Evaluation of native Earth system model output with ESMValTool v2.6.0
Authors: Schlund Manuel , Hassler Birgit , Lauer Axel , Andela Bouwe , Jöckel Patrick , Kazeroni Rémi , Loosveldt Tomas Saskia , Medeiros Brian , Predoi Valeriu , Sénési Stéphane , Servonnat Jérôme , Stacke Tobias , Vegas-Regidor Javier , Zimmermann Klaus , Eyring Veronika 
Published: January 2023

Abstract. Earth system models (ESMs) are state-of-the-art climate models that allow numerical simulations
of the past, present-day, and future climate. To extend our understanding of the Earth system
and improve climate change projections, the complexity of ESMs heavily increased over the last decades.
As a consequence, the amount and volume of data provided by ESMs has increased considerably.
Innovative tools for a comprehensive model evaluation and analysis are required to assess
the performance of these increasingly complex ESMs against observations or reanalyses.
One of these tools is the Earth System Model Evaluation Tool (ESMValTool), a community diagnostic
and performance metrics tool for the evaluation of ESMs. Input data for ESMValTool needs to be
formatted according to the CMOR (Climate Model Output Rewriter) standard, a process that
is usually referred to as “CMORization”. While this is a quasi-standard for large model
intercomparison projects like the Coupled Model Intercomparison Project (CMIP), this complicates
the application of ESMValTool to non-CMOR-compliant climate model output. In this paper, we describe an extension of ESMValTool introduced in v2.6.0
that allows seamless reading and processing of
“native” climate model output, i.e., operational output produced by running the climate
model through the standard workflow of the corresponding modeling institute. This is
achieved by an extension of ESMValTool's preprocessing pipeline that performs a CMOR-like reformatting
of the native model output during runtime. Thus, the rich collection of diagnostics provided by
ESMValTool is now fully available for these models. For models that use unstructured grids, a
further preprocessing step required to apply many common diagnostics is regridding to a regular
latitude–longitude grid. Extensions to ESMValTool's regridding functions described here
allow for more flexible interpolation schemes that can be used on
unstructured grids. Currently, ESMValTool supports nearest-neighbor, bilinear, and first-order
conservative regridding from unstructured grids to regular grids. Example applications of this new native model support are the evaluation of new model setups
against predecessor versions, assessing of the performance of different simulations against
observations, CMORization of native model data for contributions to model intercomparison projects,
and monitoring of running climate model simulations. For the latter, new general-purpose
diagnostics have been added to ESMValTool that are able to plot a wide range of variable types.
Currently, five climate models are supported: CESM2 (experimental; at the moment, only surface
variables are available), EC-Earth3, EMAC, ICON, and IPSL-CM6. As the framework for the CMOR-like
reformatting of native model output described here is implemented in a general way, support for
other climate models can be easily added.