Urban heat islands during heat waves: a comparative study between Boston and Phoenix

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  • 1 Department of Earth and Environment, Boston University, MA, 02215, USA.
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Abstract

In this study, we simulate the magnitude of urban heat islands (UHIs) during heat wave (HWs) in two cities with contrasting climates (Boston and Phoenix) using the Weather Research and Forecasting (WRF) model and quantify their drivers with a newly developed attribution method. During the daytime, a surface UHI (SUHI) is found in Boston mainly caused by the higher urban surface resistance (rs) that reduces the latent heat flux, and the higher urban aerodynamic resistance (ra) that inhibits convective heat transfer between the urban surface and the lower atmosphere. In contrast, a surface urban cool island (SUCI) is found in Phoenix mainly due to the lower urban ra that facilitates convective heat transfer. In terms of near-surface air UHI (AUHI), there is almost no daytime AUHI in either city. At night, a SUHI and an AUHI are identified in Boston due to the stronger release of heat storage in urban areas. In comparison, the lower urban ra in Phoenix enhances convective heat transfer from the atmosphere to the urban surface at night, leading to a positive SUHI but no AUHI. Our study highlights that the magnitude of UHIs or UCIs is strongly controlled by urban-rural differences in terms of aerodynamic features, vegetation and moisture conditions, and heat storage, which show contrasting characteristics in different regions.

Corresponding author: lidan@bu.edu

Abstract

In this study, we simulate the magnitude of urban heat islands (UHIs) during heat wave (HWs) in two cities with contrasting climates (Boston and Phoenix) using the Weather Research and Forecasting (WRF) model and quantify their drivers with a newly developed attribution method. During the daytime, a surface UHI (SUHI) is found in Boston mainly caused by the higher urban surface resistance (rs) that reduces the latent heat flux, and the higher urban aerodynamic resistance (ra) that inhibits convective heat transfer between the urban surface and the lower atmosphere. In contrast, a surface urban cool island (SUCI) is found in Phoenix mainly due to the lower urban ra that facilitates convective heat transfer. In terms of near-surface air UHI (AUHI), there is almost no daytime AUHI in either city. At night, a SUHI and an AUHI are identified in Boston due to the stronger release of heat storage in urban areas. In comparison, the lower urban ra in Phoenix enhances convective heat transfer from the atmosphere to the urban surface at night, leading to a positive SUHI but no AUHI. Our study highlights that the magnitude of UHIs or UCIs is strongly controlled by urban-rural differences in terms of aerodynamic features, vegetation and moisture conditions, and heat storage, which show contrasting characteristics in different regions.

Corresponding author: lidan@bu.edu
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