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

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 small green spaces placed 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.

The Weather Research and Forecasting (WRF) model is used to simulate the Kansas City, MO region using a triple nested grid with an inner domain grid spacing of 300 m so that analysis can occur at a neighborhood-level scale. WRF is run with the Noah Land Surface Model (Noah-LSM), Building Energy Parameterization (BEP), and Building Energy Model (BEM) to account for the combined radiative effects of natural land cover, vegetation, impervious cover, building surfaces, and anthropogenic heat release. Four simulations of extreme heat events between 2011 and 2013 are investigated, and model output is validated with satellite and surface observations. Using vacant property data obtained from the city that has a high fraction of impervious surfaces, several of the most suitable locations for converting vacant lots to green spaces are determined. WRF geographic datasets are modified to simulate conversion of urban to green spaces in these areas. The four control cases are run again with the modified geographic datasets, and the local cooling effect of each location is compared to the initial control runs. Preliminary results show that a downwind cooling impact of over 1°C is possible up to approximately 1 km from 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.