Short-range diffusion experiments in unstable conditions over inhomogeneous terrain

Equipment to measure the dispersion of a release of the tracer sulphurhexafluoride (SF6) was developed and nine experiments were then performed at the Risø National Laboratory in the period April to July 1976. The tracer was released at 60 m from the Risø meteorological tower, situated on a peninsula in the Roskilde Fjord, Denmark, and was sampled at ground-level positions distributed over a crosswind line about 1 km downwind. During the experiments, the turbulent wind velocity components were measured at the height of release with a three-dimensional ultrasonic anemometer. The experiments were performed in westerly winds, hence the dispersion was influenced by the internal boundary layer developing downwind from the inhomogeneity in surface roughness between the fjord and the site. Five of the experiments, performed in neutral and slightly unstable conditions, were accepted as cases of purely turbulent dispersion of the tracer. Because of wind direction shifts the others were discarded.

The concentration measurements allowed a direct estimate of the lateral dispersion parameter σ_y. The Taylor diffusion formula using the wind velocity measurements at the height of release was found to work well in predicting σ_y, despite the fact that shortcomings in fulfilling the requirements for its application existed. σ_y, predicted by an empirical formula characteristic for an open area (Bultynck and Malet, 1972), was found to be systematically larger than the experimental value. Excellent agreement was found by multiplying the predicted σ_y values by the factor 0.6. This indicates that the effect of surface roughness can be separated from the effect of stability.

Because the estimates of the vertical dispersion parameter, σ_z, were only indirectly inferred from continuity considerations, they could not, although fair agreement was obtained, be used to check the validity of calculating σ_z from Taylor's formula or from the empirical formula mentioned above.