Using Numerical Simulations to Assess Urban Heat Island Mitigation by Converting Vacant Areas into Green Spaces

Abstract

Impervious surfaces and buildings in the urban environment alter the radiative balance and energy exchange in the boundary layer, increasing sensible heat flux and decreasing latent heat flux near the surface. This typically results in a positive temperature anomaly known as the urban heat island (UHI). The UHI has been attributed to increases in heat related-illness and mortality. Continued urbanization and anthropogenic warming will enhance the magnitude of UHIs worldwide in the coming decades, raising the need for viable mitigation strategies. Observational studies indicate that green spaces within urban areas can reduce local surface temperature by increasing evaporative cooling and latent heat flux, suggesting that implementing such spaces on a widespread scale may be a viable option to lessen the impacts of the UHI. This work explores the potential impact on the UHI if existing vacant lots are converted to green spaces. The Weather Research and Forecasting (WRF) model was used to simulate the Kansas City, MO region with an inner domain grid spacing of 300 m that allows for block-level analysis. Within WRF, the Single Layer Urban Canopy Model (SLUCM) accounts for the combined radiative effects of natural land cover, vegetation, impervious cover, and building surfaces. Three simulations of summertime heat wave events between 2011 and 2013 are investigated, and model output was validated with surface observations. Using vacant property data and identifying places with a high fraction of impervious surfaces, the most suitable "focus area" for converting vacant lots to green spaces was determined. WRF geographic datasets were modified to simulate varying degrees of realistic conversion of urban to green spaces in these areas. The three control cases under each greening strategy were repeated with the modified geographic datasets, and the local cooling effect using each strategy was compared to each initial control run. Results show that under more aggressive greening strategies, a mean local cooling impact of 0.5 to 1.0 ◦C was present within the focus area itself during the nighttime hours following the development of the stable nocturnal boundary layer. Furthermore, additional cooling via the "park cool island" is of up to 1.0 ◦C possible up to 1 km downwind of the implemented green spaces. Quantifying the thermal impact of converting vacant lots with impervious surfaces to green spaces is an additional factor that can be taken into consideration by policy makers when considering the abatement of the UHI. It is hoped that the focus of this study will serve as guidance to both planners and atmospheric scientists alike as part of the effort to promote future sustainable cities.