Abstract
A probability density function (PDF) dispersion model is presented for buoyant plumes in the convective boundary layer (CBL), where the mean concentration field C is obtained from the PDFs p_{y} and p_{z} of tracer particle position in the lateral y and vertical z directions. The p_{y} is assumed to be Gaussian, whereas the p_{z} is derived from the the vertical velocity PDF, which is skewed. Three primary sources contribute to the modeled C field: 1) the “direct” or real source at the stack, 2) an “indirect” source to account for the slow downward dispersion of lofting plumes from the CBL top, and 3) a “penetrated” source to treat material that initially penetrates the elevated inversion but later fumigates into the CBL. Image sources are included to satisfy the zero-flux conditions at the ground and the CBL top.
Comparisons between the modeled crosswind-integrated concentration fields C^{y} and convection tank data show fair to good agreement in the lower half of the CBL. In particular, the C^{y} profiles at the surface agree with the data over a wide range of the dimensionless buoyancy flux F∗ and show a systematic decrease in C^{y} with F∗.
Comparisons between the modeled and observed ground-level concentrations around several power plants exhibit good agreement on average and are considerably better than those obtained with a standard Gaussian plume model. A residual analysis suggests some areas for future model development.
Corresponding author address: Dr. Jeffrey C. Weil, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307-3000.