A PDF Dispersion Model for Buoyant Plumes in the Convective Boundary Layer

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 py and pz of tracer particle position in the lateral y and vertical z directions. The py is assumed to be Gaussian, whereas the pz 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 Cy and convection tank data show fair to good agreement in the lower half of the CBL. In particular, the Cy profiles at the surface agree with the data over a wide range of the dimensionless buoyancy flux F? and show a systematic decrease in Cy 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.