J3.5 OUTDOOR SCALE MODEL EXPERIMENT TO EVALUATE THE COMPLETE URBAN SURFACE TEMPERATURE Sarah M. Roberts 1,* , J.A. Voogt 2 , and T. R. Oke 1 1 University of British Columbia, Vancouver, B.C., Canada; 2 University of Western Ontario, London, ON, Canada 1. INTRODUCTION 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. Insight is also gained from wind tunnel modeling because it allows isolation and simplification of climatic processes through control of the impinging flow and the properties of surface structures (Plate, 1999). Data from both approaches are used to construct, evaluate, and validate numerical models which further aid our understanding of urban climate processes. However, the inherent complexity of surface morphology and energetic exchanges of real-world urban environments still poses challenges to numerical modeling of urban environments. Outdoor physical scale modeling is a potentially powerful compromise between wind tunnel modeling and full-scale observation because it incorporates the experimental control of physical and numerical modeling but preserves some of the real complexities associated with natural environmental forcing (e.g. atmospheric turbulence and radiation loading; Mills, 1997). Here we describe the project design and preliminary results of an open-air physical scale model experiment to investigate three-dimensional (i.e., complete) facet surface temperatures within an idealized urban array. In addition to providing a detailed analysis of the variation in observed facet temperatures, the resulting dataset can be used to evaluate radiation emissions and a model to optimize sensor placement. 2. SCALE MODEL SIMILARITY Given that reduced-scale physical models are unable to fully replicate the complex morphology *Corresponding author address: Sarah Roberts, Dept. of Geography, University of British Columbia, 1984 West Mall, Vancouver, B.C. Canada V6T 1Z2; e-mail: sroberts@geog.ubc.ca of cities and their meteorological exchange processes, it is difficult for these models to satisfy all of the similarity requirements considered important in scale modeling. Geometrical similarity, which is the primary modeling objective of the present experiment, is entirely feasible so long as the collection of roughness elements is scaled to generally match the relative size, dimension, and spatial distribution of real-world urban surface elements (Kanda, 2006). To achieve similarity in reduced-scale modeling of radiation emissions from a densely built-up urban setting it is necessary to ensure the physical processes governing these exchanges are suitably scaled. The characteristic length scale of radiation (10 -7 – 10 -4 m) is considered negligible compared to the linear scale of even a small scale model (Oke, 1981). Hence placing the model outdoors, where it is subjected to the downward components of short- and longwave radiation, ensures similarity of radiation always exists. Matching of surface albedo, surface emissivity, and thermal mass is not rigorously undertaken here, however, actual building materials are used to construct the model. 3. METHODS 3.1 Site The array was constructed on a rooftop at Arizona State University (ASU) in Tempe, Arizona and observations were conducted from November 2006 – January 2007, a period characterized by clear skies, modest precipitation and low atmospheric humidity. These conditions provide a robust regime of daytime heating-nighttime cooling of surfaces representative of those observed at many full-scale arid urban sites. The model assembly consisted of 40 cm x 40 cm x 25 cm scaled “buildings” constructed of hollow concrete masonry blocks with solid capping slabs (Figure 1) situated on a surface constructed of 1.2 m x 1.3 cm x 2.4 m polystyrene sheets overlain with 1.2 m x 1.3 cm x 2.4 m fiberboard sheets. Experimental configurations of the 13 x 13 m array included: three different canyon aspect ratios (H/W = 1.25,