Urban Influence on Nighttime Airflow Estimated from Tetroon Flights

Constant-volume balloon (tetroon) flights tracked by radar across Columbus, Ohio, in March 1969, illustrate the effect of a city on the nighttime airflow at heights of 100–200 m. On the average, the urban influence on wind direction is small at a height of 100 m, but an anticyclonic turning of 10° is observed at 200 m. The anticyclonic turning is greater under inversion than under lapse conditions and greater after midnight than before; it appears to result both from an increase in the frictional force due to increased vertical mixing and from a mesoscale high pressure system formed aloft as the result of the warmer temperatures within the city. The decrease in wind speed across the city averages nearly 20% of the upwind speed under lapse conditions but is very small under inversion conditions. In both cases the region of maximum deceleration tilts downwind with height. The average upward air motion exceeds 4 cm sec−1 above the down-town area under light wind conditions, and increases to 1 m sec−1 as the wind speed approaches 20 m sec−1. In the case of strong winds, alternating regions of upward and downward motion occur downwind of the city.