The contributions of fronts, lows and thunderstorms to southern Australian rainfall
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The contributions of fronts, lows and thunderstorms to southern Australian rainfall Acacia S. Pepler1 · Andrew J. Dowdy1 · Peter van Rensch1 · Irina Rudeva1,2 · Jennifer L. Catto3 · Pandora Hope1 Received: 28 January 2020 / Accepted: 13 June 2020 © The Author(s) 2020
Abstract A systematic analysis of the main weather types influencing southern Australian rainfall is presented for the period 1979– 2015. This incorporates two multi-method datasets of cold fronts and low pressure systems, which indicate the more robust fronts and lows as distinguished from the weaker and less impactful events that are often indicated only by a single method. The front and low pressure system datasets are then combined with a dataset of environmental conditions associated with thunderstorms, as well as datasets of warm fronts and high pressure systems. The results demonstrate that these weather types collectively account for about 86% of days and more than 98% of rainfall in Australia south of 25° S. We also show how the key rain-bearing weather systems vary throughout the year and for different regions, with the co-occurrence of simultaneous lows, fronts and thunderstorm conditions particularly important during the spring and summer months in southeast Australia. Keywords Cyclone · Front · Thunderstorm · Rainfall · Reanalysis
1 Introduction A number of studies in recent years have used reanalysis data to associate global and regional rainfall with specific weather systems, including fronts (Catto et al. 2012, 2015; Catto and Pfahl 2013; Blázquez and Solman 2017; Raut et al. 2017) and low pressure systems (Dare et al. 2012; Pfahl and Wernli 2012; Hawcroft et al. 2012; Lavender and Abbs 2013). However, recent research has highlighted that, rather than analysing synoptic systems in isolation, for extreme rainfall and wind the co-occurrence of fronts and lows as well as the interaction of these events with smallerscale convective systems such as thunderstorms becomes increasingly important (Dowdy and Catto 2017). Dowdy and Catto (2017), hereafter ‘DC17’, referred to cases with more than one of these weather systems in a given locations as "concurrent" events, and showed that while they occur relatively infrequently they are disproportionately likely to cause extreme wind and extreme rainfall. In many areas of * Acacia S. Pepler [email protected] 1
Bureau of Meteorology, Melbourne, Australia
2
University of Melbourne, Melbourne, Australia
3
University of Exeter, Exeter, UK
the globe concurrent storm types are responsible for more extreme events than any weather system in isolation. For this reason, it is useful to understand the interactions between different weather systems when explaining the drivers of regional rainfall. Southern Australian rainfall is influenced by a large number of weather systems including extratropical and subtropical cyclones, low pressure troughs, thunderstorms, cold and warm fronts, and transient and blocking high pressure systems (Sturman and Tapper 1996). Several previous studies hav
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