Porting a namelist (recipe) or diagnostic to ESMValTool v2.0

This guide summarizes the main steps to be taken in order to port an ESMValTool namelist (now called recipe) and the corresponding diagnostic(s) from v1.0 to v2.0, hereafter also referred as the “old” and the “new version”, respectively. The new ESMValTool version is being developed in the public git branch master. An identical version of this branch is maintained in the private repository as well and kept synchronized on an hourly basis.

In the following, it is assumed that the user has successfully installed ESMValTool v2 and has a rough overview of its structure (see Technical Overview).

Create a github issue

Create an issue in the public repository to keep track of your work and inform other developers. See an example here. Use the following title for the issue: “PORTING <recipe> into v2.0”. Do not forget to assign it to yourself.

Create your own branch

Create your own branch from master for each namelist (recipe) to be ported:

git checkout master
git pull
git checkout -b <recipe>

master contains only v2.0 under the ./esmvaltool/ directory.

Convert xml to yml

In ESMValTool v2.0, the namelist (now recipe) is written in yaml format (Yet Another Markup Language format). It may be useful to activate the yaml syntax highlighting for the editor in use. This improves the readability of the recipe file and facilitates the editing, especially concerning the indentations which are essential in this format (like in python). Instructions can be easily found online, for example for emacs and vim.

A xml2yml converter is available in esmvaltool/utils/xml2yml/, please refer to the corresponding README file for detailed instructions on how to use it.

Once the recipe is converted, a first attempt to run it can be done, possibly starting with a few datasets and one diagnostics and proceed gradually. The recipe file ./esmvaltool/recipes/recipe_perfmetrics_CMIP5.yml can be used as an example, as it covers most of the common cases.

Do not forget to also rewrite the recipe header in a documentation section using the yaml syntax and, if possible, to add themes and realms item to each diagnostic section. All keys and tags used for this part must be defined in ./esmvaltool/config-references.yml. See ./esmvaltool/recipes/recipe_perfmetrics_CMIP5.yml for an example.

Create a copy of the diag script in v2.0

The diagnostic script to be ported goes into the directory ./esmvaltool/diag_script/. It is recommended to get a copy of the very last version of the script to be ported from the version1 branch (either in the public or in the private repository). Just create a local (offline) copy of this file from the repository and add it to ../esmvaltool/diag_script/ as a new file.

Note that (in general) this is not necessary for plot scripts and for the libraries in ./esmvaltool/diag_script/ncl/lib/, which have already been ported. Changes may however still be necessary, especially in the plot scripts which have not yet been fully tested with all diagnostics.

Check and apply renamings

The new ESMValTool version includes a completely revised interface, handling the communication between the python workflow and the (NCL) scripts. This required several variables and functions to be renamed or removed. These chagnes are listed in the following table and have to be applied to the diagnostic code before starting with testing.

Name in v1.0

Name in v2.0

Affected code



all .ncl scripts


diag_script_info@att or config_user_info@att

all .ncl scripts



all scripts



all .ncl scripts



all .ncl scripts



all .ncl scripts



all .ncl scripts



all .ncl scripts



all .ncl scripts



all .ncl scripts

info_output(arg1, arg2, arg3)

log_info(arg1) if arg3=1

all .ncl scripts

info_output(arg1, arg2, arg3)

log_debug(arg1) if arg3>1

all .ncl scripts

verbosity = config_user_info@verbosity

remove this statement

all .ncl scripts

enter_msg(arg1, arg2, arg3)

enter_msg(arg1, arg2)

all .ncl scripts

leave_msg(arg1, arg2, arg3)

leave_msg(arg1, arg2)

all .ncl scripts


appropriate if-else statement

all .ncl scripts


appropriate if-else stsatement

all .ncl scripts



all .ncl scripts

get_output_dir(arg1, arg2)


all .ncl scripts



all .ncl scripts

inlist(arg1, arg2)


all .ncl scripts

load interface_scripts/*.ncl

load $diag_scripts/../interface_scripts/interface.ncl

all .ncl scripts


<varname>_info.ncl in preproc dir

all .ncl scripts


settings.ncl in run_dir and interface_scripts/interface.ncl

all .ncl scripts

load diag_scripts/lib/ncl/

load $diag_scripts/shared/

all .ncl scripts

load plot_scripts/ncl/

load $diag_scripts/shared/plot/

all .ncl scripts

load diag_scripts/lib/ncl/rgb/

load $diag_scripts/shared/plot/rgb/

all .ncl scripts

load diag_scripts/lib/ncl/styles/

load $diag_scripts/shared/plot/styles

all .ncl scripts

load diag_scripts/lib/ncl/misc_function.ncl

load $diag_scripts/shared/plot/misc_function.ncl

all .ncl scripts


lwcre, swcre

some yml recipes



all .ncl scripts



all .ncl scripts



all .ncl scripts

The following changes may also have to be considered:

  • namelists are now called recipes and collected in esmvaltool/recipes;

  • models are now called datasets and all files have been updated accordingly, including NCL functions (see table above);

  • run_dir (previous interface_data), plot_dir, work_dir are now unique to each diagnostic script, so it is no longer necessary to define specific paths in the diagnostic scripts to prevent file collision;

  • input_file_info is now a list of a list of logicals, where each element describes one dataset and one variable. Convenience functions to extract the required elements (e.g., all datasets of a given variable) are provided in esmvaltool/interface_scripts/interface.ncl;

  • the interface functions interface_get_* and get_figure_filename are no longer available: their functionalities can be easily reproduced using the input_file_info and the convenience functions in esmvaltool/interface_scripts/interface.ncl to access the required attributes;

  • there are now only 4 log levels (debug, info, warning, and error) instead of (infinite) numerical values in verbosity

  • diagnostic scripts are now organized in subdirectories in esmvaltool/diag_scripts/: all scripts belonging to the same diagnostics are to be collected in a single subdirectory (see esmvaltool/diag_scripts/perfmetrics/ for example). This applies also to the aux_ scripts, unless they are shared among multiple diagnostics (in this case they go in shared/);

  • the relevant input_file_info items required by a plot routine should be passed as argument to the routine itself;

  • upper case characters have to be avoided in script names, if possible.

As for the recipe, the diagnostic script ./esmvaltool/diag_scripts/perfmetrics/main.ncl can be followed as working example.

Move preprocessing from the diagnostic script to the backend

Many operations previously performed by the diagnostic scripts, are now included in the backend, including level extraction, regridding, masking, and multi-model statistics. If the diagnostics to be ported contains code performing any of such operations, the corresponding code has to be removed from the diagnostic script and the respective backend functionality can be used instead.

The backend operations are fully controlled by the preprocessors section in the recipe. Here, a number of preprocessor sets can be defined, with different options for each of the operations. The sets defined in this section are applied in the diagnostics section to preprocess a given variable.

It is recommended to proceed step by step, porting and testing each operation separately before proceeding with the next one. A useful setting in the user configuration file (config-private.yml) called write_intermediary_cube allows writing out the variable field after each preprocessing step, thus facilitating the comparison with the old version (e.g., after CMORization, level selection, after regridding, etc.). The CMORization step of the new backend exactly corresponds to the operation performed by the old backend (and stored in the climo directory, now called preprec): this is the very first step to be checked, by simply comparing the intermediary file produced by the new backend after CMORization with the output of the old backend in the climo directorsy (see “Testing” below for instructions).

The new backend also performs variable derivation, replacing the calculate function in the variable_defs scripts. If the recipe which is being ported makes use of derived variables, the corresponding calculation must be ported from the ./variable_defs/<variable>.ncl file to ./esmvaltool/preprocessor/_derive.py.

Note that the Python library esmval_lib, containing the ESMValProject class is no longer available in version 2. Most functionalities have been moved to the new preprocessor. If you miss a feature, please open an issue on github [https://github.com/ESMValGroup/ESMValTool/issues].

Move diagnostic- and variable-specific settings to the recipe

In the new version, all settings are centralized in the recipe, completely replacing the diagnostic-specific settings in ./nml/cfg_files/ (passed as diag_script_info to the diagnostic scripts) and the variable-specific settings in variable_defs/<variable>.ncl (passed as variable_info). There is also no distinction anymore between diagnostic- and variable-specific settings: they are collectively defined in the scripts dictionary of each diagnostic in the recipe and passed as diag_script_info attributes by the new ESMValTool interface. Note that the variable_info logical still exists, but it is used to pass variable information as given in the corresponding dictionary of the recipe.

Make sure the diagnostic script writes NetCDF output

Each diagnostic script is required to write the output of the anaylsis in one or more NetCDF files. This is to give the user the possibility to further look into the results, besides the plots, but (most importantly) for tagging purposes when publishing the data in a report and/or on a website.

For each of the plot produced by the diagnostic script a single NetCDF file has to be generated. The variable saved in this file should also contain all the necessary metadata that documents the plot (dataset names, units, statistical methods, etc.). The files have to be saved in the work directory (defined in cfg[‘work_dir’] and config_user_info@work_dir, for the python and NCL diagnostics, respectively).

Test the recipe/diagnostic in the new version

Once complete, the porting of the diagnostic script can be tested. Most of the diagnostic script allows writing the output in a NetCDF file before calling the plotting routine. This output can be used to check whether the results of v1.0 are correctly reproduced. As a reference for v1.0, it is recommended to use the development branch.

There are two methods for comparing NetCDF files: cdo and ncdiff. The first method is applied with the command:

cdo diffv old_output.nc new_output.nc

which will print a log on the stdout, reporting how many records of the file differ and the absolute/relative differences.

The second method produces a NetCDF file (e.g., diff.nc) with the difference between two given files:

ncdiff old_output.nc new_output.nc diff.nc

This file can be opened with ncview to visually inspect the differences.

In general, binary identical results cannot be expected, due to the use of different languages and algorithms in the two versions, especially for complex operations such as regridding. However, difference within machine precision are desirable. At this stage, it is essential to test all datasets in the recipe and not just a subset of them.

It is also recommended to compare the graphical output (this may be necessary if the ported diagnostic does not produce a NetCDF output). For this comparison, the PostScript format is preferable, since it is easy to directly compare two PostScript files with the standard diff command in Linux:

diff old_graphic.ps new_graphic.ps

but it is very unlikely to produce no differences, therefore visual inspection of the output may also be required.

Clean the code

Before submitting a pull request, the code should be cleaned to adhere to the coding standard, which are somehow stricter in v2.0. This check is performed automatically on GitHub (CircleCI and Codacy) when opening a pull request on the public repository. A code-style checker (nclcodestyle) is available in the tool to check NCL scripts and installed alongside the tool itself. When checking NCL code style, the following should be considered in addition to the warning issued by the style checker:

  • two-space instead of four-space indentation is now adopted for NCL as per NCL standard;

  • load statements for NCL standard libraries should be removed: these are automatically loaded since NCL v6.4.0 (see NCL documentation);

  • the description of diagnostic- and variable-specific settings can be moved from the header of the diagnostic script to the recipe, since the settings are now defined there (see above);

  • NCL print and printVarSummary statements must be avoided and replaced by the log_info and log_debug functions;

  • for error and warning statments, the error_msg function can be used, which automatically include an exit statement.

Update the documentation

If necessary, add or update the documentation for your recipes in the corrsponding rst file, which is now in doc\sphinx\source\recipes. Do not forget to also add the documentation file to the list in doc\sphinx\source\annex_c to make sure it actually appears in the documentation.

Open a pull request

Create a pull request on github to merge your branch back to master, provide a short description of what has been done and nominate one or more reviewers.