A physical scale model to study urban surface temperatures and radiation

Much of our current understanding of the physical processes that contribute to urban climate is derived from field studies conducted in real cities using a range of ground-based and airborne measurement techniques. Much insight has also been gained from wind tunnel modeling, as this technique allows for the isolation and simplification of climatic processes by controlling the impinging flow and surface structures. Data from both types of study serve as an important input to the construction, evaluation, and validation of numerical models which further enhance our appreciation of urban processes. However, despite expanding computational abilities the inherent complexity of surface morphology and energetic exchanges of real-world urban environments still pose many numerical modeling challenges. Outdoor physical scale modeling is a potentially powerful compromise between wind tunnel modeling and full-scale observation that incorporates the experimental control possessed by both physical and numerical modeling and the real complexities associated with natural environmental forcing (atmospheric turbulence and radiation loading).

Here we describe the project design and preliminary results from open-air scale model experiments intended to investigate three-dimensional surface facet temperatures and radiative exchanges within and above the urban canopy layer. The model array consists of scaled “buildings” constructed of hollow concrete masonry blocks with solid capping slabs situated on a rooftop on the campus of Arizona State University in Tempe, Arizona. Three experimental configurations (canyon aspect ratios of 1.0, 0.5, and 0.33) of the 13 m x 13 m array are tested, as is the impact of modifying the roof albedo. The measurement period (November 2006–January 2007) is characterized by clear skies, modest precipitation, low atmospheric humidity, and a large range in diurnal temperature. The performance of the scale model to achieve thermal and radiative similarity requirements is discussed.