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

Abstract

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.

Evaluation of the dynamic core of the PALM model system 6.0 in a neutrally stratified urban environment: comparison between LES and wind-tunnel experiments

“…Also, we recommend a high grid resolution to represent structures as close to the reference as possible. To achieve this, future validations could utilize PALM's nesting feature in order to cope with increasing computational demand of reduced grid size (Hellsten et al, 2020). A higher resolution also reduces the errors introduced by shifting locations of measurements of PALM when comparing against reference data.…”

“…These margins are FAC2 > 0.5, |FB| < 0.3, 0.7 < MG < 1.3, NMSE < 4, and VG < 1.6. Hertwig et al (2017) recommend to evaluate the shape parameters of the wind speed distributions of u and v when comparing LES and wind-tunnel results. Figure 11 shows the skewness γ and the excess kurtosis β of u and v. Between both experiments, γ u mostly agrees and shows either symmetrical distributions (γ u ≈ 0) or a positive skew.…”

Crowdsourcing air temperature data for the evaluation of the urban microscale model PALM—a case study in central Europe

“…The online nesting allows the simulation of a large domain with a coarse resolution combined with a small domain containing the area of interest with a fine resolution. The one-way coupled mode of the online nesting was applied here where influences from the parent domain are interpolated on the child domain without feedbacks from the child to the parent domain [48]. A detailed description of the model is given by Maronga et al [20].…”

“…A detailed description of the model is given by Maronga et al [20]. Further information on the model components can be found in Gehrke et al [41] for the LSM, Resler et al [42] for the BSM, Salim et al [43] for the RTM, Kadasch et al [47] for the offline nesting and Hellsten et al [48] for the online nesting. The mesoscale simulation was run on 126 CPU cores.…”

Correlated spatiotemporal downscaling of Euro‐CORDEX climatic data for infrastructure resilience assessment

“…I G U R E 3 Example of a wind field that is computed via LES simulations around the Metsovo valley in Greece. The results of a vertical cross section across the highway axis are shown (adopted fromHellsten et al, 2021a).T A B L E 2 Weather station parameters used for generating the Frankenstein days for the Metsovo-Panagia highway segment in Greece.…”