Regional Climate Impacts of the Chicago Urban Heat Island

Climate change will likely have significant environmental and societal impacts on the Chicago metropolitan area. Trends in temperature and humidity within the city are projected to be exacerbated by urban surface characteristics. This study uses an atmospheric transport model to characterize how air parcels moving through Chicago’s urban setting are affected by the city’s anomalous urban heat island (UHI). These patterns are related to variations in land use and urban morphology through a comparison to non-urban sites. The methodology developed provides an opportunity to explore the vertical structure of the urban atmosphere and evaluate the simulation of the urban heat island effect within numerical models used for weather prediction. The analyses presented refine the conceptual model of the vertical atmospheric structure of the UHI proposed by Oke (1982) and helps define the scale of heat transport from the Chicago urban environment to the region as an urban heat “plume” (Clarke, 1969). The results suggest seasonal differences in the intensity of the Chicago UHI as related to the behavior of the atmospheric boundary layer over the urban environment. Heat transport from the Chicago urban environment over land is most pronounced at night and can persist for up to ~70 km during the summer and winter. This movement of heat is defined in the results by a temperature changes in the atmosphere by up to ~8°C during the summer. Lake Michigan is estimated to reduce this advection of heat by up to ~40 km over the lake surface. The vertical scale of the UHI is detected in winter nighttime results, and warming is present within the boundary layer up to ~400 m AGL. These findings contribute to the definition of the urban heat island at a spatial and temporal resolution beyond traditional measurement techniques. Quantification of the scale of the urban heat island contributes to the understanding the impact of urban development on regional climatology and has implications for urban planning, air quality, pollution dispersion, weather forecasting, thermal comfort and human health.