Beginner’s Guide to ESGF

This guide is targetted at users who are new to obtaining CMIP data from ESGF. While many people work hard to provide the community access in an intuitive fashion, ESGF remains a data source for researchers who have some prior understanding about the data they wish to find and how they are organized. This tutorial is meant to gently expose the uninitiated to key concepts and step you through your first searches using intake-esgf.

Which Variable Do We Need?¶

At the highest level, ESGF stores data in projects such as CMIP5 and CMIP6. While there are some similarities between projects, the control vocabulary, that is the metadata used to identify unique datasets, varies. In this tutorial we will explain some of the CMIP6 vocabulary, which is the default project for intake-esgf.

Perhaps the most important search criteria to determine is the name of the variable you wish to use. intake-esgf has some functionality to assist. First, import and instantiate the catalog.

from intake_esgf import ESGFCatalog
cat = ESGFCatalog()

Then you can use the catalog to perform a free text search for any word that may be related to the variable for which you are searching. In this case, we will search for air temperature surface.

cat.variable_info("air temperature surface")

This function returns a pandas dataframe which lists the name of several variables along with their units and standard names. From a perusal of this list, it appears that tas is the variable we want for this search. The dataframe index also shows us that the name of the control vocabulary is variable_id.

Control Vocabulary¶

While we could now perform a search for variable_id=tas, this search will take quite some time. intake-esgf currently works better if we give it a better idea of what we wish to find. Simply put, we recommend constraining the search.

One of the more useful search facets is the experiment_id, a unique identifier corresponding to the experiment. As part of the planning phase of the CMIP process, groups of researchers write papers detailing the specific method that a model is to be run to be included in an experiment. This allows modeling centers to follow the protocol if they wish to be part of the experiment. You can browse the experiments to see the indentifiers and some basic information.

One commonly used experiment is historical, where models are run using reconstructions of the historical earth state from 1850 until 2015. We will use this in our example search.

cat.search(variable_id="tas",experiment_id="historical")

cat

This will populate an underlying pandas dataframe with the search results. The columns of that dataframe and unique values are presented . This exposes more of the control vocabulary for CMIP6. We have already explored variable_id and experiment_id. Now we explain more of the control vocabulary emphasizing what we find to be the more useful facets.

• source_id - The identifier of the model. We use the term source instead of model in an attempt to make the control vocabulary more general and in the future unify vocabularies among projects. Each model or model version will have a unique string identifying which model and/or configuration was run, which can be browsed.

• member_id - The label for the variant of the model run (also known as variant_label). The precise meaning of these labels is specific to each model group. For CMIP6 these take the form r...i...p...f... where integers after each character reflect a separate run. Usually (but not with all models) the main result will be r1i1p1f1.

  • r stands for the realization. Models can be run with small pertubations of the initial conditions to produce an ensemble. Model runs with the same r number started with the same initial conditions.

  • i stands for the initialization. Models use different methods to spin up their states into quasi-equilibrium. This integer reflects the method that was used by the model.

  • p stands for the physics. Modern models have many configuration options and while most submit results in a single configuration, this designation provides a method to distinguish among them if desired.

  • f stands for the forcing. When multiple methods for forcing an experiment are possible, this label distinguishes among them.

• table_id - Variables are organized into what CMIP refers to as tables. This tends to be a juxtaposition of a problem realm (A for atmosphere, O for ocean) along with time frequency (mon for month, day for day). Note that a variable can exist in several tables. In our search we see that there is day temperature data as well as monthly Amon.

Downloading Data¶

We will refine our search to select a single model CanESM5, variant r1i1p1f1, and table Amon.

cat.search(
    variable_id="tas",
    experiment_id="historical",
    source_id="CanESM5",
    member_id="r1i1p1f1",
    table_id="Amon"
)

cat

Once your search has been sufficiently narrowed, you may download into a dictionary of xarray datasets.

dsd = cat.to_dataset_dict()

dsd

Note that you do not need to explicitly search for cell measures such as areacella. These will be included automatically. The files are downloaded locally to a cache directory which mirrors the directory structure of the remote storate. So while the above code is how you download data, it is also how you load it into memory for your analysis scripts. There is no need to handle files in your working directory or write complicated code to load them into memory.

Plotting¶

In this example, we will just take a temporal mean and plot the result using matplotlib.

import matplotlib.pyplot as plt
fig, ax = plt.subplots(figsize=(6, 4), tight_layout=True)
ds = dsd["tas"]["tas"].mean(dim="time") - 273.15  # to [C]
ds.plot(ax=ax, cmap="bwr", vmin=-40, vmax=40, cbar_kwargs={"label": "tas [C]"});