A COMPUTATIONAL MODEL FOR THE RISE AND DISPERSION OF WIND-BLOWN, BUOYANCY-DRIVEN PLUMES .3. PENETRATION OF ATMOSPHERIC INVERSION

The computational model of Zhang and Ghoniem (1993, Atmospheric Enviro nment 27, 2295-2311; 1994, Atmospheric Environment, 28, 3005-3018) is applied to simulate the dispersion of a windblown, buoyancy-driven plu me in a stratified atmosphere characterized by a sharp density drop ac ross a thin horizontal inversion layer. Results show that the plume ma y completely penetrate, partially penetrate or get fully trapped below the inversion layer depending on the plume buoyancy, the height, stre ngth, and thickness of the inversion layer. Plume penetration is favor ed by the lower height, weaker strength and larger thickness of the in version layer. The presence of inversion accelerates the plume bifurca tion into two diverging, downwind drifting material lumps, supported b y the formation of two counter-rotating streamwise eddies, below the i nversion. As a plume impinges on an inversion, internal gravity waves are generated along the layer, absorbing some of the plume energy and reducing its penetration potential. Concomitant with the wave activiti es, re-entrainment plays an important role in determining the final eq uilibrium height and the generation of weak oscillations in the plume trajectory, ambient circulation and trapping fraction. The mechanism o f baroclinic-vorticity generation is used to interpret various buoyanc y-related phenomena in this problem. The plume trapping fraction, defi ned as the percentage of plume material held below the initial inversi on height, is calculated under different conditions. Comparison with l aboratory experiments shows that the predicted terminal trapping fract ion agrees well with the measurements.