In the previous study of air pollution over Nashville, Tennessee, during the southern Oxidants Study campaign in July 1995, we found significant differences in mixing height between the subregion to the northwest of the city and that in the other quadrants. These results were based on airborne lidar and ground-based radar wind-profiler measurements of mixing height during a 3-day stagnation episode. The differences were ascribed to differences in the land-use and geology between the subregions.

If the mixing-height variation was indeed due to surface heat-flux differences resulting from land-use variability, then these differences should diminish as wind speeds increase. We used profiler data for all dry days of the campaign, when we could get good mixing height measurements from the profiler in each subregion. The results were that the difference essentially vanished when the boundary-layer wind speed reached 6 m/s. Explanations based on blending-height arguments did not convincingly explain why the mixing-height differences went away at these wind speeds. Explanations involving the increase in mechanical mixing at higher wind speeds over the rough forest canopy where of the right magnitude, however. Mechanical mixing is more uniform over the region and does not respond to local variations in heat flux.