Latest supercomputers run truer simulations of extreme weather

A high-resolution simulation of the global climate provides a much better representation of extreme weather events than previous lower-resolution models (Image: Department of Energy/Berkeley Lab)

High-resolution simulations of the global climate can now perform much closer to actual observations, and they perform far better at reproducing extreme weather events, a new Berkeley Lab study has found. Lead author Michael Wehner heralds this news as evidence of a golden age in climate modeling, as not only did the simulation closer match reality but it also took a fraction as long to complete as it would have in recent history – just three months compared to several years.

"These kinds of calculations have gone from basically intractable to heroic to now doable," Wehner said. "I've literally waited my entire career to be able to do these simulations."

The researchers used a CRAY XE-6 supercomputer at the National Energy Research Supercomputing Center to conduct their analysis for the period 1979 to 2005 at three spatial resolutions – 25 km (15.5 mi), 100 km (62 mi), and 200 km (124 mi) – and then compared the results to both each other and to real-world observations. The highest resolution case required 7,680 cores, while the entire output of all three cases amounted to just over 100 terabytes of data.

The higher-resolution simulation, which you can see in action in the video below, gave a much more accurate representation of weather areas with a lot of topography (i.e. mountainous regions), since altitude in the simulation grid is averaged over all terrain in each square – 25 sq km (9.6 sq mi) for the higher-resolution, 100 and 200 sq km (37 and 77 sq mi), respectively, for the lower resolutions.

The higher fidelity particularly produced stronger and more frequent storms in locations where mountains and hills are prevalent, in keeping with actual observations, and it provided significantly more realistic results for midlatitude winter and spring over land. But it was not always superior in its realism.

The simulations revealed that many deficiencies in the Community Atmospheric Model 5.1 they used are made significantly worse at higher resolutions, such as a bias towards forming a "double intertropical convergence zone." More uniform regions were often better represented at the 100 km resolution, and extreme precipitation over land in the midlatitude summer was too high.

Even so, future modeling at high resolution like this will be a huge help in projecting long-term climate change. Wehner was a lead author on the chapter concerning this in the fifth Intergovernmental Panel on Climate Change assessment report, which concluded that the contrast between wet and dry seasons will increase with global mean temperatures – leading to more extreme (in both magnitude and frequency) precipitation events in the wetter regions.

"Knowing it will increase is one thing," Wehner said, "but having a confident statement about how much and where, as a function of location, requires the models do a better job of replicating observations than they have." And the key to this is more high-resolution modeling.

His next project is to run the 25 km-resolution model for a future-case scenario, though he argues that it's just a matter of time before scientists get to one kilometer (0.4 sq mi) resolutions. Before they can, however, they must improve their understanding and modeling of cloud behavior.

"That will be a paradigm shift in climate modeling," Wehner said. "We’re at a shift now, but that is the next one coming."

A paper describing the research was published in the Journal of Advances in Modeling Earth Systems.

Source: Berkeley Lab

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