Vertically nested LES for high-resolution simulation of the surface layer in PALM (version 5.0)

Abstract

Abstract. Large-eddy simulation (LES) has become a well-established tool in the atmospheric boundary-layer research community to study turbulence. It allows three-dimensional realizations of the turbulent fields, which large-scale models and most experimental studies cannot yield. To resolve the largest eddies in the mixed layer, a moderate grid resolution in the range of 10 to 100 m is often sufficient, and these simulations can be run on a computing cluster with few hundred processors, or even on a workstation for simple configurations. The desired resolution is usually limited by the computational resources. However, to compare with tower measurements of turbulence and exchange fluxes in the surface layer a much higher resolution is required. In spite of the growth in computational power, a high-resolution simulation LES of the surface layer is often not feasible: to fully resolve the energy containing eddies near the surface a grid spacing of O(1 m) is required. One way to tackle this problem is to employ a vertical grid nesting technique, where the surface is simulated at the necessary fine grid resolution, and it is coupled with a standard, coarse, LES that resolves the turbulence in the whole boundary-layer. We modified the LES model PALM (Parallelized Large-eddy simulation Model) and implemented a two-way nesting technique, with coupling in both directions between the coarse and the fine grid. The coupling algorithm has to ensure correct boundary conditions for the fine grid. Our nesting algorithm is realized by modifying the standard third order Runge-Kutta time stepping to allow communication of data between the two grids. The two grids are concurrently advanced in time while ensuring that the sum of resolved and subgrid-scale kinetic energy is conserved. We design a validation test and show that the temporal averaged profiles from the fine grid agree well compared to the reference simulation with high-resolution in the entire domain. The overall performance and scalability of the nesting algorithm is found to be satisfactory. Our nesting results in more than 80 percent savings in computational power for 5 times higher resolution in each direction in the surface layer.

The Atmospheric Boundary Layer and the “Gray Zone” of Turbulence: A Critical Review

“…The grid refinement method does not really differentiate grid spacings aloft, and can be applied as a general nesting method. The method is of limited use to LES because the turbulent flows are already well resolved in the CBL, although Sullivan et al (1996) and Huq et al (2014) did apply a similar method with LES as an improved wall model to better resolve fine‐scale surface layer turbulence. The method is also unnecessary for mesoscale models, because however well resolved the thermals are in the nested high‐resolution surface grids, they are not expected to have any impact on the coarse mesoscale grids where they are entirely subgrid scale.…”

Overview of the PALM model system 6.0

“…For a detailed description of this self-nesting as well as sensitivity studies concerning, e.g., flow adjustment lengths within the child models, see Hellsten et al (2019, to be submitted to this special issue). Furthermore, note that, while the 3-D selfnesting also can work in a 1-D manner, a separate method for pure 1-D nesting is implemented in PALM as described by (Huq et al, 2018). In both cases, child and parent domains have identical horizontal dimensions and where the child obtains boundary conditions from the parent only at its top boundary.…”