Flow Across an Urban Area Determined from Double-Theodolite Pilot Balloon Observations

Abstract

Double-theodolite pilot balloon observations at 6 min intervals at two stations upwind and two stations downwind of Oklahoma City provide new information concerning the influence, at heights up to 700 m, of an isolated urban area on a strong (12 m s−1 daytime air flow. Constant volume balloons (tetroons) flown along the line of the pilot balloon stations provide vertical velocity information. During the three days of the experiment the wind speed was a maximum downwind of the city, and the tetroon height traces suggest that this speed maximum results from the downward transport of the faster moving air aloft due to sinking motion in the lee of the city induced by the barrier or heat island effect of the city. The derived wind speed and direction fluctuations (of at least 15 min period) are a maximum near city-center, with low-level speed fluctuations 60% greater than over rural areas. Lagged correlations between stations suggest that, over and upwind of the city, these speed fluctuations are transported approximately with the mean wind, but downwind of the city the fluctuations occur nearly simultaneously at different locations, presumably indicating the effect of the city in breaking up coherent, traveling flow structures.