Modeling of thermotopographic flows in forested terrain

Thermotopographic flows are winds that develop from the interaction of local thermal gradients and sloping terrain. Strong heating or cooling of the air near the surface alters local temperature, density, and pressure gradients. During nighttime hours, the air near the surface cools more rapidly than that aloft due to radiative loss at the surface, and near-surface downslope winds may develop. During the day, surface heating by solar radiation may drive flow upslope. Recently, there has been an increased interest in thermotopographic flows in forested areas, largely because these flows may affect the accuracy of measurements of ecosystem-atmosphere exchange of carbon dioxide. In some forests, under some conditions, the diel pattern of thermotopographic flows differs from expected: at night, strong radiative cooling in the canopy layer may drive upslope sub-canopy flows, and daytime downslope flows may occur below the canopy due to heating of the canopy. There is still uncertainty as to whether thermotopographic flows will occur in a given forest, what diel pattern they will exhibit, what drivers (e.g., terrain or canopy characteristics, ambient winds) influence the flow, and what effects these flows have on measurements of forest-atmosphere exchange. As observational studies are limited, numerical modeling provides an attractive option for studying thermotopographic flows in forests.;The aim of this research was to develop a numerical model which may be used to study thermotopographic flows in hilly forested terrain. The model is based on existing large-eddy simulation software (Advanced Regional Prediction System, ARPS) which is used to model flow in hilly terrain. Adaptations were made to the ARPS model to simulate the dynamic, radiative, and thermal influences of canopy elements (leaves, branches, and boles).;Major contributions of this research are methodological advances and several outcomes from the results of the model application. Methodological contributions are concerned with the radiative and thermal complexities that occur in forests in sloping terrain. Firstly, fully three-dimensional simulations of the radiation transfer through a forest in sloping terrain are computationally intensive; however, neglecting the heterogeneity of the terrain removes spatial variability in cooling and flow. To address this, the concept of view factor was used to adjust radiative fluxes obtained through computationally-efficient calculations, which assume horizontal homogeneity, to account for the heterogeneous terrain in which slope flows develop. Secondly, it was recognized that accurate simulation of cooling of air within a canopy can not be done without consideration of the differences in cooling rates of canopy elements and air. A parameterization was developed that implicitly accounts for radiative cooling of the canopy elements, heat storage in the canopy elements, and heat transfer between the canopy elements and the air. Limitations of these parameterizations and future developments are discussed.;The model was used to investigate the impact of a range of slope angles and vegetation densities on thermotopographic flows. Simulated rates of cooling and resulting flows were unsteady or pulsing in nature, particularly within a more dense canopy. In thinner canopies, as expected, cooling was more rapid, resulting in stronger temperature inversions and faster downslope flows. Flows also were more rapid on steeper slopes, due to a greater along-slope component of the driving buoyancy force. These results demonstrate the future potential of modeling thermotopographic flows in forested terrain.