Synoptic comparison of cold events in winter and summer in Melbourne and Perth

Unseasonably cold weather episodes have the potential to cause dislocation to many aspects of society, regardless of the season in which they occur. L n this work we devise a method for quantitatively identifying extreme cold events in such a way that it is not biased to the winter season (as is usua l in most other studies). We have applied this method to the daily maximum temperatures (over the period January 1972 to June 1991) in the southern Au stralian cities of Melbourne and Perth. We identify 10 cold events in winte r and summer for the cities. Analyses were performed to determine the synoptic environment in which thes e events occurred. The most common synoptic type in these samples was the ' classic', which is characterised by, amongst other factors, the passage of a cold front over the city on the day of the outbreak, and the transport of air from subantarctic latitudes. Melbourne recorded five such events in su mmer and six in winter, while seven and eight occurred in the two seasons f or Perth. The circulation features and characteristics of other synoptic ty pes identified with these episodes is also examined. The mean synoptic anomalies which are coincident with these cold events are analysed. For both cities and seasons there is a 'high-low' anomalous dipo le in the regional MSLP pattern, with the high located in the 'upstream' qu adrant from the anomalous cyclone. Having said this, the relative importanc e of the two features of the dipole in being associated with the cold event strongly depended on the city and season under consideration. The research shows that the regional structures associated with cold events in Melbourn e and Perth bear some similarity, but also display a number of significant differences. These differences are associated partly with the different cli matological and synoptic settings in which these cities find themselves, an d the nature of their seasonality.