Large-Scale Influences on the Evolution of Winter Subtropical Maritime Cyclones Affecting Australia’s East Coast

East Coast Lows (ECLs) are intense low-pressure systems which occur over the subtropical east coasts of southern and northern hemisphere continents. ECLs are typically associated with gale force winds, large seas, storm surges, heavy rainfall and flooding. While ECL impacts are typically seen as negative the rainfall associated with ECLs is also very important for urban water security within the heavily populated eastern seaboard of Australia (ESA). This study investigates historical ECLs to gain insights into the timing, frequency, intensity and location of ECL occurrence as well as the magnitude and spatial extent of ECL impacts on rainfall. The different characteristics and impacts associated with different ECL sub-types are highlighted and it is proposed that this spatial and temporal variability in ECL behaviour at least partially explains why the ESA is hydroclimatically different to the rest of Australia and why different locations within the ESA have such different rainfall patterns-and therefore different levels of flood and drought risk. These insights are critical to the objectives of the New South Wales government funded Eastern Seaboard Climate Change Initiative (ESCCI), in particular Project 5 which focuses on the water security impacts of ECLs. The results of this work will be used to produce climate-informed stochastic daily rainfall simulations that are more realistic than existing stochastic rainfall simulation methods at preserving the statistics important for catchment-scale hydrology (e.g. clustering of extreme events, long-term persistence, frequency/duration/magnitude of wet and dry spells). These simulated rainfall sequences, that incorporate the spatial and temporal hydroclimatic variability caused by ECLs and other climate phenomena, are important inputs into the hydrological models used to determine current and future urban water security within the ESA.

Section: Defining and Categorising Eclsmentioning

confidence: 99%

Links between East Coast Lows and the spatial and temporal variability of rainfall along the eastern seaboard of Australia

Kiem¹,

Twomey²,

Lockart³

et al. 2016

“…Hunt 1914;Visher and Hodge 1925;Bond and Wiesner 1955;Brunt 1956Brunt , 1958Public Works Department, NSW 1985;Shepherd and Coloquhoun 1985;Holland et al 1987;Whetton 1990;Hopkins and Holland 1997;Speer 2008;Speer et al , 2011Vernon-Kidd et al 2010;Wilson et al 2013;Mackay 2014;Browning and Goodwin 2013;CP2014). However, fewer studies have looked at the detailed structure of the synoptic systems linked to either extreme rainfall or major flooding (Bond and Wiesener 1955;Browning and Goodwin 2013;Wilson et al 2013;Dowdy et al 2013). While these studies did not examine winds, they did examine both Mean Sea Level Pressure (MSLP) patterns and the structure of middle to upperlevel troughs linked to heavy rainfall.…”

mentioning

confidence: 99%

A vertical wind structure that leads to extreme rainfall and major flooding in southeast Australia

Callaghan¹,

Power²

2017

Here we examine winds associated with extreme rainfall and major flooding in coastal catchments and more broadly over southeastern Australia. Both radiosonde and re-analysis data are examined. In every case (i) atmospheric moisture content is high and (ii) the low-level winds are onshore, and in almost every case (iii) the wind-direction turns anti-cyclonically with increasing height up to 500 hPa. Data from Brisbane extending back more than 50 years is consistent with this behavior: winds turn anti-cyclonically with increasing height on days with heavy rainfall, whereas winds turn cyclonically with increasing height on days with light or no rainfall. In the coastal zone, extreme rainfall rarely occurs without (i), (ii) and (iii). In eastern Australia beyond the coastal zone, conditions (i) and (iii) are also associated with extreme rainfall. We found very few cases where such conditions were not associated with extreme rainfall in this broader region. This study extends previous work by showing that the link between turning winds and rainfall exists in both the tropics and subtropics, and the link applies in cases of extreme rainfall and associated major flooding.

“…The detection and tracking algorithm is based on the method developed by Browning and Goodwin (2013) with modifications that were detailed in Di . The main characteristics of the detection algorithm used in this study are described below.…”

Section: Detection and Tracking Algorithmmentioning

confidence: 99%

“…The ∇p is computed by averaging differences between the central SLP and the SLP in grid points located within a radius of 200 km around the central pressure (∇p 200 ). The value of the 200-km SLP gradient threshold was chosen to be 0.6 hPa/100 km and is thus very similar to the threshold used by Browning and Goodwin (2013) of 1 hPa/1.5 degree.…”

Section: Detection and Tracking Algorithmmentioning

confidence: 99%

“…The analysis includes the evaluation of various characteristics of ECLs including their frequency, intensity, size and duration. We focus on maritime cyclones that appear over the Tasman Sea to the east of Australia, using a single automated cyclone detection and tracking algorithm that employs 6-hourly sea level pressure (SLP) fields as input data (Browning andGoodwin 2013, Di Luca et al 2015). While ECL climatologies are somewhat sensitive to the choice of tracking method and identification criteria, we expect that these choices are of second order importance when looking at differences between datasets for which ECLs were identified using the same method.…”

Section: Introductionmentioning

confidence: 99%

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Evaluating the representation of Australian East Coast Lows in a regional climate model ensemble

Luca¹,

Evans²,

Pepler³

et al. 2016

Due to their large influence on both severe weather and water security along the east coast of Australia, it is increasingly important to understand how East Coast Lows (ECLs) may change over coming decades. Changes in ECLs may occur for a number of reasons including changes in the general atmospheric circulation (e.g. poleward shift of storm tracks) and/or changes in local conditions (e.g. changes in sea surface temperatures). Numerical climate models are the best available tool for studying these changes however, in order to assess future projections, climate model simulations need to be evaluated on how well they represent the historical climatology of ECLs. In this paper, we evaluate the performance of a 15-member ensemble of regional climate model (RCM) simulations to reproduce the climatology of cyclones obtained using three high-resolution reanalysis datasets (ERAInterim, NASA-MERRA and JRA55). The performance of the RCM ensemble is also compared to results obtained from the global datasets that are used to drive the RCM ensemble (four general circulation model simulations and a low resolution reanalysis), to identify whether they offer additional value beyond the driving data. An existing cyclone detection and tracking algorithm is applied to derive a number of ECL characteristics and assess results at a variety of spatial scales. The RCM ensemble offers substantial improvement on the coarse-resolution driving data for most ECL characteristics, with results typically falling within the range of observational uncertainty, instilling confidence for studies of future projections. The study clearly highlights the need to use an ensemble of simulations to obtain reliable projections and a range of possible future changes.