Weather systems and extreme rainfall generation in the 2019 north Queensland floods compared with historical north Queensland record floods

Abstract

Earlier papers have addressed floods from warm-air advection (WAA) in southeast Australia and around the globe, and extreme rainfall in US hurricanes and Australian tropical cyclones (TCs). This is the first paper to address the WAA phenomena in causing monsoon and TC floods and in TC-like systems which develop over the interior of northern Australia. The inland events help explain Australia’s worst tropical flooding disaster in 1916. A disastrous series of floods during late January and early February 2019 caused widespread damage in tropical north Queensland both in inland regions and along the coast. This occurred when some large-scale climate influences, including the sea surface temperatures suggested conditions would not lead to major flooding. Therefore, it is important to focus on the weather systems to understand the processes that resulted in the extreme rainfall responsible for the flooding. The structure of weather systems in most areas involved a pattern in which the winds turned in an anticyclonic sense as they ascended from the low to middle levels of the atmosphere (often referred to as WAA) which was maintained over large areas for 11 days. HYSPLIT air parcel trajectory observations were employed to confirm these ascent analyses. Examination of a period during which the heaviest rain was reported and compared with climatology showed a much stronger monsoon circulation, widespread WAA through tropical Queensland where normally its descending equivalent of cold-air advection is found, and higher mean sea level pressures along the south Queensland coast. The monsoon low was located between strong deep monsoon westerlies to the north and strong deep easterlies to the south which ensured its slow movement. This non-TC event produced heavy inland rainfall. Extreme inland rainfall is rare in this region. Dare et al. (2012), using data from 1969/70 to 2009/10, showed that over north Queensland non-TC events produced a large percentage of the total rainfall. The vertical structure associated with one of the earlier events that occurred in 2008 had sufficient data to detect strong and widespread WAA overlying an onshore moist tropical airstream. This appears to have played a crucial role in such extreme rainfall extending well inland and perhaps gives insight to the cause of a 1916 flooding disaster at Clermont which claimed around 70 lives. Several other events over the inland Tropics with strong WAA also help explain the 1916 disaster.

Multi-week prediction of livestock chill conditions associated with the northwest Queensland floods of February 2019

“…While the Temperature Humidity Index has been successfully used to quantify the heat stress in Bos taurus (cooler climate) and Bos indicus (warmer climate) cattle breeds 5 , there is a currently a knowledge gap in our understanding of the effect of cold stress on northern Australia cattle 6 . Less is known about cattle comfort during the Australian summer months (December to February) when temperatures can rapidly drop when a monsoon depression or tropical cyclone makes landfall, as was the case in February 2019 over the Gulf 7 . Previous studies have predominantly focused on beef cattle in North America where animals have access to sheltered pens for protection from sub-zero temperatures 8 , 9 or have focused on the detrimental impacts of warmer temperatures on northern Australia forage production and animal stress 10 .…”

“…The weather system responsible for the extreme compound event was a quasi-stationary tropical low, described as a warm-air advection event (anticyclonic turning of winds with height) with cooler mid-level air drawn in from higher latitudes 7 . In the lead up to the tropical low deepening (26-27 January), a shallow inland trough stretched across tropical northern Australia, linking tropical cyclone Riley (off the northwest coast) and a tropical low over Cape York Peninsula 13 .…”

“…The fourth model, on the other hand, showed good rainfall prediction skill beyond 2 weeks, which the authors argued to result from the small ensemble size ( N = 4) limiting the robustness of the statistical significance. While previous studies focused on the rainfall prediction of the flood event 1 , and the nature of atmospheric circulation 7 , 14 , here we focus on quantifying the livestock chill conditions of this event, including the observed evolution over late January and early February 2019, and the prediction of these conditions in the S2S models and ACCESS-S1. Given the compound nature of the extreme event, our main motivation here is to verify whether the livestock chill index, which combines rainfall, temperature and wind, could be better predicted than the individual variables, using the 2019 flood as a case study.…”

Subseasonal Forecasts of the Northern Queensland Floods of February 2019: Causes and Forecast Evaluation

“…This index is a modification of the AUSSM index proposed in Yim et al [25] based on the climatological monsoon depression in Figure 2a, where the westerlies appear in the north and easterlies in the south. After comparing the difference between the wind shear and the westerly [6,7,26] monsoon indices, we found that the wind shear index has the merit of showing clear seasonal contrast separated by the onset date and the cyclonic structure of the clockwise (cyclonic) monsoon depression over the interior of northern Australia [27]. The subseasonal variations of these indices in summer 2018/19 can be easily identified in Figure 2c,d. A strong westerly surge occurred in late January and lasted until mid-February.…”

“…Therefore, the strong ER influence in 2019 seems to be unusual. The strong enhancement by other factors, such as the Warm Air Advection (WAA) described in Callaghan [27], can be important. Further research in this direction is desperately needed to find enough evidence to support the hypothesis.…”