Abstract
Abstract. Using data for a common period (1976–2005) for a set of 31 sites located in south-east Australia, variations in frequency and magnitude of intense rainfall events across durations from 6 min to 72 h were assessed. This study was driven by a need to clarify how variations in climate might affect intense rainfall and the potential for flooding. Sub-daily durations are of particular interest for urban applications. Worldwide, few such observation-based studies exist, which is mainly due to limitations in data. Analysis of seasonality in frequency and magnitude of events revealed considerable variation across the set of sites, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. Both these factors are relevant when assessing the potential effects of climate variations on intense rainfall events. The set of sites was therefore split into groups ("north cluster" and "south cluster") according to the characteristics of intense rainfall events. There is a strong polarisation in the nature of changes found for the north cluster and south cluster. While sites in the north cluster typically exhibit decrease in frequency of events, particularly in autumn and at durations of 1 h and longer; sites in the south cluster experience an increase in frequency of events, particularly for summer and sub-hourly durations. Non-stationarity found in historical records has the potential to significantly affect design rainfall estimates. An assessment of quantile estimates derived using a standard regionalisation technique and periods representative of record lengths available for practical applications show that such estimates may not be representative of long-term conditions, so alternative approaches need to be considered, particularly where short records are concerned. Additional rainfall information, in particular radar data, could be used for an in-depth spatial analysis of intense rainfall events.
Highlights
In a companion paper (Jakob et al, 2011) we outlined how design rainfall estimates are derived under the assumption of a stationary climate
Analysis of seasonality in frequency and magnitude of events revealed considerable variation across the set of sites, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. Both these factors are relevant when assessing the potential effects of climate variations on intense rainfall events
Deriving design rainfall estimates under climate variability and climate change raises two questions: firstly, are climate variations driving changes in rainfall extremes that significantly affect design rainfall estimates; and secondly, what measures would be required to adapt to such changes? Mailhot and Duchesne (2010) discuss how to define design criteria under climate change
Summary
In a companion paper (Jakob et al, 2011) we outlined how design rainfall estimates are derived under the assumption of a stationary climate. State-of-the-art approaches based on a hierarchical Bayesian framework have been developed (Lima and Lall, 2010) and Hanel et al, 2009 discuss a regionalisation approach for annual maxima of daily precipitation where the typical “index flood” approach is modified, so that all three parameters of the Generalised Extreme Value (GEV) distribution vary with time. The purpose of this two-part study is to examine rainfall extremes for evidence of non-stationarity. Synoptic situations associated with intense rainfall events in this region include easterly dips, monsoon troughs, and tropical depressions
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