Picture (above): Storm at dawn. Credit: Michael D (Unsplash).

Despite what has been a very challenging year, the Extreme Rainfall research program continues to produce high-quality research, develop deep and wide datasets, extend the reach of our citizen science, and has seen our researchers continue to achieve at a national and international level.

In early November, we announced the release of version 1 of the Aus400 data set for community use. The dataset is the culmination of a collaborative project between the ARC Centre of Excellence for Climate Extremes, Bureau of Meteorology, and National Computational Infrastructure (NCI) Australia and is based on a simulation at 400m grid spacing right across Australia covering a 60-hour period that starts on March 26, 2017. The 400-m domain has more than 12.6 billion grid points, and at its completion was the largest simulation with the Unified Model ever conducted

The output shows 3D variables every one-hour and 2D variables every 10 minutes. This incredibly detailed simulation captures the period when severe tropical cyclone Debbie made landfall, the passage of a cold front over southern Australia, and severe storms in other areas of the continent. This is the highest resolution simulation of the atmosphere ever produced over all of Australia and will be used as the basis for a range of scientific endeavours and to create a detailed animated simulation later in 2021. A workshop to discuss the first results from the analysis of the simulation is planned for Feb/Mar 2021.

Another piece of impressive research led by PhD student Chiara Holgate revealed the origin of rainfall across Australia and gave us insight into how it may change in the future. Using meteorological simulations to travel backwards in time the researchers were able to find out the original evaporation point for much of Australia’s rainfall. While it revealed that the majority came from the oceans, it was surprising to find that in some cases very small ocean regions were extremely important to rainfall in key parts of the country. For instance, 90% of the rainfall over south-western Australia came from one small region in the Southern Ocean and Indian Ocean. Meanwhile, in north and north-east Australia and south-east Australia between 18% to 25% of rainfall originated via evaporation from plants and soil. The researchers found that if soils were wet in these origin regions, it had an amplifying effect that produced even more rain as other systems passed through. This research also helped explain changes in the seasonality of rainfall for key agricultural regions, the upward trend of rainfall in north-west Australia, and the reduction in winter rainfall in south-east Australia. This has ramifications for a range of industries including the agricultural and water resource managers.

But when it comes to looking backwards in time to estimate how things may change in the future it is hard to surpass looking back 3 million years to the mid-Pliocene when temperatures were 2 to 3°C warmer than today but the concentrations of carbon dioxide were very similar. The extreme rainfall research program in concert with the drought program examined these past conditions to better understand how atmospheric circulation may change with expected global warming and what this will mean for rainfall. Together they found that there would be drier conditions in the tropics and subtropics of the southern hemisphere but that at the same time Australia itself may see an intensification of its monsoon season. This suggests the idea of Australia as a country of drought and flooding rains will only grow more extreme in the future if climate change continues at its current pace.

This study was reinforced by another that looked specifically at monsoons and how they will change using a period 6000 years ago as a reference point. The researchers modelled this past era to simulate how monsoons changed at this time and then compared this to simulations of monsoonal changes in the future under a high emissions scenario. This simulation showed that changes to atmospheric circulation led to an intensification of the monsoon season primarily caused by thermodynamic factors.

But it is not just monsoons that will intensify, importantly for Australia and a La Niña year our researchers found that tropical cyclones will also intensify in a warmer world. Using climate simulations that captured the entire life cycle of a cyclone, from a “seed disturbance” up to a category five event, they found that while there was no trend in the number of cyclones, the speed with which powerful cyclones developed increased. This suggests that rapidly intensifying storms may become more frequent in a future warmer climate and the speed of this increase in intensity will continue to grow as the world’s oceans warm.

As you can see already, much of our work involves taking real-world observations and simulating the outcomes for rainfall in our climate models. While this has given us considerable insight into changing rainfall, researchers around the world recognize that precipitation still remains one of the great challenges for climate models. In particular, they have struggled to reproduce tropical precipitation. This prompted CLEX researchers in the rainfall program to look at three generations of models used by the Intergovernmental Panel on Climate Change to see whether the representation of rainfall has improved over time. The results were mixed, with some advances while in other areas, such as the fraction of precipitation that comes from low-level cloud regimes (warm rain), the models had actually gone backwards. The researchers suggested that large-scale global climate models could be replaced by high-resolution storm resolving models to more accurately capture changes in these more difficult areas. In coming to this conclusion, they noted that improvements in computer processing power meant this was already possible today.

Another atmospheric phenomenon that is ripe for re-examination is atmospheric rivers, which are often associated with extreme flooding events. PhD student Kim Reid has been exploring how atmospheric rivers are defined and identified in climate data and how variation in these definitions explicitly change the number of atmospheric rivers that can be detected through observations and models. She found that when certain definitions were used or even the order of calculation and resolution processes were changed, some of the most powerful atmospheric rivers were not detected. This high level of detection uncertainty is something that will need to be resolved across the research community if we are to produce consistent informative research about these important events.

Equally important, is the capacity to get detailed data about those extreme rainfall events that occur over quite small areas. An example of these is supercell thunderstorms. Our citizen science project, WeatheX, has now moved into its second phase with version 2 being released in September. This improved version has proven to be more popular and, as a result, we saw 270 reports of storms that passed through south-east Queensland in early November. This information is invaluable in helping us understand how these storms develop, comparing what we see on radars with what is happening on the ground, and will provide important information that will improve our ability to forecast these rapidly developing events. Through our partnership with the Bureau of Meteorology, IAG, Risk Frontiers and the NSW Department of Planning, Industry and Environment we expect to see the WeatheX app continue to improve and provide us with valuable data for our research.

Amidst all the research we have also had a range of other successes. Steve Sherwood, Jason Evans, Fei Ji, and Andrew Dowdy received a linkage grant that seeks to better understand and predict wind gusts and their impacts to aid in planning. Lisa Alexander was named by Clarivate as a highly cited researcher in its 2020 list; Ben Henley received a Victorian Young Tall Poppy Award; Christian Jakob edited a comprehensive overview of research on clouds and their role in our present and future climate, covering theoretical, observational, and modelling perspectives, Clouds and Climate: Climate Science’s Greatest Challenge; and to cap it all Kim Reid won first place in a Haiku Thesis competition. Her thesis, Impacts of Atmospheric Rivers in Australia and New Zealand, was rendered into poetry that seems a perfect way to conclude this report. The Haiku reads: 

Rivers in the sky
Growth, life, destruction and death
What will you bring us?