Comparison of Results from a Meandering-Plume Model with Measured Atmospheric Tracer Concentration Fluctuations

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  • a Department of Environmental Engineering, Montana College of Mineral Science and Technology, Butte, Montana
  • | b Laboratory for Atmospheric Research, Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington
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Abstract

Measured wind-azimuth data are used in a simple meandering-plume model to predict observed SF6 concentration fluctuations measured downwind of a point source during a range of stability conditions. The meander component of plume diffusion is calculated as the running mean of wind-azimuth signal using travel time as the smoothing time. Instantaneous plume coefficients are derived from measured peak instantaneous concentrations in a subset of the data for distances less than 1 km. These empirical plume coefficients are shown to be a linear function of the standard deviation of the residual azimuth signal after the meander component is removed (σθI = 0.285 σ¯θR). Instantaneous concentration time series predicted with the model using the observed azimuth data are quite similar to observed time series. Sensitivity analyses indicate that the model predictions are strongly dependent upon the averaging period used to filter the azimuth signal but that the instantaneous plume width is only a weak function of the averaging period. Overall, the model predictions are within a factor of 2 or better for the concentration mean, peak-to-mean concentration ratio, plume intermittency factor, and concentration-fluctuation intensity during stable conditions when meander dominates and the instantaneous plumes are narrow.

Abstract

Measured wind-azimuth data are used in a simple meandering-plume model to predict observed SF6 concentration fluctuations measured downwind of a point source during a range of stability conditions. The meander component of plume diffusion is calculated as the running mean of wind-azimuth signal using travel time as the smoothing time. Instantaneous plume coefficients are derived from measured peak instantaneous concentrations in a subset of the data for distances less than 1 km. These empirical plume coefficients are shown to be a linear function of the standard deviation of the residual azimuth signal after the meander component is removed (σθI = 0.285 σ¯θR). Instantaneous concentration time series predicted with the model using the observed azimuth data are quite similar to observed time series. Sensitivity analyses indicate that the model predictions are strongly dependent upon the averaging period used to filter the azimuth signal but that the instantaneous plume width is only a weak function of the averaging period. Overall, the model predictions are within a factor of 2 or better for the concentration mean, peak-to-mean concentration ratio, plume intermittency factor, and concentration-fluctuation intensity during stable conditions when meander dominates and the instantaneous plumes are narrow.

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