The Categorisation of Synoptic Scale Circulations over the Australian Region and their Influence on the Urban Heat Island in Melbourne

Chris Gerbing

Melbourne (Australia) has experienced significant urban development in recent decades with a projection for more rapid growth in the immediate future. This study investigates how the Urban Heat Island (UHI) in Melbourne has changed historically over the period X to Y and specifically considers its interaction with synoptic circulations. Automatic Weather Station (AWS) data was used to reconstruct the summer and winter UHI intensities using representative urban and rural stations. An historical trend of increasing UHI was noted in both seasons above and beyond any enhanced global warming.  This suggests that Melbourne’s urban development has been progressively altering the local urban climate. Annually the mean UHI over the period X to Y was 1.79°C in winter and warmer than in summer which was measured at 1.54°C.

Synoptic conditions are known to influence the magnitude of the UHI. Their influence is highly dependent on the physical structure of the urban environment as well as the natural variation of their frequency. This study is unique in that it uses an objective Self Organising Map (SOM) classification scheme to develop seasonal synoptic cliamtologies for the Australian region which are then used as categories for investigation of UHI. The UHI mean was calculated for each synoptic category (node) and revealed that in summer highest UHI events occurred when light southerly winds were blowing and in winter when a large anticyclonic system was dominant over Melbourne. A recent change in the trend for winter was observed, with highest UHI being associated with predominantly westerly flow, which may be linked to changes in the Southern Hemisphere Annular Mode.

The UHI in Melbourne is likely formed by the differences in thermal properties of urban and rural environments as well as the increased influence of anthropogenic waste heat from combustion processes and air-conditioning. However, this study shows that the magnitude of the UHI can be greatly modulated by synoptic weather conditions.   Therefore the relationships found in this thesis between UHI and synoptic scale circulation should be further quantified and strongly considered in the future urban planning endeavours of the city of Melbourne.

Prof Jason Beringer and Prof Amanda Lynch