Tetroon Trajectories in an Urban Atmosphere

Abstract

Tetroon flights at heights near 300 m within the Los Angeles Basin illustrate the useful results to be derived from the use of Lagrangian probes in an urban environment. The tetroon trajectories depict air-flow reversals which could only be obtained with difficulty from fixed-point data, and furthermore yield evidence for a diurnal recirculation of air within the Basin. Of importance from the viewpoint of vertical dispersion is the observation that tetroon-derived root mean square vertical velocities average 0.5 m sec−1 over the land and 0.2 m sec−1 over the water during daylight hours, while the period of vertical oscillation averages 17 min over land and 9 min over water. Broadly speaking, the period of vertical oscillation appears to be a function of lapse rate, in agreement with the Brunt-Väisälä formulation. Two flights released simultaneously indicate the existence of helical circulations within the marine air over the land. These helices have lateral and vertical dimensions of about 600 m and the tetroons complete a circuit in 20–30 min. Examples are presented of tetroon vertical motions over the Palos Verdes Hills.