Synoptic comparison of cold events in winter and summerin Melbourne and Perth |
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Authors: | I Simmonds T Richter |
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Institution: | (1) School of Earth Sciences, University of Melbourne, Parkville, Australia, AU |
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Abstract: | Summary Unseasonably cold weather episodes have the potential to cause dislocation to many aspects of society, regardless of the
season in which they occur. In 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 usual 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 Australian cities of Melbourne and Perth. We identify
10 cold events in winter and summer for the cities.
Analyses were performed to determine the synoptic environment in which these 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 summer and six in winter, while seven and eight occurred in the two seasons for Perth. The circulation features and characteristics
of other synoptic types 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 dipole in the regional MSLP pattern, with the high located in the ‘upstream’ quadrant from the anomalous
cyclone. Having said this, the relative importance 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 Melbourne and Perth bear some similarity, but also display a number of significant differences. These differences
are associated partly with the different climatological and synoptic settings in which these cities find themselves, and the
nature of their seasonality.
Received October 10, 1999/Revised April 7, 2000 |
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