The Southerly Burster of South Eastern Australia: An Orographically Forced Cold Front

Abstract

This paper presents an observational study of the ?southerly burster?, or ?southerly buster?, a particularly intense type of orographically and thermally influenced cold front which occurs in spring and summer along the southern coast of New South Wales in eastern Australia. It may have counterparts elsewhere. A brief review is given of the synoptic aspects of seventeen such fronts which occurred during the period January 1972 to January 1978. These had diverse origins, only rarely being fronts of Southern Ocean origin; mostly they developed ahead of Southern Ocean fronts. In all cases orographical effects appear to play an important role in genesis. A more detailed study is made of two southerly bursters which occurred during an observational program held from 21 November 1982 until 13 December 1982, providing the synoptic background for a more detailed analysis of the structure of these phenomena in another paper. The first was particularly severe, producing wind gusts of up to 24.7 m s?1 and temperature changes of more than 21°C within an hour of its passage. Blocking of the front by the mountains of southeastern Australia was evident. A major feature of the second burster event was the multiplicity of prefrontal wind changes on the southern New South Wales coast and a roll vortex was observed in the postfrontal air. Most of the pressure changes in the postfrontal air of both fronts was attributable to the density difference between the pre- and postfrontal air masses and the rate at which the cold air deepened. Propagation speeds were also strongly affected by changes in the density difference. Twenty-four hour predictions using the ANMRC fine mesh numerical model showed favorable comparisons with mesoscale mean sea level pressure analyses, but the model was less successful in simulating the strong postfrontal pressure gradients and it underestimated the speed of fronts.