Analysis of the Dispersion of Smoke released in the Night-time Surface Layer

This study reports on observations of the dispersion characteristics of smoke released in the night-time surface layer of Cabauw (Netherlands). A smoke generator connected to a 10m stack was employed to continuously release smoke simultaneously at three heights (3, 6 and 9m respectively). The resulting smoke plumes were photographed during the night at a frame rate of max 30 frames per second. The images were later post-processed to diagnose the dispersion characteristics with the aim to relate the measured dispersion coefficients to the local meteorological conditions as observed at the Cabauw measurement tower, in particular the stability parameter z/L and gradient Richardson number. Apart from regular (Gaussian plume like) dispersion behavior, the observations also reveal striking intermittent periods with vigorous mixing events taking place when the gradient Richardson number is far above its critical value. The smoke visualizations also reveal other atmospheric phenomena (waves, inertial oscillations, etc) which would otherwise have been hard to interpret from point-measurements alone. Finally we report on our attempts to release "smoke-puffs" rather continuous smoke plumes.