More research into rainfall lows to improve predictions

“We know the benefits for cropping regions and catchments that come from these lows and their critical role in bringing substantial rains,” said a CSIRO Climate Adaptation Flagship scientist with the Centre for Australian Weather and Climate Research (a partnership between CSIRO and the Bureau of Meteorology), Dr Mike Pook.

“But we have little insight through climate modelling into how their behaviour will change in a warming world, which is something groups such as the insurance industry are particularly interested in because they cause hail storms and intense rainfall which can damage roads and infrastructure.”

Hobart-based Dr Pook specialises in understanding atmospheric patterns and processes that generate rainfall for southern Australia. He has been researching rainfall systems over the winter cropping regions of the Australian mainland and Tasmania.

In an address in Canberra today to the annual conference of the Australian Meteorological and Oceanographic Society he said the so-called ‘east coast low’ – which brings gales, heavy rain and high seas to the heavily populated NSW and southern Queensland coasts – is an extreme example of a broader class of synoptic systems known as cut-off lows.

Such lows, caused by favourable conditions in the upper atmosphere, led to: the storms that caused havoc and the loss of six lives during the1998 Sydney-Hobart yacht race, relief rainfall in recent months in Australia’s south-east and the severe 1929 floods that resulted from extreme rainfalls in Tasmania’s north-east.

Dr Pook said cut-off lows make a significant contribution to rainfall over much of southern Australia’s agricultural districts and water catchments but in Tasmania the proportion of rain attributed to these systems varies substantially from district to district because of their association with blocking highs and the complex topography of the state. 

“For example, Tasmania’s hydroelectric catchments can experience very low rainfall because of the influence of a blocking high while the normally drier eastern and midlands districts receive well above average rainfall if the associated cut-off low is in a favourable location,” Dr Pook said.

“Weather forecasting models used by meteorological and research agencies can provide excellent forecasts of individual synoptic systems for up to 10 days. On seasonal time-scales and longer, climate models can only predict the average behaviour of weather systems because of computer and data limitations and the effect of chaos.

“However, these climate models are known to have problems in accurately simulating blocking location, frequency and intensity and hence improved treatment of the blocking and cutting-off mechanisms in numerical models is critically important if progress is to made in seasonal climate prediction and in climate change projections,” Dr Pook said.