University of NSW Professor Jason Evans said the amount of rain falling in these extreme deluges was now generally between seven and 20 millimetres in 10 minutes,whereas 20 years ago,it was four to 15.
“It’s going up far faster than we imagined”,he said,despite there being little evidence of the same trends at hourly or daily scales. “The storm overall might not be producing a lot more rain,but it’s a lot more concentrated.”
The exact definition of a rapid rain burst varies depending on the area,but generally the term refers to the most extreme 5 per cent of rainfall in that location. The findings arepublished in the current issue of the academic journalScience.
Sydneysiders have endured flash-flooding all year as the city clocked up its wettest year on record. Evans said the trend his team observed was so strong that if it continues,“we’re going to see some really big issues in urban areas with flash-flooding type events”.
“These aren’t the kind of events that are causing large-scale river flooding,” he said. “It’s the kind of event that overwhelms the gutters and the drainage systems,floods the road,floods the property really quickly.”
Evans said the reason for the bigger rain bursts wasn’t yet clear,though it was consistent with what was expected from climate change. The team has ruled out changes in radar sensors and the main forms of climate variability.