Evaluating the Thermal Footprint of Rooftop Heat Island Mitigation Strategies

Extreme heat is an increasingly important challenge for cities due both to urban growth and global climate change. As a result, city and regional governments are exploring various strategies to mitigate the health, air quality, and energy consequences of extreme heat events. A commonly proposed mitigation strategy is increasing the albedo of roofs and paved surfaces. While a number of studies have attempted to quantify the potential of high-albedo roofing to cool cities at a regional scale, most have ignored or poorly represented the role of roof height and the complexity of mixing processes within the urban canopy. This study seeks to estimate the downwind cooling footprint associated with increasing albedo of roof surfaces and explores how this footprint is affected by urban morphology and local meteorological conditions. This presentation will focus on two aspects of a multi-faceted investigation of this question. Specifically, we will discuss CFD simulations constructed to model wind flow through a hypothetical urban area and parallel wind tunnel measurements being conducted in a boundary-layer environmental wind tunnel located on the Portland State University campus. In both cases the urban area is represented by a uniform array of buildings. Simulations and experiments are conducted for a control case in which all buildings have dark roofs and mitigation cases in which one or several buildings on the upwind side of the array experience a significant increase in rooftop albedo. Our results suggest that building heights play an important role in determining street-level efficacy of the implementation of high-albedo rooftop mitigation strategies. Thus, simplistic representation of high albedo roofing in regional scale atmospheric models may overestimate the ground-level cooling effects. Further research is needed, however, to more fully explore the role of turbulent-transport at neighborhood scales before drawing broad conclusions regarding the extent to which albedo modification can address thermal conditions within urban canyons.