Simulation and measurement of the heat exchange between the urban canopy and boundary layers

This thesis is concerned with the energy exchanges that occur within city street canyons and the interactions between canyon air and the overlying atmosphere. The character of both the urban canopy and urban boundary layers is partly controlled by these interactions which occur across an interface at roof level. The method employed in this study is one of numerical simulation and testing. The simulation is accomplished by considering an urban canyon which is uniform and infinite in the along-canyon direction. The processes responsible for heat exchange at canyon surfaces and into/out of the canyon air volume are simulated on a computer. Model results compare favorably to measurements of energy fluxes at a real canyon site by other researchers. A field experiment was established to validate the mechanisms on which the model is based and to evaluate its predictions. The results of the field measurements were unexpected in that they suggested that the canyon surfaces exchanged little sensible heat with their environment. This result contradicted the findings of others for wider canyons. Interaction between the urban canopy and boundary layers appears to be a function of the height to width ratio of the canyon. The measurements also failed to confirm the existence of the sensible heat exchange mechanism embodied in the model. In spite of this, the model simulated the invariant form of the measured sensible heat exchange at the canyon top and correctly concluded that the substrate heat exchange was the dominant component in the canyon energy balance. The magnitude and the form of the measured values of both net radiation at the canyon top and canyon surface temperatures were generally predicted well by the model.