Numerical Modelling Of Atmospheric Flows Over Complex Sites With Special Regard To The Forest Canopy

The work is dedicated to the investigation of the interaction between an Atmospheric Boundary Layer and a canopy (representing a forest cover). We have focused our attention to the complex problem of the generation and transformation of turbulent vortices over homogeneous, heterogeneous and sparse canopy. This problem has been studied using Large Eddy Simulation (LES) approach and High Performance Computing (HPC) technique.The numerical results reproduced correctly all the main characteristics of this flow, as reported in the literature: the formation of a first generation of coherent structures aligned transversally with the wind flow direction, the reorganization and the deformation of these vortex tubes into horse-shoe structures. The results obtained with the introduction of a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest) are compared with the experimental data collected in a wind tunnel. In this case, the results confirmed the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as the Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor. Then, the process of passive scalar transport from a forest canopy into a clear atmosphere is studied for two cases, i.e., when the concentration held by the forest canopy is either constant or variable. While this difference has little influence on the concentration patterns, results show that it has an important influence on the concentration magnitude as well as on the dynamics of the total concentration in the atmosphere.

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.

This volume is a collection of lectures given at the two colloquia on atmospheric flows over complex terrain with applications to wind energy and air pollution, organized and sponsored by ICTP in Trieste, Italy. The colloquia were the result of the recognition of the importance of renewable energy sources, an important aspect which grows yearly as the environmental problems become more pronounced and their effects more direct and intense, while at the same time, the wise management of the Earth's evidently limited resources becomes imperative.It is divided into two main parts. The first, which comprises Chaps. 1 to 4, presents the structure of the atmospheric boundary layer with emphasis in the region adjacent to the ground. The second, Chaps. 5 to 10, discusses methods for the numerical computation of the wind field on an arbitrary terrain. The unique feature of this book is that it does not stop at the theoretical exposition of the analytical and numerical techniques but includes a number of codes, in a diskette, where the mechanisms and techniques presented in the main part are implemented and can be run by the reader. Some of the codes are of instructional value while others can be utilized for simple operational work.Some of the lecturers are: D N Asimakopoulos, C I Aspliden, V R Barros, A K Blackadar, G A Dalu, A de Baas, D Etling, G Furlan, D P Lalas, P J Mason, C F Ratto and F B Smith.

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.

The present work is not exactly a "course", but rather is presented as a monograph in which the author has set forth what are, for the most part, his own results; this is particularly true of Chaps. 7-13. Many of the problems dealt with herein have, since the school year 1975-76, been the subject of a series of graduate lectures at the "Universire des Sciences et Techniques de Lille I" for students preparing for the "Diplome d'Etudes Ap profondies de Mecanique (option fluides)". The writing of this book was thus strongly influenced by the author's own conception of meteorology as a fluid mechanics discipline which is in a privi leged area for the application of singular perturbation techniques. It goes without saying that the modeling of atmospheric flows is a vast and complex problem which is presently the focal point of many research projects. The enonnity of the topic explains why many important questions have not been taken up in this work, even among those which are closely related to the subject treated herein. Nonetheless, the author thought it worthwhile for the development of future research on the modeling of atmospheric flows (from the viewpoint of theoretical fluid mechanics) to bring forth a book specifying the problems which have already been resolved in this field and those which are, as yet, unsolved.

This volume covers aspects of numerical modeling of the atmosphere and climate from the microscales of turbulence to the very large scales associated with climate and climatic change. Each of the three major spatio-temporal scales of the atmosphere, namely, the microscale, the mesoscale, and the macroscale is addressed through a hierarchy of models. Results of model simulations are illustrated throughout the text, with many of these examples based on the author's original research work. For each type of model discussed here, the theoretical background, including governing equation sets, simplifying assumptions, and advantages and limits of the models, is provided. The topic of coupled, or nested, modeling systems as a promising approach to air pllution embedded in regional atmospheric flows, as well as to the regional atmospheric response to global climate forcings, is also addressed. An attempt is made throughout the book to highlight the highly interdisciplinary nature of atmospheric modeling, particularly in those sections dealing with climatic change issues.