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The ICON model can be used for weather forecasting as well as climate predictions and long-term projections. So far, however, the different applications have been developed separately. An initiative that aims to bring the two closer together is now presenting first results.
Climate modeling once grew out of numerical weather prediction. Since then, meteorologists and climate researchers have developed models for weather and climate largely separately from one another. The reason for this is the different requirements of the two applications: Weather forecasting requires high temporal and spatial resolution, makes intensive use of the state of the atmosphere at the start of the forecast, and may neglect processes with long-term effects, such as heat transport in the ocean. Conversely, climate research considers processes on long time scales and uses information from the slower components of the climate system, such as the ocean. This can be done at a coarser spatial resolution.
The ICON weather forecast and climate model, which has been jointly developed by the Max Planck Institute for Meteorology (MPI-M), the Deutscher Wetterdienst (DWD), and other partners for around 20 years, has also been available in different versions – so-called configurations – depending on the application. They were all based on the same dynamical core, which includes the model grid, the fundamental equations, and the mathematical solution methods. However, weather forecasting and climate modeling have used different atmospheric and land components. In addition, depending on the application, different simplifications (parameterizations) were made for processes that were not explicitly represented.
Bringing weather and climate back together in modeling
However, current research questions, such as those concerning the regional impacts of anthropogenic climate change, require that the gap between weather forecasting and climate modeling be gradually closed. This is made possible by modern supercomputers, which can calculate longer time periods with high spatial resolution.
A team of researchers from the MPI-M, the DWD, and other partners has taken on this task and is bringing together what belongs together: the ICON developments for numerical weather prediction and climate applications. “Our project has benefited greatly from the fact that the components use the same model structure,” says Wolfgang Müller, group leader at MPI-M and lead author of the recently published paper presenting first successes of the project. Roland Potthast, head of the Department of Meteorological Analysis and Modeling at the DWD, highlights the societal relevance of the work: “The developments help us to take an integrated approach to weather and climate and to provide well-coordinated services ranging from high-resolution weather forecasts to seasonal and decadal climate forecasts.”
Successful integration of different components
The most important step was to couple the atmospheric component ICON-NWP, previously used for weather forecasting, with the ocean component ICON-O, which is essential for climate research. In addition, the researchers have developed a concept for a unified treatment of the different parameterizations. The same applies to the methods of data assimilation – a procedure commonly used in weather forecasting to initialize or adjust model states using observational data. This has resulted in a model configuration for global weather prediction by the DWD in which the atmosphere and ocean are coupled, and a configuration for climate simulations (ICON XPP – eXtended Predictions and Projections), which explicitly incorporates the developments in numerical weather forecasting. These models are more similar than any previous ICON configurations for weather and climate. This allows weather and climate processes to be compared more directly and accurately in the configurations.
The coupled weather configuration is currently being tested. In the long term, it could become part of the operational weather prediction portfolio. ICON XPP allows, among other things, to produce simulations for the Coupled Model Intercomparison Project, whose seventh phase (CMIP7) is already underway and is expected to deliver its first results in early 2027. These results will be included in the next report of the Intergovernmental Panel on Climate Change (IPCC). At the same time, ICON XPP is being used for novel high-resolution simulations. The scientists are working on a model resolution that is standard for weather forecasts but has not yet been possible for climate studies over long periods of time. This allows them to explore, for example, the role of small-scale ocean eddies in weather and climate variations.
Dr. Barbara Früh, Deutscher Wetterdienst: Barbara.Frueh@dwd.de
Dr. Peter Korn, Max Planck Institute for Meteorology: peter.korn@mpimet.mpg.de
Dr. Wolfgang Müller, Max Planck Institute for Meteorology: wolfgang.mueller@mpimet.mpg.de
Prof. Dr. Roland Potthast, Deutscher Wetterdienst: Roland.Potthast@dwd.de
Müller, W. A., et al. ICON: Towards vertically integrated model configurations for numerical weather prediction, climate predictions, and projections. Bulletin of the American Meteorological Society 106 (6) E1017–E1031, https://doi.org/10.1175/BAMS-D-24-0042.1, 2025.
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