The characteristics of the Chicago Lake breeze and its effects on trace particle transport: Results from an episodic event simulation

The lake-breeze circulation that forms over Lake Michigan during the summer influences the Chicago, Illinois, metropolitan area's weather in several ways. Of particular significance is the circulation's effect on the dispersion of pollutants such as ozone and aerosols produced in and around the city. To investigate these effects, the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5) was used to perform numerical simulations for two lake-breeze events-one in July 1999 and another in July 2002. The model runs were verified with data from several locations around the Chicago area. The simulated breeze circulation decreased the rate of increase in air temperature while penetrating roughly 12 km inland and lasting about 8 h, in reasonable agreement with observations. Furthermore, the inland penetration distance was related to the strength of the maximum vertical velocity within the front. Calculations of trajectories and transport of particles showed that the breeze tended to transport particles trapped within it to the north when release occurred before the circulation came ashore, whereas particles released at the time of the breeze's landfall or afterward moved more northeasterly, in the direction of the prevailing wind. Thirty-four percent of all released particles were trapped by the circulation and raised to a height of at least 300 m, and 20% of the particles remained in the lowest 100 m above the surface. In addition, sensitivity tests showed little change in the modeled breeze when measured surface temperatures for Lake Michigan were used as initial conditions and boundary conditions in the place of surface skin temperature ( as derived by the National Centers for Environmental Prediction). Raising the lake temperatures significantly in the simulation yielded a more elongated vertical circulation and a briefer lake-breeze event that did not reach as far inland.