Forward-in-time and Backward-in-time Dispersion in the Convective Boundary Layer: the Concentration Footprint |
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| Authors: | Xuhui Cai Monique Y Leclerc |
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| Institution: | (1) Department of Environmental Sciences, Peking University, Beijing, 100871, China;(2) Lab for Environmental Physics, The University of Georgia, Griffin, GA 30223, USA |
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| Abstract: | The turbulence field obtained using a large-eddy simulation model is used to simulate particle dispersion in the convective
boundary layer with both forward-in-time and backward-in-time modes. A Lagrangian stochastic model is used to treat subgrid-scale
turbulence. Results of forward dispersion match both laboratory experiments and previous numerical studies for different release
heights in the convective boundary layer. Results obtained from backward dispersion show obvious asymmetry when directly compared
to results from forward dispersion. However, a direct comparison of forward and backward dispersion has no apparent physical
meaning and might be misleading. Results of backward dispersion can be interpreted as three-dimensional or generalized concentration
footprints, which indicate that sources in the entire boundary layer, not only sources at the surface, may influence a concentration
measurement at a point. Footprints at four source heights in the convective boundary layer corresponding to four receptors
are derived using forward and backward dispersion methods. The agreement among footprints derived with forward and backward
methods illustrates the equivalence between both approaches. The paper shows explicitly that Lagrangian simulations can yield
identical footprints using forward and backward methods in horizontally homogeneous turbulence. |
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| Keywords: | Concentration footprint Convective boundary layer Forward and backward dispersion Lagrangian stochastic model Large-eddy simulation |
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