The Thermodynamic and Kinematic Structure of the Planetary Boundary Layer for a Summer Lake Breeze Day in Chicago

Abstract The planetary boundary layer (PBL) over Chicago is strongly modulated by interactions between the lake breeze and the urban heat island (UHI). This study focuses on characterizing the thermodynamic and kinematic processes governing the Chicago UHI and its interaction with the cooler air from Lake Michigan. The study uses remote sensing and in situ measurements collected during the CROCUS Urban Canyons campaign that took place in July 2024. Regular rawinsonde launches, together with the University of Wisconsin–Madison University of Wisconsin–Madison Space Science and Engineering Center Portable Atmospheric Research Center (SPARC) trailer equipped with Doppler and high‐spectral‐resolution LiDARS, as well as a thermodynamic profiler, provided a detailed characterization of the PBL structure during a clear‐air summer day with a lake breeze initiating in the late morning hours. Observations revealed a complex evolution of the boundary layer throughout the day, which ultimately resulted in a two‐layer structure: a lake breeze layer forming during the late morning and an inversion layer formed by the lake breeze circulation lifting warm urban air to the top of the PBL. In general, the TROPOe‐derived planetary boundary layer height and Doppler LiDAR layer heights captured the expected behavior of the PBL. These findings underscore the need for careful interpretation and method selection of PBL retrievals in urban lake systems, with implications for BL research and operational applications. While the weather model can capture the general PBL height evolution associated with the lake breeze, the detailed vertical structure influenced by various factors such as haze are not identified.