A Study of Concentration Fluctuations in Instantaneous Clouds Dispersing in the Atmospheric Surface Layer for Relative Turbulent Diffusion: Basic Descriptive Statistics

A series of tracer experiments studying the statistical properties of concentration fluctuations in clouds dispersing in the atmospheric surface layer is described and analyzed. Experiments were conducted at downwind fetches between about 200 and 1200 m, under a wide range of atmospheric conditions ranging from very unstable to moderately stable stratification. The present experiments have addressed basic requirements not met by past field experiments involving instantaneously released clouds; namely, the experiments provided repeat realizations of instantaneously released clouds measured with high-resolution concentration detectors, accompanied by the contemporaneous acquisition of high-quality meteorological and turbulence measurements.Extensive analyses are performed on the cloud concentration data in the framework of relative diffusion. Ensembles of cloud concentration realizations have been constructed. From these ensembles, crosswind and time profiles of the ensemble-mean concentration, concentration variance, ensemble-mean dosage, and dosage variance are obtained. The behaviour of the time profiles of the integral time scale of cloud concentration fluctuations is studied. The use of surface-layer similarity theory for the analysis of the downwind variation of a number of cloud quantities (e.g., cloud size and duration, cloud centre ensemble-mean concentration and dosage, cloud centre concentration and dosage variance, cloud centre integral time scale) is shown to be an effective basis for ordering these quantities. Furthermore, a number of approximate universal relationships describing the behavior of these cloud quantities has been derived. Finally, it is shown that the scaled crosswind and time profiles of ensemble-mean concentration and concentration variance as well as the scaled time profiles of the concentration fluctuation integral time scale exhibit self-similar forms that are independent of atmospheric stratification and downwind fetch.

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