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 during this period ranged from similar to 0.1 to 0.3, indicating that flow in the area should be strongly stratified. Wind patterns, determined by using data from 13 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 m.s.l.( mean sea level) pressure data from the region for the same period of time indicated perturbations in that field that were consistent with the observed wind patterns. Overnight, the eddy patterns were replaced at the surface by katabatic winds, as cooling occurred and a ground-based inversion became reestablished. However, m.s.l. pressure data and the anemograph records suggest that the eddy pattern may have continued aloft overnight. 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.