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S1: Diagnosis and Metrics in Climate Models

Principal investigators: Prof. Valerio Lucarini (University of Reading/Universität Hamburg), Prof. Thomas Jung (MARUM/AWI)

Mean systematic errors of 500 hPa geopotential height fields (shading in dam) for winters (December–February) of the period 1962–2005 and various versions of the ECMWF model: (a) 29R2, (b) 30R1, (c) 31R1, (d) 32R1, (e) 32R3 and (f) 33R1. Also shown are mean fields (contours) obtained from a combination of ERA-40 (1962–2001) and operational ECMWF analysis data (2002–2005). Mean systematic errors significant at the 95% confidence level are hatched. Notice that negative values are contoured (as well as shaded). (From Jung et al. 2010; Q. J. R. Meteorol. Soc. 136: 1145–1160).

The hypothesis underlying the proposed Collaborative Research Centre is that a more consistent representation of parameterised processes and numerical formulations will result in better climate models. To test this hypothesis, therefore, it is critical to thoroughly evaluate the influence of enhanced model formulations, developed in the other project areas, on the realism of climate models. Here, we will focus on two next-generation climate models developed and used by the German climate research community, namely ICON-a/ICON-o and ECHAM6/FESOM.

Efforts towards developing a new generation of energetically consistent models require the availability of tools that can help to identify short-comings and quantify progress (e.g. Figure 1). Therefore, we will develop and apply new diagnostics and metrics that help us to understand the pathways of energy transport and energy conversion, analyse the energetic consistency of existing models and formulate measures taken up by the other subprojects to advance the energetic consistency and quality of parameterisations of climate models. Furthermore, we will develop effective metrics and diagnostics that can be used to quantify the realism of models and unravel the origin of model error. Metrics describing systematic model error in atmosphere, ocean and sea ice using some of the most advanced observational data sets for reference, will be elaborated. Furthermore, dedicated metrics will be developed that capture important weather and climate phenomena resulting from instabilities such as extratropical storms, Euro-Atlantic blocking, or monsoon dynamics. Finally, we will construct metrics able to analyse consistently a large class of extremes occurring across a wide range of spatial and temporal scales.

An important aspect will be the practical implementation of new tools that can be used for model evaluation. We will build on the newly developed Earth System Model Validation Tool (ESMValTool)—a community diagnostic and performance metrics tool for routine evaluation of climate models. By doing so, we can also benefit from other community efforts in developing evaluation tools. Moreover, we can add our new metrics and diagnostics to ESMValTool thereby increasing the applicability and international visibility—and hence the impact—of the research coming out of this subproject.

To aid effective evaluation of the model development activities, coming out of the other research areas, and to guide further efforts, we will design protocols for numerical experimentation, including common model setups, together with the PIs from subproject S2. These protocols will allow direct comparability between different modelling experiments, ensure availability of model output needed to compute all relevant metrics and diagnostics and provide a framework to evaluate model changes in the context of CMIP6-type experiments with ICON-a/ICON-o and ECHAM6/FESOM.

The results will be made available through a web-based tool—the Diagnostics and Metrics Explorer—for effective analysis. We will take advantage of direct collaboration with ongoing Horizon2020 projects (CRESCENDO and PRIMAVERA) dealing with evaluation of Earth System Models in the context of CMIP6.

FESOM model

One of the two ocean models used in the TRR181 project is FESOM (Finite-Element/volumE Sea ice-Ocean Model), which is part of the AWI-CM climate model of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).

FESOM is the only ocean model participating in CMIP6, that is formulated on an unstructured mesh. This allows scientists to flexibly increase horizontal resolution of the model in more energetically active areas of the ocean like the Gulf stream or Agulhas current. The resulting model resolves important details of ocean circulation, but is still computationally efficient.

FESOM ocean currents in Indian and Pacific oceans (100m)

Report 4th COMMODORE workshpop in Trieste by Hugo Lévy

Last September, I participated in the 4th COMMODORE workshop: the COMmunity for the numerical MODeling of the glObal, Regional and coastal ocEan – the kind of convoluted acronym that we all love. After Paris, Hamburg and Boulder, the 2026 edition – my first – was hosted by the ICTP in Trieste, Italy, overlooking the Adriatic Sea. Beyond this charming setting, I was quite excited about the scientific programme and its strong focus on numerical techniques for ocean models. Over the years, I have come to realise that this type of workshop is probably the most relevant kind of scientific gathering for me, mainly because of its manageable size (a little over 50 participants in the present case) and sufficiently narrow scope. These workshops also give me, as a presenter, the opportunity to receive external and often spot-on feedback on my own work.

As anticipated, getting there was not so easy. No longer keen on flying for business trips, I travelled by train from Bremen. The long and somewhat hectic journey was made much more enjoyable thanks to Steffen Maaß, another TRR 181 member who was also attending the workshop and travelling with me. We had planned to arrive two days early so that we could enjoy the area and the Adriatic Sea a little, as summer was coming to an end.

The workshop itself lived up to expectations. There were many familiar faces and just as many new encounters over the four intensive days. The topics covered in the oral presentations and posters ranged from the subtle yet crucial details of numerical schemes and how they are implemented in ocean GCMs, to subgrid-scale parameterisations and the use of machine learning in ocean models. As one prominent participant put it, this was a great setting for discussing all the “nerdy” stuff that keeps us entertained as scientists. I particularly enjoyed learning about the history of model development, thanks to the many contributions from people who have been working in this field for a long time now.

Each day ended with an open discussion session on specific issues, such as the perspective of the Global South on ocean modelling, or taking stock of where we currently stand in terms of Machine Learning in ocean modelling. In my opinion, the topic that sparked the most discussion was, unsurprisingly, AI and its now-ubiquitous use across all fields of science — ours being no exception. We openly shared our experiences with AI, discussing both the opportunities it creates and the concerns it raises, and speculated on what the (near) future of our field might look like. It was insightful to hear other scientists' perspectives on what feels to me like a major upheaval in the way we work. To be continued…

On the last day, I gave a talk myself on eddy parameterisation over the Antarctic continental slope. I explored whether traditional approaches are well suited to this region, looking both at their underlying assumptions and at how well they perform in ocean models. The talk connected naturally with some of the earlier presentations. It also followed nicely from the discussion session the previous evening, which focused on whether traditional mesoscale parameterizations are still needed in eddy-permitting models, given recent advances in kinetic energy backscatter and numerical methods.

After one last coffee break, it was already time to wrap up the workshop. We did so in a friendly atmosphere, with a walk along the seashore (including a quick dip for some of us), followed by dinner at a large pizzeria in the city centre of Trieste. Overall, I am very grateful to TRR181 for supporting this trip. I came back with the feeling that I had learnt new things, gained some fresh ideas and found extra motivation to continue my research!

The Earth’s energy budget and other funny aspects of the thermodynamics of the climate system

State-of-the-art climate models still struggle to reproduce a reasonably energetically consistent system, even though outstanding improvements have been achieved in the recent past.

Valerio Lembo, Postdoc in S1

The idea of this subproject is assessing the impact of introducing new numerical schemes and physical parametrizations developed in the TRR181 for the energy closure of state-of-the-art climate models. We provide diagnostic tools that allow for evaluation and intercomparison of climate models, starting from their outputted datasets.

It might sound trivial, expecting that the climate system, if in steady state, is also in thermodynamic equilibrium. This is at least what our studies of classical thermodynamics suggest. The problem is that the system constantly exchanges energy with its exterior, i.e. the outer space, and within its interior. In steady state conditions, the net exchange of energy with the exterior has to be null. In other words, the climate system is in thermodynamic equilibrium, once we averaged out the modulation of the solar energy input to an appropriately long timescale and all the energy exchanges occurring in its interior, shaping the solar “reflection” and the thermal energy output. This is a clear example of what is called a “non-equilibrium dissipative steady state thermodynamical system”.

State-of-the-art climate models still struggle to reproduce a reasonably energetically consistent system, even though outstanding improvements have been achieved in the recent past. This points to the very basic reasons for climate modeling, on one hand reflecting the lack of understanding of some processes involving energy exchanges and the limits of the discretization/truncation of the real world in finite dimension models, on the other hand preventing us from correctly evaluating the impact of the various forcings for reconstructed and projected climate change.

As TRR181, we are participating to the community effort called “ESMValTool”, whose aim is providing a set of standardized diagnostics for the evaluation of state-of-the-art and forthcoming multi-model ensembles. In our diagnostics, we try to address specifically the Earth’s energy budget and its atmospheric and oceanic components, and the atmospheric energy exchanges, including the Lorenz Energy Cycle, which describes the energy exchanges in the extratropical synoptic eddies. We also provide an estimate of the atmospheric material entropy production, i.e. the entropy production through irreversible processes, and the water mass budget, which is known to be one of the main sources of uncertainty for the modeled energy budget.

The diagnostic tool is currently being ported from version 1 to version 2 of ESMValTool, and will be hopefully soon publicly released. A report for the ESMValTool version 2 is being written, with contributions by all groups in the community, and another paper, focused on potential applications of the tool in various fields of climate science, will be submitted.