Observations and Simulations of the Diurnal Variation of the Urban Heat Island Circulation and Associated Variations of the Ozone Distribution: A Case Study

The observed surface and upper air temperature and wind field patterns on 8 June 1976 in St. Louis, Missouri, were analyzed and compared with simulation results from a three-dimensional hydrodynamic model. An urban heat island (1–2°C temperature difference between the urban and rural regions) persisted during the day. The daytime temperature differential was relatively weak compared to that at night (∼5°C difference). In contrast, the urban heat island circulation was more intense during the day. This is thought to be due to the heating being distributed through a deeper layer. In the early evening, the heat island circulation dissipated due to the development of a surface-based inversion in the boundary layer. The highest concentrations of O3 at the surface were found in the zone of convergence associated with the urban heat island circulation immediately downwind of the center of the city. As the heat island circulation dissipated during the early evening, the area of high O3 concentration was displaced further downstream.