Three-dimensional aerodynamic structure of a tree shelterbelt: Definition, characterization and working models

In order to make recommendations to landowners with regard to the design and management of tree shelterbelts, it is necessary to understand and predict the wind flow patterns associated with shelterbelt structure. A structural description is a prerequisite for any prediction of wind flow. Optical porosity (percentage of open spaces on the side view of a shelterbelt) has been used as a structural descriptor of a shelterbelt; however, it is a 2-dimensional measure unable to fully represent the aerodynamic influence of a tree shelterbelt. Based on numerous studies observing the wind fields associated with shelterbelt structure, the overall aerodynamic structure of a tree shelterbelt in three dimensions is defined by its external structural characteristics (length, height, width, and crosssectional shape) and by its internal structural components (amounts and arrangements of vegetative surface area and volume, and geometric shape of individual vegetative elements). In order to associate the defined structure with wind speed, turbulent stress, and pressure, it is characterized using two structural descriptors the spatial functions of vegetative surface area density (vegetative surface area per unit canopy volume) and cubic density (vegetative volume per unit canopy volume). For field estimation, the two structural descriptors are expressed in three dimensions using two working models in terms of 1- or 2- dimensional sub-functions capable of being defined with field measurements. This paper discusses the rationale behind the definition, characterization, and working models for the 3-dimensional aerodynamic structure of a tree shelterbelt.