Urban Heat Mitigation Under Realistic Constraints: A Case Study in Austin, TX

As urban heat stress affects both the physiological and psychological health of people, many city administrations are planning to implement heat mitigation strategies that include built and green infrastructure as a part of their action plan. Using Austin, TX, as a representative metropolitan area, this study explored the potential for realistically reducing 2-meter air temperature (T2M) and the Universal Thermal Climate Index (UTCI) in cities through heat mitigation strategies (HMS) implemented in the Weather Research and Forecasting model coupled with urban physics (WRF-Urban). We tested cool roofs, green roofs, and solar photovoltaics under two scenarios: full rooftop coverage and a more realistic implementation based on available flat roofs. In addition to rooftop-based HMS, we also evaluated the effectiveness of urban gardens and street trees in reducing T2M and UTCI. All experiments, including the control scenario (without any HMS applied), were conducted during clear-sky days in August 2020, which is one of the hottest months.      For the simulation of realistic rooftop-based HMS, we introduced a new I/O feature in the building effect parameterization model within WRF-Urban to handle 2-D fields specifying the fractional rooftop areas available. Results showed that while cool and green roofs are effective in some neighborhoods, their realistic implementation led to negligible changes in city-wide T2M. Combining rooftop-based HMS with gardens and trees also showed similar results and had limited impact. Further, HMS had a minimal impact on reducing the city-wide UTCI. Based on these findings, we argue that heat mitigation strategies must be hyper-locally targeted to areas with high heat risk (e.g., streets or playgrounds), as mitigating heat risk at the city scale may be extremely challenging and costly.