NUMERICAL-SIMULATION OF POLLUTANT DISPERSION IN A SMALL VALLEY UNDER CONDITIONS WITH SUPERCRITICAL RICHARDSON NUMBERS

The numerical drainage wind model of Wong et al. (1987) is used togeth er with a Lagrangian particle model in the simulation of carbon monoxi de (CO) dispersion within a small urban valley in Edmonton, Alberta, C anada. The conditions studied are those of strong static stability whe n vertical mixing is suppressed, These are conditions with the Richard son number exceeding its critical value (hereafter referred to as supe rcritical conditions). Observations showed that under such conditions, vertical turbulence is suppressed but horizontal turbulence still exi sts. The effects of turbulence in the dispersion and transport of poll utants under such conditions are small. However, in the present simula tion, a simple turbulence parameterization based on observations is us ed for supercritical conditions, Some field experiments were performed and the observations are compared with model results, For a location downwind of the CO source, two peaks can be observed during the course of the drainage flow regime, The model results suggest that these rep resent an initial flux from the drainage flow and a second flux later from drainage wind recirculation. Another main feature of the model-pr edicted concentration field is zones of maximum concentration at and a bove the valley floor. There is a drainage wind cell on each side of t he valley slope and the cells are effectively decoupled from the preva iling wind above. The present modelling results show that when the pre vailing wind exists before the development of the drainage wind, it ca n be instrumental in transporting CO from one drainage wind cell to th e other. Otherwise, the CO released within one drainage wind cell is w ell contained.