizer:

# Fix/add coordinate information and metadata
cube.coord('lat').standard_name = 'latitude'
cube.coord('lon').standard_name = 'longitude'

With some additional refactoring, our cmorization function might then look something like this:

def cmorization(in_dir, out_dir, cfg, _):
    """Cmorize the dataset."""

    # Get general information from the config file
    attributes = cfg['attributes']
    variables = cfg['variables']

    for short_name, variable_info in variables.items():"CMORizing variable: %s", short_name)

        # 1a. Find the input data (one file for each year)
        filename_pattern = cfg['filename']
        matches = Path(in_dir).glob(filename_pattern)

        for match in matches:
            # 1b. Load the input data
            input_file = str(match)
  "found: %s", input_file)
            cube = iris.load_cube(input_file)

            # 2. Apply the necessary fixes
            # 2a. Fix/add coordinate information and metadata
            cube.coord('lat').standard_name = 'latitude'
            cube.coord('lon').standard_name = 'longitude'

            # 3. Save the CMORized data
            all_attributes = {**attributes, **variable_info}
            utils.save_variable(cube=cube, var=short_name, outdir=out_dir, attrs=all_attributes)

Have a look at the netCDF file, and confirm that the coordinates now have much more metadata added to them. Then, run the test recipe again with the latest CMORizer output. The next error is:

esmvalcore.cmor.check.CMORCheckError: There were errors in variable GPP:
Variable GPP units unknown can not be converted to kg m-2 s-1 in cube:

Okay, so let’s fix the units of the “GPP” variable in the CMORizer. Remember that you can find the correct units in the CMOR table. Add the following three lines to our CMORizer:

# 2b. Fix gpp units"Changing units for gpp from gc/m2/day to kg/m2/s") = cube.core_data() / (1000 * 86400)
cube.units = 'kg m-2 s-1'

If everything is okay, the test recipe should now pass. We’re getting there. Looking through the output though, there’s still a warning.

WARNING There were warnings in variable GPP:
Standard name for GPP changed from None to gross_primary_productivity_of_biomass_expressed_as_carbon
Long name for GPP changed from GPP to Carbon Mass Flux out of Atmosphere Due to Gross Primary Production on Land [kgC m-2 s-1]

ESMValTool is able to apply automatic fixes here, but if we are running a CMORizer script anyway, we might as well fix it immediately.

Add the following snippet:

# 2c. Fix metadata
cmor_table = cfg['cmor_table']
cmor_info = cmor_table.get_variable(variable_info['mip'], short_name)
utils.fix_var_metadata(cube, cmor_info)

You can see that we’re using the CMOR table here. This was passed on by ESMValTool as part of the CFG input variable. So here we’re making sure that we’re updating the cubes metadata to conform to the CMOR table.

Finally, the test recipe should run without errors or warnings.

4. Finalizing the CMORizer

Once everything works as expected, there’s a couple of things that we can still do.

Fill out the header for the “FLUXCOM” dataset

Fill out the header of the new CMORizer. The different parts that need to be present in the header are the following:

  • Caption: the first line of the docstring should summarize what the script does.
  • Tier
  • Source
  • Last access
  • Download and processing instructions


The header for the “FLUXCOM” dataset could look something like this:

"""ESMValTool CMORizer for FLUXCOM GPP data.

    Tier 3: restricted dataset.


Last access

Download and processing instructions
    From the website, select FLUXCOM as the data choice and click download.
    Two files will be displayed. One for Land Carbon Fluxes and one for
    Land Energy fluxes. The Land Carbon Flux file (RS + METEO) using
    CRUNCEP data file has several data files for different variables.
    The data for GPP generated using the
    Artificial Neural Network Method will be in files with name:
    A registration is required for downloading the data.
    Users in the UK with a CEDA-JASMIN account may request access to the jules
    workspace and access the data.
    Note : This data may require rechunking of the netcdf files.
    This constraint will not exist once iris is updated to
    version 2.3.0 Aug 2019
# 2d. Update the cubes metadata with all info from the config file
utils.set_global_atts(cube, attributes)

Some final comments

Congratulations! You have just added support for a new dataset to ESMValTool! Adding a new CMORizer is definitely already an advanced task when working with the ESMValTool. You need to have a basic understanding of how the ESMValTool works and how it’s internal structure looks like. In addition, you need to have a basic understanding of NetCDF files and a programming language. In our example we used python for the CMORizing script since we advocate for focusing the code development on only a few different programming languages. This helps to maintain the code and to ensure the compatibility of the code with possible fundamental changes to the structure of the ESMValTool and ESMValCore.

More information about adding observations to the ESMValTool can be found in the documentation.

Key Points

  • CMORizers are dataset-specific scripts that can be run once to generate CMOR-compliant data.

  • ESMValTool comes with a set of CMORizers readily available, but you can also add your own.