By: Thijs Bon, Raúl Bayoán Cal & Johan Meyers
One of the key challenges in atmospheric and environmental sciences is understanding of the stably stratified boundary layer, which is relevant to the renewable energy sector, air quality and pollutant dispersion, numerical weather prediction, and climate modeling. A flow is called stably stratified if the upper layers are warmer, and therefore less heavy, than the lower layers, which inhibits vertical mixing and reduces the turbulence intensity. The atmospheric boundary layer (ABL) commonly exhibits stable stratification during nighttime over land, when the ground surface is cooler than the overlying air, and in polar areas.
A particular process that can influence the structure of the stable boundary layer (SBL) is subsidence, large-scale downward movement of air. This phenomenon is often associated with high-pressure regions, fair weather conditions, and clear skies, but can also arise from planetary-scale circulations. In a stably stratified atmosphere, subsidence transports warm air downwards, and can therefore significantly affect the heat balance in the SBL. Nevertheless, the effects of subsidence on the SBL have been rarely studied, and are often neglected.
The `Turbulent Flow Simulation and Optimization’ (TFSO) group of the Department of Mechanical Engineering, KU Leuven, has investigated this topic in detail. In this research, we performed a systematic analysis of the SBL subjected to subsidence, using large-eddy simulations (LES). This type of computational fluid dynamics (CFD) method uses fine spatial grids to resolve the majority of the three-dimensional chaotic turbulent motions, while the effect of the smallest scales is represented by a `subgrid-model’. In the present study, the three-dimensional computational domain of 4003 m was discretized in grid boxes of 3.125x3.125x1.56 m, resulting in a mesh of 4.2 million grid points.
Figure 1: Horizontally- and time-averaged mean profiles of horitontal wind speed (left) and the difference in potential temperature with respect to the surface temperature. Cases with different subsidence rates are displayed, with darker blue lines indicating simulations with stronber subsidence.
A total of 17 different simulation cases were performed, where the effect of input variables such as subsidence velocity, surface temperature, and surface roughness were varied. Using the simulation results, we could systematically investigate the complex relationship between subsidence, SBL depth, surface fluxes of heat and momentum, and the internal distribution of mean temperature and velocity profiles. Moreover, it was found that all simulations of the SBL reached a truly steady state, in which thermal equilibrium is established by a balance between subsidence heating and surface cooling. This steady state does not occur when subsidence is not included in the simulations.
This research would not have been possible without the computational resources and services provided by the Flemish Supercomputer Center (VSC). LES of stably stratified turbulence is known to require very high spatial grid resolutions, presenting significant computational challenges.
The latest version of the scientific LES code that we used, named SP-Wind3, employs a full 3-dimensional domain decomposition for efficient parallel computation. This means that each processor computes the flow in a small part of the computational domain, while for certain global operations, they need to communicate through MPI protocols. Due to this efficient parallelization, we were allowed to run the large-eddy simulations on 12 nodes, each containing 128 AMD Epyc 7763 cores, on the Tier-1 ``Hortense’’ partition in the HPC cluster of the VSC. Therefore, the runtime for one simulation was `only’ about two days.
Figure 2: Instantaneous two-dimensional visualization slice of the three-dimensional turbulent potential temperature field. The `layered' structure of the stable boundary layer is clearly visible.
This work was published in the journal Boundary-Layer Meteorology, and the full article can be found here