Researchers Develop High-Resolution Models for Weather and Climate Research

September 04, 2023 - by Simone Ulmer

The effects of climate change are contributing to more frequent extreme weather events, such as sustained high temperatures, destructive thunderstorms, and torrential rainfall, or sometimes even hailstones the size of a fist. Identifying and accurately predicting such extreme events in weather forecasts is both societally and economically relevant. However, classical formulations, so-called hydrostatic models, are often too coarse for this task. For example, they are not able to map the details of thunderstorm cells, because they can only calculate weather forecasts with a spatial grid spacing of several kilometres or more. 

For more precise forecasts of extreme weather events, the models must calculate small-scale physical processes with even higher resolution. For example, for central Europe, models need to account for how air moves vertically within thunderstorms and interacts with the complex orography of the Alps, i.e. the topographical arrangement with steep slopes. “Here we need so-called non-hydrostatic models with grid spacing of one kilometre or less,” explains Heini Wernli, professor of atmospheric dynamics at ETH Zurich. “This is the only way we can represent the acceleration of air parcels in thunderstorms and near mountains as a result of the non-linear interaction of various processes."

Less than one hour for the 10-day forecast

For such complex calculations on a kilometre and sub-kilometre scale, the models must be better implemented on modern computer architectures. That means they need to be adapted to the architecture in a way that better utilises the supercomputer’s capabilities in order to calculate the forecasts cost-efficiently and significantly faster than in real time. 

"The current world-leading model is an absolute success story," says Wernli, referring to the Integrated Forecasting System (IFS) introduced in the 1980s at the European Centre for Medium-Range Weather Forecasts (ECMWF). It was programmed in the computer language Fortran and has been continuously adapted and improved to keep up with the latest developments in research and technology. The model solves the hydrostatic equations with a so-called spectral method and operates highly efficiently on CPU-based computers. "The efficiency of the model is crucial, because we only have a short time window of less than one hour to calculate our global medium-range forecasts," says Christian Kühnlein, Senior Scientist at ECMWF.

Separating domain science and hardware aspects

During the transfers of the original Fortran programming of the FVM into the Python programming language, the KILOS team inserts certain instructions so that the toolchain of the GT4Py library, part of the GridTools Framework, implements and optimises the code on the available hardware. GT4Py separates the code components dedicated to the physical-mathematical model from those needed for optimal hardware use. "This makes it possible to develop and maintain the model more productively," says Kühnlein. More precisely, a hardware-specific programming language is automatically generated according to GT4Py’s instructions that ensures that the system is utilised efficiently, for example the programming language CUDA is used on GPUs made by NVIDIA. These GPUs will be installed in the new flagship system of CSCS, "Alps", which will replace "Piz Daint" in 2024. "Besides NVIDIA GPUs, however, there are others, such as those from AMD. GT4Py can generate specific hardware-optimised codes for different architectures," says Kühnlein. This efficient portability of the model on different GPUs is an overarching goal of the project. 

According to the researchers, the newly formulated numerical equations in the dynamical core should also expand the capabilities of the model. "The dynamical core describes and calculates purely the fluid dynamics in the atmosphere, for example how the three-dimensional wind field develops as a function of time," says Wernli. Many physical phenomena outside of fluid mechanics, on the other hand, are described via parameterisations: If, for example, the flow causes moist air to rise and clouds to form that lead to precipitation, then additional routines are activated that are added to the dynamical core as parameterisations — these include cloud processes, turbulence, and radiation. 

The researchers in the KILOS project are working closely with ETH Professor Torsten Hoefler and his team from the Scalable Parallel Computing Lab on linking the parameterisations with the dynamical core and the transformation from Python via GT4Py to CUDA or another hardware-specific language. The new developments are being tested at CSCS, on "Piz Daint" and its successor "Alps", as well as on the Finnish pre-exascale system LUMI, currently Europe's fastest supercomputer and one in which Switzerland holds shares that it can provide to Swiss researchers.

For the time being, the newly developed model will be used for such research projects by Wernli and his team at ETH Zurich. Additionally, the FVM will be further developed until it is as reliable as the IFS model currently used by the ECMWF. For Wernli, however, the project has already fulfilled a long-standing wish: "It is a matter of concern to me that climate and weather researchers work together more," says Wernli. The KILOS project and its team are therefore in close exchange with the EXCLAIM research initiative, launched by ETH Zurich together with CSCS, MeteoSwiss, and other partners with the aim of significantly increasing the spatial resolution of weather and climate models in order to increase their precision.  

Parallel to the code development in KILOS, other models are also being optimised in EXCLAIM. In both projects, the focus is on high-resolution and highly efficient models that are also useful for climate research, as they represent elementary processes such as storms and other weather systems in much greater detail. This makes it possible to investigate how climate change affects specific weather events, and it also promises a future with more accurate, timely, hyper-local weather forecasts.

Image above: Monday, 28 August 2023 - The Rhine in Diepoldsau, Switzerland, carries a lot of water after torrential rains. (Image: MeteoSwiss App, Meteo Meldung)

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