Intense Rain Bursts Rise with Heat, Forecast More Flash Flooding

The heaviest rain bursts within a storm happen when it’s warmest, according to new research that suggests rising temperatures could exacerbate flooding as intense downpours are concentrated into smaller windows.

The analysis by two Australian engineers shows that storms are becoming more unruly. They’re prone to fits of faster rainfall during condensed periods of severity, and lessening rainfall during calmer, cooler times within the same event.

In other words, storms are being reorganized, said Ashish Sharma, a professor at the University of New South Wales who co-authored the study. So even if the overall volume of rain doesn’t rise under climate change, storms could be more damaging as increased precipitation drums the ground in shorter amounts of time, he said.

But his example is just an illustration. Other studies show that the strongest storms are already dropping more water overall. So the findings by Sharma and Conrad Wasko, a doctoral candidate at the same university, suggest that taken together, those dual effects could pose even greater tests for stormwater systems.

“What we find is on colder days the storm is more uniform, and on warmer days, more peaked,” Sharma said in an email. “Given the future is likely to have more warmer days, it means the storms will get more peaked. This peakiness of the storm will increase floods and create problems for existing infrastructure.”

The , published last week in the journal , analyzed data from nearly 40,000 storms at 79 weather stations across Australia. The researchers dissected each event into five time periods and compared the amount of rain with the average temperature in each section. They found that the warmest periods corresponded with the most intense rainfall.

Sharma said that the consistency of their findings was “unexpected,” because of the size and climatic diversity of the Australian continent, which ranges from tropical to temperate.

In Darwin, a tropical area in the north, rainfall volume increased 4.9 percent with every rise of 1 degree Celsius in temperature. That was the largest jump, but the researchers found increases across the board. In Perth, in the extreme west, it was 1.3 percent; in Sydney, in the east, it was 1.7 percent; and in Hobart, far to the south, it was 3.3 percent.

“What we are saying here is that you will have greater flooding simply because of these within-storm pattern changes—and when you factor in the increased volume (because a warmer atmosphere will hold more moisture), you will have a double impact,” Sharma said.

Stationarity is ‘dead’

U.S. scientists have found that the strongest downpours are dropping more rain since the 1960s. The Northeast is seeing the biggest increase, with a 71 percent rise, followed by the Midwest (37 percent) and the Southeast (27 percent). Other regions are seeing more rain, but at smaller increases. The Southwest, for example, is seeing a 5 percent intensification.

“We know the temperature has been rising,” Kunkel said. “We know that heavy rain has been rising. So there is a relationship.”

So what does that mean for flooding? It’s intuitive to say that more rain will lead to greater inundation. But the answer is made much more complex when the water hits the ground. Every city has different surface qualities. More developed areas will likely see more flooding, especially if their stormwater infrastructure isn’t maintained well.

One thing is clear: It’s getting more difficult for planners to predict how a storm will behave. To some, that means historical risks related to floods can no longer be used to sufficiently plan for the future. Extremes are changing, but infrastructure to handle them is not.

Paul “Chris” Milly, a hydrologist at the U.S. Geological Survey, describes this as “stationarity,” the idea that the capacity of a drainage system, for example, can be estimated by looking at past rainfalls. From there, you calculate the size of the pipe that will be buried underground.

Instead, he says, stationarity is “dead.”

“When one does that now, one is increasingly invoking an error, because we think that runoff or precipitation characteristics are changing,” Milly said recently.

But there’s another problem. He says no one is quite sure how to measure the new envelope of possibilities. Some planners are doubling the capacity of their projects, while others assume the status quo will be tough enough. It’s just a guess, he said.

“Really, there are not alternative methods in general use, because they haven’t been developed,” Milly said. “That in turn is because we don’t know enough about the climate change signal and what it means on a local scale. That’s where all this stuff happens.”

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