The Wave Propagation Laboratory (WPL) dual-Doppler radar system has been used in METROMEX to study the effect of a large urban area (St. Louis) on air motions within the planetary mixing layer during clear conditions. It was demonstrated that aircraft-dispensed chaff could be used to trace the air motion over a several hundred square kilometer area and through the entire depth of the developed mixing layer. Data from two days having similar temperature and wind profiles have been selected for discussion. Both data sets suggest the presence of horizontal roll vortices existing throughout the duration of the observations (20–70 min) and beyond the extent of the chaff cloud (8–12 km). A comparison with the temperature field reveals that a line of convergence (∼10−3 s−1) 300 m above the surface was found to coincide with a thermal ridge of about 0.2°C measured by instruments on the University of Wyoming aircraft. Differences in the apparent roll structures on these two days may be explained in terms of the cross-roll shear and buoyancy resulting from the elongated urban heat island. Also, a persistent localized region of relatively high convergence was observed on the convergence line several kilometers downwind from an area of concentrated industrial activity at Granite City.