Designing and Documenting Experiments in CMIP6

Earth system modelling relies on contributions from groups who develop models and from those involved in devising, executing, and exploiting numerical experiments. Often these people work in different institutions, and they may communicate primarily via published information (whether journal papers, technical notes, or websites). The complexity of the models, experiments, and methodologies, along with the diversity (and sometimes inexact nature) of information sources can easily lead to misinterpretation of what was actually intended or done. In this paper we introduce a taxonomy of terms for more clearly defining numerical experiments, put it in the context of previous work on experimental ontologies, and describe how we have used it to document the CMIP6 experiments. We describe how this process involved iteration with a range of CMIP6 stakeholders to rationalise multiple sources of information and add clarity to experimental definitions. We demonstrate how this process has added value to CMIP6 itself by a) helping those devising experiments to be clear about their goals and expected methodology, b) making it easier for those executing experiments to know what was intended, c) exposing inter-relationships between experiments, and d) making it clearer for third parties (data users) to understand the CMIP6 experiments. We conclude with some lessons learned, and how these may be applied for any modelling campaign as well as future CMIP phases.