Surface Urban Heat islands during Heat Waves: a Comparative Study between Boston and Phoenix

Abstract:

Urban heat islands (UHIs), coupled with heat waves (HWs), exacerbate thermal risks for the soaring urban population. Understanding the drivers of UHIs during HWs can help design strategies for mitigating extreme heat in urban environments. In this study, we simulate the magnitude of surface UHIs (SUHIs), or surface urban cool islands (SUCIs), during HWs in Boston and Phoenix using the Weather Research and Forecasting (WRF) model and quantify their drivers with a newly developed attribution method. Results show that during the daytime, a strong SUHI is observed in Boston but a moderate SUCI effect is observed in Phoenix. We find that the daytime SUHI in Boston is mostly caused by the higher aerodynamic and surface resistances of urban land compared to rural land (forests), while the daytime SUCI in Phoenix is primarily caused by the lower aerodynamic resistance of urban land compared to rural land (shrubs). At night, SUHIs are observed in both cities but the dominant drivers are different. In Boston, the nighttime SUHI is predominantly caused by the stronger release of urban heat storage; while in Phoenix it is largely due to the lower urban aerodynamic resistance. Our study highlights that the magnitude of SUHIs or SUCIs is strongly controlled by urban-rural differences in terms of aerodynamic features, moisture availability, and heat storage, which show contrasting characteristics in different metropolitan regions.