An investigation of a dramatic cold outbreak over southeast Australia

Abstract

This paper investigates an episode of three successive cold outbreak events that took place over southeast Australia during May-June 2000. Of these three cold events, the most intense occurred on 27-28 May and was associated with a Melbourne maximum temperature of only 10.2°C. This is the second lowest daily maximum temperature recorded in Melbourne for May since 1958. The other two events, with slightly higher daily maximum temperatures, followed the first event closely. Various data processing techniques along with an air-parcel trajectory model and a sophisticated vortex tracking scheme are utilised to examine several aspects of these cold events, with a particular emphasis on the major event of 27 May. Results show that the synoptic pattern and its temporal evolution leading to the major event were characterised by a persistent anticyclone-cyclone dipole, located south of Australia. On the onset day, the configuration and position of this anticyclone-cyclone pair was such that a strong cold advection from the interior of Antarctica was implied, which was confirmed by computing air-parcel trajectories at the 1000 hPa level. Of the anticyclone and cyclone, the former was found to have a dominant influence on the occurrence and evolution of cold outbreaks over southeast Australia. This was demonstrated by analysing the tracks and the changes in intensity, 'height', and effective radius of the anticyclones that were associated with two of the three outbreak events. Analysis shows that the anticyclones, which were in rapid translational motion initially, became quasi-stationary near southern Australia during the later stages of their lifecycles. At these times, the anticyclones also strengthened significantly, as revealed by increases in their intensity, height, and effective radius. Analyses of the 500 hPa height field show that the above anticyclone-cyclone pair was indeed a part of a large-amplitude hemispheric wave train with a remarkable degree of structural organisation. Much of this organisation was due to the amplification of wavenumber 4 prior to the onset of the cold events. Initially, the wave energy was concentrated mainly in the eastern hemisphere. However, concurrent with the decay of the first cold event, the energy propagated downstream very quickly, resulting in an amplified wave train in the western hemisphere. This downstream wave development was found to continue for an unusually long time, making two revolutions around the hemisphere and, also, triggering two of the three cold outbreak events studied in this paper.