Trends in extreme rainfall indices for an updated high quality data set for Australia, 1910-1998

Abstract

Daily rainfall was analysed at 91 high quality stations over eastern and southwestern Australia to determine if extreme rainfall had changed between 1910 and 1998. Three indices of extreme rainfall were examined: the number of events above an extreme threshold (extreme frequency); the average intensity of rainfall from extreme events (extreme intensity); and the proportion of total rainfall from extreme events (extreme percent). Several problems are discussed associated with designing such indices under a climate with significant trends in the number of raindays. Three different methods are used for calculating the extreme intensity and extreme percent indices to account for such trends in raindays. A separate analysis was carried out for four separate regions with significant results including a decrease in the extreme frequency and extreme intensity in southwest Western Australia and an increase in the extreme percent in eastern Australia. Trends in the extreme intensity and extreme percent are largely dependent on the method used to calculate the index. Total rainfall is strongly correlated with the extreme frequency and extreme intensity indices, suggesting that extreme events are more frequent and intense during years with high rainfall. Due to an increase in the number of raindays during such years, the proportional contribution from extreme events to the total rainfall depends on the method used to calculate this index. Copyright © 2000 Royal Meteorological Society