Multi-scale modelling of Chicago urban heat island and climate change impacts

Urban heat island (UHI) affects densely built cities such as Chicago through acute modification of the local climate across a swath of spatiotemporal scales. UHI causes warmer temperatures in the urban core compared to adjacent rural areas. It negatively affects energy consumption, human comfort, and air quality within urbanized areas. Global climate change may heighten these negative effects of UHI, understanding of which requires studies on the interaction of global climate change and UHI spanning all relevant scales. To our knowledge, none of the available numerical models address the UHI problem from climate to urban canyon scales, and the work to be described attempts to undertake this task by applying a multi-model chain for a comprehensive study of Chicago's UHI. The coupling of global circulation, mesoscale, and micro-scale models has allowed for a dynamical downscaling platform from global to regional to city and finally to neighbourhood scales. In this nesting, output from the Community Atmosphere Model (CAM) of Community Climate System Model (CCSM5) was fed into a Weather Research and Forecasting (WRF) model. WRF model output at 0.333 km resolution drove the microscale ENVI-met model, which provided output at much finer (2 m) resolution within a representative Chicago neighbourhood. A simplified building energy model was developed to evaluate building cooling loads using ENVI-met output. This model chain was validated with observational data from existing meteorological stations and a dedicated field experiment conducted in July and August 2013. The resulting multi-scale modelling tool was applied to analyse climate-change impacts at all the relevant scales.