Cut-off low pressure systems over southern Australia: Climatology and casestudy

Cut-off low-pressure systems are among the most important weather systems t hat affect southern Australia. They can bring moderate to heavy rainfall ov er large areas and as such are extremely important to the Australian agricu ltural industry. There are two components of this study. First, a statistic al analysis is carried out on 14 years of archived data (1983-1996) to prod uce a climatology. The emphasis in the climatology is on the frequency, dur ation and tracks (location and movement) of cut-off lows in southern Austra lia. Second is a case study of the cut-off low of late August 1997, in whic h the distinctive features of cut-off lows are highlighted. The results fro m the climatology reveal the following: there is a maximum in the frequency of cut-off lows in southern Australia during May-October; the most active area for the genesis of cut-off lows in southern Australia is the southwest ern region; and the vast majority of cut-off lows move either eastward or s outheastward. The case study is aimed at revealing details of the formation and structure of one particular 'classical' example of a cut-off low. It i s found that there is strong baroclinicity in the mid-upper troposphere, fa vourable for the intensification of the low. The synoptic analysis also sho ws that the moisture supply for the precipitation was transported from the tropical ocean (the Coral Sea) to higher latitude regions by a low level je t (LLJ), forming a narrow moist tongue which interacted with the cut-off lo w and its associated cold front. The cut-off low was located at the right-f ront quadrant of an upper level jet (ULJ), which is a favourable position f or low system development. Copyright (C) 1999 Royal Meteorological Society.