A Stochastic Time Series Model for Threshold Crossing Statistics of Concentration Fluctuations in non-Intermittent Plumes |
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Authors: | Shuming Du David J Wilson Eugene Yee |
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Institution: | (1) Earth Tech. Inc., 196 Baker Avenue, Concord, MA, 01742-2167, U.S.A.;(2) Department of Mechanical Engineering, University of Alberta, Edmonton, Canada, T6G 2G8;(3) Defence Research Establishment Suffield, Medicine Hat, Canada, T1A 8K6 |
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Abstract: | A numerical stochastic model is developed for the upcrossing rate across a specified threshold concentration. The model assumes that the concentration time series at a given spatial point within a dispersing plume can be approximated as a first-order Markovian process designed to be consistent with a given time-invariant concentration probability density function (pdf). The model requires only the specification of a concentration pdf with a given mean and variance and a concentration fluctuation integral time scale. Predicted upcrossing rates are compared with atmospheric plume concentration data obtained from a point source near the ground. For this data set, a log-normal pdf is found to give better estimates of the threshold crossing rate than a gamma pdf. |
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Keywords: | Concentration time series Concentration fluctuations Upcrossing rate |
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