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

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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.