Overwater Atmospheric Diffusion: Measurements and Parameterization

Abstract

A series of ten atmospheric tracer experiments provided 62 hours of overwater atmospheric dispersion data. Sulfur hexafluoride (SF6) was released as the tracer gas at a height of 13 m from a ship positioned about 7 km off the central California coast in the vicinity of Pismo Beach. Surface and upper air meteorological measurements were made at the release point and near the shoreline. Horizontal crosswind tracer concentration profiles were obtained at the shoreline, both at the surface and multiple levels aloft through the mixed layer. Tracer data were analyzed to determine the lateral diffusion parameter σy for hourly periods as the best fit to a Gaussian distribution. The vertical diffusion parameter σz was determined from the vertical profile of the hourly crosswind-integrated tracer concentration. Four classification schemes of atmospheric stability were intercompared and showed wide variation in their estimates. The lateral diffusion measurements are reasonably well gratified by three of the four stability classification methods, while the vertical diffusion measurements are not. Four diffusion-parameterization methods were evaluated at the shoreline. Two of the four methods provided consistently good estimates of σy. The vertical diffusion estimates were poor with all methods.