Indications of topographically-induced eddies in stratified flow during a severe air pollution event

During the last week of June 1978, Melbourne experienced a prolonged period of stagnation over the city, resulting in high levels of air pollutants. Internal Froude numbers through the period ranged from approximately 0.1 to 0.3, indicating that flow in the area should be strongly stratified. Wind patterns determined using data from thirteen anemographs within and immediately adjacent to the city revealed a pattern of eddies across the city each afternoon in a cavity in the lee of upstream topography. That pattern evolved with time, and changed between days as the controlling meteorological conditions altered. The available mean sea level pressure data from the region for the same times indicated perturbations in that field which were consistent with the observed wind patterns.The eddy patterns were replaced at the surface overnight by katabatic winds as cooling took place and a ground-based inversion became reestablished. However, mean sea level pressure data and the anemograph records suggest that the eddy pattern may have continued overnight, aloft.Considerable recycling of air occurred, both within individual circulations and by wind reversal between daytime and nocturnal regimes. This aided the persistence of high pollutant concentrations.