A three-dimensional immersed boundary method was implemented into a Large Eddy Simulation (LES) with advanced subgrid-scale modeling capability. In this way, obstacles in the urban atmospheric boundary layer such as buildings and hills could be represented without changing the Cartesian grid. These numerical methods are applied in two urban environment investigations. The first explores the effect of hilly urban morphology on dispersion characteristics in the urban boundary layer. The second investigate the application of wall functions for building convection heat transfer in large eddy simulation. Air flow and dispersion in urban areas are strongly affected by the presence of buildings. In natural settings hills strongly impact dispersion. Five simulations of flow over building arrays over flat terrain and witch of Agnesi hills with maximum slope of 0.26 were conducted to study turbulence and dispersion properties in and above the canopy. While the small hill reduces the shear stress and velocity variance above the urban canopy compared to the flat urban array, the shear stress increases for larger hills. The TKE in the canopy downwind of the hill decreased below the value for the flat urban case, but canopy ventilation for the hilly cases was several times larger than for the flat case, especially near the hill crest. Therefore, urban dispersion models should account for these relatively moderate terrain changes to produce accurate results. In urban energy balance models, convection heat transfer model is often over-simplified by using a uniform convection heat transfer coefficient (CHTC) for each building surface. We consider more complex flow patterns by implementing a wall function to calculate the local CHTC from local velocities provided by LES. Simulations consisting of single building, 3 x 3 building arrays and 6 x 6 building arrays with neutral and unstable conditions were performed. Validation showed similar results as a low Reynolds number simulation resolving the viscous region, but both simulations disagreed with measurements in a wind tunnel. The log-law relation, which is a fundamental assumption underlying many wall models, was found to be accurate for the roof surface velocity and temperature for high building density, but it does not apply to windward and leeward surfaces. Density of buildings also acts as one of most important factors in determining the temperature distribution and buoyancy force in the urban canyon and roughness layer.

This volume presents peer-reviewed papers from the NATO Advanced Research Workshop on Atmospheric Boundary Layers held in April 2006. The papers are divided into thematic sessions: nature and theory of turbulent boundary layers; boundary-layer flows: modeling and applications to environmental security; nature, theory and modeling of boundary-layer flows; air flows within and above urban and other complex canopies: air-sea-ice interaction.

A stable atmospheric boundary layer (ABL) develops over land at night due to radiative surface cooling. The state of turbulence in the stable boundary layer (SBL) is determined by the competing forcings of shear production and buoyancy destruction. When both forcings are comparable in strength, the SBL falls into an intermittently turbulent state, where intense turbulent bursts emerge sporadically from an overall quiescent background. This usually occurs on clear nights with weak winds when the SBL is strongly stable. Although turbulent bursts are generally short-lived (half an hour or less), their impact on the SBL is significant since they are responsible for most of the turbulent mixing. The nighttime SBL can be modeled with large-eddy simulation (LES). LES is a turbulence-resolving numerical approach which separates the large-scale energy-containing eddies from the smaller ones based on application of a spatial filter. While the large eddies are explicitly resolved, the small ones are represented by a subfilter-scale (SFS) stress model. Simulation of the SBL is more challenging than the daytime convective boundary layer (CBL) because nighttime turbulent motions are limited by buoyancy stratification, thus requiring fine grid resolution at the cost of immense computational resources. The intermittently turbulent SBL adds additional levels of complexity, requiring the model to not only sustain resolved turbulence during quiescent periods, but also to transition into a turbulent state under appropriate conditions. As a result, LES of the strongly stable SBL potentially requires even finer grid resolution, and has seldom been attempted. This dissertation takes a different approach. By improving the SFS representation of turbulence with a more sophisticated model, intermittently turbulent SBL is simulated, to our knowledge, for the first time in the LES literature. The turbulence closure is the dynamic reconstruction model (DRM), applied under an explicit filtering and reconstruction LES framework. The DRM is a mixed model that consists of subgrid scale (SGS) and resolved subfilter scale (RSFS) components. The RSFS portion is represented by a scale-similarity model that allows for backscatter of energy from the SFS to the mean flow. Compared to conventional closures, the DRM is able to sustain resolved turbulence under moderate stability at coarser resolution (thus saving computational resources). The DRM performs equally well at fine resolution. Under strong stability, the DRM simulates an intermittently turbulent SBL, whereas conventional closures predict false laminar flows. The improved simulation methodology of the SBL has many potential applications in the area of wind energy, numerical weather prediction, pollution modeling and so on. The SBL is first simulated over idealized flat terrain with prescribed forcings and periodic lateral boundaries. A wide range of stability regimes, from weakly to strongly stable conditions, is tested to evaluate model performance. Under strongly stable conditions, intermittency due to mean shear and turbulence interactions is simulated and analyzed. Furthermore, results of the strongly stable SBL are used to improve wind farm siting and nighttime operations. Moving away from the idealized setting, the SBL is simulated over relatively flat terrain at a Kansas site over the Great Plains, where the Cooperative Atmospheric-Surface Exchange Study - 1999 (CASES-99) took place. The LES obtains realistic initial and lateral boundary conditions from a meso-scale model reanalysis through a grid nesting procedure. Shear-instability induced intermittency observed on the night of Oct 5th during CASES-99 is reproduced to good temporal and magnitude agreement. The LES locates the origin of the shear-instability waves in a shallow upwind valley, and uncovers the intermittency mechanism to be wave breaking over a standing wave (formed over a stagnant cold-air bubble) across the valley. Finally, flow over the highly complex terrain of the Owens Valley in California is modeled with a similar nesting procedure. The LES results are validated with observation data from the 2006 Terrain-Induced Rotor Experiment (T-REX). The nested LES reproduces a transient nighttime warming event observed on the valley floor on April 17 during T-REX. The intermittency mechanism is shown to be through slope-valley flow transitions. In addition, a cold-air intrusion from the eastern valley sidewall is simulated. This generates an easterly cross-valley flow, and the associated top-down mixing through breaking Kelvin-Helmholtz billows is analyzed. Finally, the nesting methodology tested and optimized in the CASES-99 and T-REX studies is transferrable to general ABL applications. For example, a nested LES is performed to model daytime methane plume dispersion over a landfill and good results are obtained.

The book gives a comprehensive and lucid account of the science of the atmospheric boundary layer (ABL). There is an emphasis on the application of the ABL to numerical modelling of the climate. The book comprises nine chapters, several appendices (data tables, information sources, physical constants) and an extensive reference list. Chapter 1 serves as an introduction, with chapters 2 and 3 dealing with the development of mean and turbulence equations, and the many scaling laws and theories that are the cornerstone of any serious ABL treatment. Modelling of the ABL is crucially dependent for its realism on the surface boundary conditions, and chapters 4 and 5 deal with aerodynamic and energy considerations, with attention to both dry and wet land surfaces and sea. The structure of the clear-sky, thermally stratified ABL is treated in chapter 6, including the convective and stable cases over homogeneous land, the marine ABL and the internal boundary layer at the coastline. Chapter 7 then extends the discussion to the cloudy ABL. This is seen as particularly relevant, since the extensive stratocumulus regions over the subtropical oceans and stratus regions over the Arctic are now identified as key players in the climate system. Finally, chapters 8 and 9 bring much of the book's material together in a discussion of appropriate ABL and surface parameterization schemes in general circulation models of the atmosphere that are being used for climate simulation.

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.

An introduction to the Large-Eddy-Simulation (LES) method, geared primarily toward hydraulic and environmental engineers, the book covers special features of flows in water bodies and summarizes the experience gained with LES for calculating such flows. It can also be a valuable entry to the subject of LES for researchers and students in all fields of fluids engineering, and the applications part will be useful to researchers interested in the physics of flows governed by the dynamics of coherent structures.