Heavy versus extreme rainfall events in southeast Australia

Abstract

Focussing on the major cities of Brisbane, Sydney, and Melbourne in southeast Australia, this study seeks to determine the environmental factors that distinguish between heavy rainfall events (HREs) and extreme rainfall events (EREs). Using daily rain gauge observations, HREs and EREs are defined for each domain based, respectively, on the 95th and 99th percentiles of wet‐day rainfall for the period 1979–2018. K‐means clustering is applied to mean sea‐level pressure, data from ERA5 to obtain a set of representative large‐scale circulation patterns associated with these events. Composite synoptic maps, mean vertical profiles, and a series of column‐integrated diagnostics are then examined for each cluster and used to compare the environmental characteristics of HREs and EREs. For all three cities, HREs are associated with an upper‐level trough to the west, with large‐scale ascent, positive column water vapour (CWV) anomalies, and strong moisture transport over the analysis domain. For Brisbane and Sydney, the clusters are characterised by a coastal trough/low with moist onshore flow from the Tasman and Coral Seas. For Melbourne, circulation patterns are more distinct, with clusters characterised by a front, a cut‐off low, and an inland trough. Compared with HREs, EREs show a more amplified upper‐level trough, with stronger vertical motion and larger CWV anomalies over the analysis domain. In Brisbane and Sydney, EREs also feature stronger and deeper onshore flow, promoting enhanced moisture transport. A diagnostic termed upward vapour transport, which combines the key ingredients of high CWV and large‐scale ascent, is shown to discriminate well between HREs and EREs in all three domains. In contrast, surface temperature, which is frequently linked to rainfall extremes via Clausius–Clapeyron scaling, shows significant overlap between the different event categories, particularly for Brisbane and Sydney.

Fundamental Ingredients of Australian Rainfall Extremes

“…Unlike previous studies of localized Australian rainfall events, which have tended to focus heavily on southern Australia and the eastern coast (e.g., Ashcroft et al, 2019;Avila et al, 2015;Dey et al, 2019Dey et al, , 2021King et al, 2013b;Pepler et al, 2019Pepler et al, , 2021Warren et al, 2021), we extend our focus from the tropics to mid-latitudes and choose three regions of study centered on the cities of Darwin (tropical), Brisbane (subtropical) and Melbourne (mid-latitude, although Melbourne still has some sub-tropical aspects to its climate), respectively. These areas are chosen to develop a methodology that is universally applicable from tropical to mid-latitude regions (given that the findings of Norris et al (2019); Wood et al (2021), indicate the importance of understanding precipitation extremes from the tropics to mid-latitudes).…”

“…More recently, other studies have attempted to classify the characteristics of extreme weather events by employing methodologies to group extreme rainfall events based on their large‐scale circulation patterns. These studies often employ automated grouping techniques such as K ‐means clustering, principal component analysis, and self‐organizing maps, and have been applied to the study of rainfall extremes across the world including the United States (e.g., Maddox et al., 1979; Swales et al., 2016), Europe (e.g., Allan et al., 2020; Champion et al., 2019; Houssos et al., 2008; Nuissier et al., 2011), China (e.g., Hu et al., 2019), and Australia (e.g., Callaghan & Power, 2014; Fiddes et al., 2015; Fiddes et al., 2021; Pook et al., 2006; Risbey et al., 2013; Speer et al., 2009; Warren et al., 2021).…”

Identification of Rainfall Events and Heavy Rainfall Events From Radar Measurements in Southeastern Australia

“…Melbourne rainfall events also have a strong seasonal cycle with a cool season peak that differs from Brisbane and Sydney, with events most common in July and August, and least common from January to April. Warren et al (2021) similarly found that rainfall events in Brisbane were most common in March and least common in August, rainfall events in Sydney were most common in warmer months, and rainfall events in Melbourne were most common in July and least common in February.…”

“…Gridded data sets can also be created by interpolating rain gauge data. The majority of studies examining heavy rainfall in Australia utilize rain gauges or gridded data sets derived from gauges and focus on how rainfall is affected by climate change (Alexander et al., 2007; Bao et al., 2017; Osburn et al., 2021), climate variability (Ashcroft et al., 2019; Boschat et al., 2015; Dey et al., 2021; Fan et al., 2020; King et al., 2013, 2014, 2020), or synoptic‐scale processes (Fiddes et al., 2015; Pepler et al., 2020; Pook et al., 2014; Warren et al., 2021). The long record period available from rain gauges enables investigation of the long‐term behavior of rainfall in Australia.…”

The dynamics of slow‐moving coherent cyclonic potential vorticity anomalies and their links to heavy rainfall over the eastern seaboard of Australia

“…Moreover, the easterlies are to some extent lifted orographically as they encounter the Great Dividing Range, which runs parallel to the coastline of the ESB (Speer et al, 2011). Although orography plays a role in the vertical motion required for rainfall in the ESB, dynamically driven ascent and the associated lifting are essential for heavy precipitation and convection (Taschetto and England, 2009; Warren et al, 2021; White et al, 2022).…”