A HYBRID LAGRANGIAN DISPERSION MODEL FOR ELEVATED SOURCES IN THE CONVECTIVE BOUNDARY-LAYER

A hybrid Lagrangian dispersion model is presented for predicting the m ean (C) and root-mean-square fluctuating (sigma(c)) concentrations fro m elevated sources in the convective boundary layer (CBL). The model i s based on Gifford's (1959, Adv. Geophys. 6, 117-138) meandering plume concept and includes: (1) a Lagrangian stochastic model for treating the plume meander caused by the large-scale convective turbulence, and (2) an entrainment model to determine the plume rise and growth relat ive to the meander. The hybrid model is currently limited to weakly-to -moderately buoyant plumes as defined by F()less than or similar to 0 .1, where F- is the dimensionless buoyancy flux: F-*=F-b/U(*)(2)h, wh ere F-b is the stack buoyancy flux, U is the mean wind speed, w() is the convective velocity scale, and h is the CBL depth. The modeled cro sswind-integrated concentration (C-y) and sigma(c)/C frelds showed fai r agreement with the laboratory convection tank measurements of Deardo rff and Willis (1984, Atmospheric Environment 18, 1297-1309; 1988, Lec tures on Air Pollution Modeling, pp. 357-384). The C-y fields were mod eled both with and without relative dispersion (sigma(r)). The C-y fie lds with sigma(r) included showed better agreement with the measuremen ts in the lower half of the CBL than those without sigma(r), and they demonstrated the importance of including entrainment due to ambient tu rbulence in the inertial subrange. Plume rise and buoyancy effects wer e most significant for dimensionless distances X = w()x/Uh less than or similar to 2-3, where the near-surface sigma(c)/C ranged from 1 to 7; x is the downwind distance. For the same X range, measurements in p ower plant plumes showed that sigma(c)/C less than or similar to 2 and hence smaller than the values in the laboratory. A possible reason fo r the difference is the small Reynolds number in the convection tank.