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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/gmd-2020-222
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gmd-2020-222
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: development and technical paper 07 Sep 2020

Submitted as: development and technical paper | 07 Sep 2020

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This preprint is currently under review for the journal GMD.

A Nested Multi-Scale System Implemented in the Large-Eddy Simulation Model PALM model system 6.0

Antti Hellsten1, Klaus Ketelsen2, Matthias Sühring3, Mikko Auvinen1, Björn Maronga3,6, Christoph Knigge4, Fotios Barmpas5, Georgios Tsegas5, Nicolas Moussiopoulos5, and Siegfried Raasch3 Antti Hellsten et al.
  • 1Finnish Meteorological Institute, P.O.Box 503, 00101, Helsinki, Finland
  • 2Software Consultant, Beethovenstr. 29A, 12247 Berlin, Germany
  • 3Leibniz University Hannover, Institute of Meteorology and Climatology, Herrenhäuser Strasse 2, 30419 Hannover, Germany
  • 4Deutscher Wetterdienst, Frankfurter Straße 135, 63067 Offenbach, Germany
  • 5Aristotle University Thessaloniki, P.O.Box 483, 54124, Thessaloniki, Greece
  • 6University of Bergen, Geophysical Institute, Postboks 7803, 5020 Bergen, Norway

Abstract. Large-eddy simulation provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES-LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by simulating first a purely convective boundary layer flow system and then three different neutrally-stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

Antti Hellsten et al.

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Dataset: A Nested Multi-Scale System Implemented in the Large-Eddy Simulation Model PALM model system 6.0 Antti Hellsten, Klaus Ketelsen, Matthias Sühring, Mikko Auvinen, Björn Maronga, Christoph Knigge, Fotios Barmpas, Georgios Tsegas, Nicolas Moussiopoulos and Siegfried Raasch https://doi.org/10.25835/0090593

Pre- and Post-Processing Python Library for Urban LES Simulations Mikko Auvinen, Sasu Karttunen and Mona Kurppa https://doi.org/10.5281/zenodo.4005687

Antti Hellsten et al.

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Short summary
Large-eddy simulation of urban atmospheric boundary layer involves a large separation of turbulent scales leading to prohibitive computational costs. An online LES-LES nesting scheme is implemented into the PALM model system 6.0 to overcome this problem. Test results show that the accuracy within the high-resolution nest domains approach the non-nested high-resolution reference results. The nesting can reduce the CPU time up to 80% compared to the fine-resolution reference runs.
Large-eddy simulation of urban atmospheric boundary layer involves a large separation of...
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