Urban Influence on a Strong Daytime Air Flow as Determined from Tetroon Flights

Abstract

Tetroon flights across Oklahoma City indicate the influence of an isolated urban area on the horizontal and vertical air velocity at heights near 400 m in relatively strong (13 m sec?1) daytime flow. The Lagrangian measurements so obtained are collated with fixed-point measurements of horizontal and vertical velocity on a 460 m television tower. Above the city in the morning there is a mean trajectory turning toward lower pressure of 10°. This turning, presumably fractionally induced, is noted only weakly in the afternoon and not all in the evening, but there is slight evidence for a bending of the trajectories around the city at these later times. During the day the city appears as the source of a plume of ascending air motion extending at least 30 km downwind of the city, with both tetroon and tower measurements indicating a mean upward velocity of almost 0.4 m sec?1 ten kilometers downwind of city-center at heights near 400 m. On the average the magnitude of the stress determined from the covariance of the eddy velocity components along the tetroon flights is about 70% of the magnitude measured on the tower, and there is a correlation of nearly 0.5 between individual measurements of stress by the two techniques. The magnitude of the tetroon stress is intimately related to building height and density, with a stress maximum of at least 3 dyn cm?2 located 10 km downwind of city-center in comparison with stress values near 1 dyn cm?2 beyond the city outskirts. The fraction of the stress associated with Lagrangian oscillations of 1?10 min period (in comparison with 1?30 min period) increases from 20% upwind of the city to 80% downwind of the city in the daytime average.