UBC Theses and Dissertations

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UBC Theses and Dissertations

Three-dimensional radiation flux source areas in urban areas Roberts, Sarah Marie


Outdoor physical scale modeling is a potentially powerful compromise between the more common numerical and observational techniques to understand urban climates, because it incorporates the experimental control of physical and numerical modeling but is subject to the real complexities associated with natural environmental forcing. An outdoor physical model of simple concrete “buildings” was constructed to simulate an “urban” array in three different configurations. Observations of both facet surface temperatures and of radiation fluxes within and above the urban canopy layer investigate the impact of surface-sensor-sun relations on measured radiation flux source areas. Field measurements from the scale model were complemented by analyses using two numerical models. The combined results guide development of a protocol to guide the optimal siting of radiation sensors in measurement projects of the urban surface energy balance. The siting protocol considers both the influence of surface structure and orientation on radiation source areas and on the streamwise dimensions of the equivalent turbulent flux source area ‘seen’ by turbulence sensors. Observed thermal patterns from the scale model compare well with those at similar full-scale urban sites. Measurements of the spatial variation in radiation fluxes reveal that measurement heights above approximately 2.5 times the mean building height generate uniform flux density patterns. An agreement index used to quantify the strength of correspondence between what a sensor ‘sees’ of an urban surface and the actual surface morphology demonstrates that measurement locations near a street intersection give results closest to those sought to represent the local scale. Relations derived to describe the measurement height at which the radiation flux source area of a radiation sensor encompasses the source area of a turbulent flux sensor, reveal the common practice of co-locating tower-based radiation and turbulence flux sensors is often not sufficient to ensure their overlap. To match radiation flux source areas to the streamwise extent of turbulence flux source areas, it is recommended that radiation flux sensors be sited so as to contain the location of the maximum turbulent flux source weight. This suggests that radiation sensor(s) should be about two- to three times higher in elevation than the turbulence sensor(s).

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