Effects of Climate Change on Cities: Dynamical Downscaling to Pedestrian Scale (Invited Presentation)

The confluence of urbanization and climate change may have adverse effects on the quality of urban life as well as the sustainability of cities, where environmental and anthropogenic factors appear to exacerbate climate change impacts on urban areas. A downscaling study from climate (~ 100 km) to pedestrian (~ 2m) scales was conducted to assess the climate change impacts on Chicago metropolitan area, with focus on the Urban Heat Island (UHI). A model chain was used, which included one way nesting from global climate (Community Atmosphere Model, CAM), to regional climate (Weather Research and Forecasting model, WRF, and its urban version), to microscale (ENVI-met) models. The performance of the nested mesoscale and microscale models were evaluated against the present-day observations, with mesoscale observations obtained from publicly available data sites and microscale measurements from a dedicated field study conducted in downtown Chicago. A simple building-energy model is developed and used in conjunction with the microscale model output to calculate future energy demands of individual buildings. A substantial increase of energy consumption was noted for future (~ 2080) climate.  Potential UHI mitigating strategies such as modifying the rooftops (green and cool roofs) were investigated. Results show that the efficacy of green and cool roofs is dependent on the fraction of roof modification, degree of urbanization as well as background meteorology and turbulence. The lowered wind speeds and vertical mixing near the rooftops during daytime cooling at roofs may lead to stagnation of air close to the surface, causing potential air quality deterioration. The selection of cool and green roofs for UHI mitigation therefore should be done with circumspection, considering the competing attributes of their influence.