Contrasting Effects of Lake Breeze and Urbanization on Heat Stress in Chicago Metropolitan Area
Schematic picture of the coastal urban system (Chicago in particular) and local circulations caused by temperature difference across different land cover types during late afternoon.
Image courtesy of an Argonne National Laboratory in coordination with Jiali Wang.
Cities are more vulnerable to extreme heat than rural areas because of pre-existing urban heat island (UHI) effects. Chicago, surrounded by Lake Michigan and flat farmland, offers an excellent example to see how city life and lake breezes affect temperature. During the summer, the land areas are hotter than Lake Michigan, especially in the afternoon. This temperature difference results in a lake breeze with cooler near-surface air flowing from the lake to the land. This study uses a regional climate model coupled with an urban canopy model and explores the relative effects of the lake and urbanization on heat stress over Chicago. Heat stress describes human physiological response to heat depending on both air temperature and relative humidity, i.e. how hot and humid conditions impact human health.
Models that combine urban landscape and infrastructure features with regional climate patterns help us better understand daily changes in temperature, humidity, and how warm the ground feels. These results show that it's important to consider the complex interactions between the city's surfaces, the lake, and the air above to effectively simulate the urban climate. Although this method is computationally expensive, it provides valuable insights. We also recommend improving direct measurements in cities and enhancing our models, such as including more details about urban greenery. Efforts supported by DOE BER through urban Integrated Field Laboratories are already underway to improve these models.
Results show that the presence of Lake Michigan impacts and widens the difference between the air temperature over the lake and land, driving the lake breeze during the afternoon. This brings cool, moist air to Chicago urban and suburban areas, and disrupts the vertical evolution of the urban boundary layer. Urbanization also enlarges (although not as much as the presence of the lake) the air temperature gradient between lake and urban areas and between rural and urban areas, intensifying the wind speeds over lake, but decelerating the penetration of lake breeze into suburban and rural areas. The lake's cooling effect is strongest when it also raises humidity the most, while urban areas tend to become warmer and drier. At night, urban areas experience more intense heat stress, extending the times when caution is needed due to heat by up to four hours. In contrast, the lake breeze reduces heat stress in the afternoons, shortening caution periods by one to four hours depending on the area.