Detection of centripetal heat-island circulations from tower data in St. Louis

Hourly averaged meteorological data gathered by a 25-tower network about St. Louis during 1976 are used in a search for centripetal circulations generated by the urban heat island. Considering data collected when the network resultant speed was less than 1.5 m s-1, two data classes of several hundred hours each are formed. One class is associated with weak heat islands, daytime hours, and convective instability, while the other class is associated with strong heat islands, nighttime hours, and extreme rural stability. Mean centripetal flows are clearly discernible from data of both classes, but the convergence is stronger for the flows associated with the weaker heat islands. This unexpected result is explained in terms of the ease with which sustained vertical motions can be generated over the city by the available forcing under different stability regimes. The detectability of the heat-island influence diminishes very rapidly with increasing speed of the large-scale flow.