Urban heat mitigation under realistic roof and land area constraints

Abstract To simulate realistic impact of rooftop-based heat mitigation strategies (HMS), we introduce a new capability in the Weather Research and Forecasting model coupled with the building effect parameterization urban physics module. This enhancement allows the model to ingest two dimensional fields of grid aggregated roof albedo and fractional rooftop area availability for HMS implementation. By incorporating these spatially varying inputs, the model overcomes previous limitations that relied on idealized scenarios. Using Austin, Texas, as a testbed metropolitan area, we assessed the realistic potential for cooling the city by reducing the 2 m air temperature ( T 2M ) and universal thermal climate index (UTCI) through various HMS. Specifically, we evaluated cool roofs, green roofs, and solar photovoltaics under two scenarios: (i) complete roof area coverage and (ii) realistic implementation based on the available flat (non-pitched) roof area. Additionally, we implemented urban gardens and street trees and assessed their effectiveness in reducing T 2M and UTCI. All experiments, including a control scenario without any HMS, were conducted during clear-sky days in August 2020, which was one of the hottest months recorded in Austin. The results indicate that while cool and green roofs are effective, their effectiveness is obfuscated in the city-wide mean T 2M and UTCI reduction. Further, the realistic implementation of HMS had a negligible impact on city-wide mean T 2M and UTCI. Nevertheless, these realistic rooftop simulations demonstrated potential for localized cooling in some areas of the city. Combining realistic rooftop-based HMS with urban gardens and street trees similarly resulted in limited city-wide cooling while yielding noticeable cooling in some grid cells. Street trees showed a large potential for locally reducing UTCI. These findings have direct implications for urban planning and heat mitigation approaches, as they highlight the need for evaluating HMS based on local (e.g. urban-block-scale) impacts using realistic implementation constraints, rather than emphasizing city-wide reductions in T 2M or UTCI.