Forest canopies cover about 30% of the land surfaces some of which are hilly or mountainous. Both complex terrain and forest canopies influence mean flow and turbulence, playing a critical role in affecting momentum and scalar transfer. Although recent advances have been made in numerical simulations of canopy flows, few have been conducted over steep slope terrain and considered full sets of physical and physiological processes in sub-canopy layers, which greatly limits our understanding of canopy flows and scalar transfer. To address this limitation, we upgrade the Weather Research and Forecasting model with the large-eddy simulations module (WRF-LES) by incorporating the immersed-boundary method (IBM) to improve the simulations over steep slope terrain. In addition, an advanced multiple layer canopy module (MCANOPY) is developed based largely on the Community Land Model version 4.5 and coupled with WRF-LES to simulate sources and/or sinks of momentum, heat, water vapor, and CO2 across multiple canopy layers. Both IBM and MCANOPY are evaluated against field measurements, demonstrating good performances compared with observations. The updated modeling system (i.e., WRF-LES-IBM-MCANOPY) is then applied over a forest edge to investigate the effects of foliage distributions and scalar distributions on canopy flows and scalar transfer. The results show that foliage distributions have a significant impact on the flow dynamics and scalar transfer, mainly due to the sub-canopy jet. The scalar distributions affect the scalar field with the ground source being the most important. The modeling system also simulates flows over forested hills. Flow dynamics over a three-dimensional hill show a different feature to those over a two-dimensional hill, where much large turbulence structures are simulated in the lee of a three-dimensional hill. Simulations over different slope hills demonstrate significant impacts of slopes. The lee side turbulence is enhanced as the hill slope increases. A new flow feature is the nearly unvarying flow field within the canopy in the lee of a steep slope hill, subjected to the influence of foliage distributions. Our upgraded WRF-LES-IBM-MCANOPY system has demonstrated promising capacities in many applications such as wind-turbine siting, wildfire propagation prediction, and interpretation of eddy covariance data over complex landscapes.

High-fidelity Large-Eddy Simulation (LES) of fluid flow over complex terrain has long been a challenging computational problem. Complex terrain leads to increased velocity gradients, turbulence production, and complex turbulent wakes. Body-fitted grids need a high resolution to deal with additional effects of highly skewed cells that follow a terrain of steep slope. Immersed boundary methods need special techniques like wall models to numerically resolve the associated drag force. In flow over complex terrain, the characteristic scale decreases locally which makes it a challenging endeavour for LES to mimic the turbulent energy cascade, particularly when steep terrain produce vortices and streaky structures that sustain turbulence away from the surface. This thesis presents the canopy stress method in which the terrain is immersed into the fluid, cutting the cells of a Cartesian grid, where the effects of terrain are treated by the form drag and the skin friction drag. Heat transfer analysis of flow in pipes and porous media is considered to study the sensitivity of canopy drag coefficients. A scale-adaptive methodology is proposed to model the subgrid-scale terrain effects. The analysis of wind tunnel measurements over mountains and forests shows that the scale-adaptive model dynamically adjusts the dissipation rate by the scale of energetic eddies near complex terrain. In regions without terrain effects, where subgrid turbulence is locally isotropic, the model also provides accurate dissipation rate. These results suggest that combining the rotation tensor and the vortex stretching vector with the strain tensor through the second-invariant of the square of the velocity gradient tensor is a novel approach to improve the fidelity of LES over complex terrain in which the dissipation becomes scale-aware; i.e. the rate of turbulence dissipation is adjusted with the changes in the characteristic scales. The numerical analysis of four distinct flow regimes (e.g., Chapters 3-6) illustrates the accuracy, simplicity, and cost-effectiveness of the proposed LES methodology.

Airflow over complex terrain throughout the atmospheric boundary layer (ABL) governs the transport and mixing of mass, momentum, and heat. Topography causes obstruction of the airflow and generates airflow distortion and turbulence. Perturbations in land-atmosphere interactions cause various weather phenomena like cold-air pools (CAPs) leading to changes in many aspects of weather and climate that impact the optimal position of wind-turbine, forest-fire behavior, and forecasting, as well as trace-gas and pollutant dispersion. This thesis investigates the flow over complex terrain, specifically double-hill terrain, with new numerical model approaches. The first study utilizes the Weather Research and Forecasting (WRF) model with large eddy simulations (LES) and the immersed-boundary method (IBM) to improve the simulations of the flow and recirculation regions over steep double-hill terrain. The gap distance controls the flow distribution behind both hills. The upwind hill has a significant influence on the second hill. When the gap distance is too small, the flow after the upwind hill cannot regain its momentum. The second study examines the flow distribution over a forested double-hill and the impact of the gap distance between two hills on scalar transport (CO2 and H2O). This study uses the WRF-LES model coupled with a new multiple-layer canopy module (MCANOPY module). We find that flow recirculation is the primary factor dominating scalar transport. Scalars are transported and trapped in both recirculation regions and accumulated on the lee sides of both hills. Our simulation shows the occurrence of two vortices on the lee side of the upstream hill enhances the accumulation of scalars in the valleys. In the end, we extend our work from the first study to understand flow patterns over a realistic double-hill topography. Results show that the valley gap distance is so small that the recirculation region in the valley between two hills cannot fully develop. Additionally, the WRF-IBM captures the structure of microscale flows that other models have not captured in the previous studies.

Starting with the description of meteorological variables in forest canopies and its parameter variations, a numerical three-dimentional model is developed. Its applicability is demonstrated, first, by wind sheltering effects of hedges and, second, by the effects of deforestation on local climate in complex terrain. Scientists in ecology, agricultural botany and meteorology, but also urban and regional lanners will profit from this study finding the most effective solution for their specific problems.

Research was performed using a turbulence boundary layer model to study the behavior of cold, dense flows in regions of complex terrain. Results show that flows develop a balance between turbulent entrainment of warm ambient air and dense, cold air created by surface cooling. Flow depth and strength is a function of downslope distance, slope angle and angle changes, and the ambient air temperature.

This book addresses both the fundamentals and the practical industrial applications of Large Eddy Simulation (LES) in order to bridge the gap between LES research and the growing need to use it in engineering modeling.