Measurements of Lagrangian Atmospheric Dispersion Statistics over Open Water

Abstract

Atmospheric dispersion statistics in the Lagrangian frame have been evaluated over open water by using a double-theodolite system to track neutrally buoyant balloons released a few kilometers off-shore during onshore winds. Analysis of the trajectories recorded in various atmospheric stabilities finds Lagrangian integral time scales corresponding to Pasquill stability categories C, D and E equal, respectively, to 9.0, 7.3 and 8.1 s for lateral dispersion and 2.3, 5.3 and 6.6 s for vertical dispersion. Normalized standard deviations of component velocity fluctuations (i.e., σv/u* and σw/u*) for stability categories C, D and E are found to be 3.7, 1.8 and 2.4 for lateral motion and 2.2, 1.3 and 1.2 for vertical motion. Equivalent dispersion coefficients (σy and σz) appropriate to flow over water are observed to undergo relatively less variation with stability than do those measured in flow over land. When compared to estimates derived from the Pasquill-Gifford curves for estimating dispersion over flat grassland, the dispersion coefficients over water are, in effect, shifted about two categories toward the stable side for the vertical component and between one and two categories toward the stable side for the lateral component.