Associations between varying magnitudes of the urban heat island and the synoptic climatology in Melbourne, Australia

This paper examines the characteristics of the urban heat island (UHI) in a large Australian city (Melbourne) using local area weather data and the US National Centers for Environmental Prediction (NCEP) reanalyses data of me an sea level pressure (MSLP). Conventional methods to determine the UHI fro m climatological data often involve comparing minimum temperature data betw een urban and rural environments. The findings presented in this paper show that comparing simultaneous measurements of temperature at one urban and t hree nearby airport sites at 0600 (Australian eastern standard time (EST)) provided a better estimate of the UHI magnitude. Analyses of these data bet ween 1973-1991 were grouped according to the daily UHI magnitude. For each UHI group of the daily data, mean anomalous synoptic conditions from the 19 -year mean monthly MSLP were examined for the influence of the different MS LP conditions on varying UHI magnitudes. These synoptic conditions included the entire range of weather conditions over the study period. Over the 19- year period, daily analyses of the regional climatological 0600 EST tempera ture data revealed a UHI between - 3.16 degrees and 6.0 degreesC. The reana lyses of the NCEP MSLP data, in association with the local area climatologi cal data, suggest that statistically significant anomalous anticyclonic con ditions were associated with the warmest 17% and coolest 1% of UHI events. The position of the centre of the anticyclone was critical to UHI genesis a nd development. Statistically significant mean low pressure anomalies were associated with UHI values between 0 degrees and 1 degreesC. These occurred on 40% of the days between 1973-1991. Melbourne's urban area, independent of the topography, was found to inhibit early morning advection events of w arm continent air and result in an urban cool island (UCI). Copyright (C) 2 000 Royal Meteorological Society.