Many general-purpose computational fluid dynamics packages offer unstructured grid systems, but according to our experience such solvers are usually computationally decidedly less efficient than ABL-tailored LES models, such as PALM ( Raasch and Schröter, 2001 Maronga et al., 2015, 2020), the Weather Research and Forecasting Model (WRF) ( Skamarock et al., 2008) with its LES option, and the Dutch Atmospheric Large-Eddy Simulation (DALES) ( Heus et al., 2010) that are based on structured orthogonal grid system with constant horizontal resolution.
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However, only unstructured grid systems allow full advantage to be taken of spatially variable resolution. Many conventional continuum-based numerical solution methods (e.g., finite-element and finite-volume methods) allow variable resolution so that the resolution can be concentrated to the area of principal interest and relaxed elsewhere. Moreover, the uncertainty related to the lateral boundary conditions usually decreases as the domain is made larger.
To adequately capture processes on the street scale and to simultaneously capture large ABL-scale turbulence, sufficiently large model domains at small grid sizes are required, posing high demands on the computational resources in terms of CPU time and memory. However, the vertical extent of the LES domain should scale with the ABL height, and the horizontal size should span over several ABL heights in order to capture the ABL-scale turbulent structures ( de Roode et al., 2004 Fishpool et al., 2009 Chung and McKeon, 2010 Auvinen et al., 2020 a). This requirement typically leads to grid spacings on the order of 1 m. Concerning urban LES, Xie and Castro ( 2006) have shown that at least 15 to 20 grid nodes are needed across street canyons to satisfactorily resolve the most important turbulent structures within the canyons. Nowadays, it is possible to carry out LES for complex built areas (e.g., Letzel et al., 2008), but this is still limited to relatively small areas because of the high spatial resolution requirement. Until recent years, there were no ABL LES models capable of modeling detailed surface structures, such as buildings or steep complex terrain shapes in the ABL. At present it is becoming an important method in applied research on realistic, very detailed, and complicated flow systems such as urban ABL problems ( Britter and Hanna, 2003 Tseng et al., 2006 Bou-Zeid et al., 2009 Tominaga and Stathopoulos, 2013 Giometto et al., 2016 Buccolieri and Hang, 2019 Auvinen et al., 2020 a). Large-eddy simulation (LES) has been used for basic research of atmospheric boundary layer (ABL) phenomena using idealized model setups for decades. 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. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. 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. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The nesting system is evaluated by first simulating a purely convective boundary layer flow system and then three different neutrally stratified flow scenarios with increasing order of topographic complexity.
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. To overcome this problem, an online LES–LES nesting scheme is implemented into the PALM model system 6.0. 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. Large-eddy simulation (LES) provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows.