New method to predict heavy rainfall, extreme storms

MIT scientists have developed a new technique that can predict extreme rainfall and storm events by identifying telltale large-scale patterns in atmospheric data.

The technique significantly reduces the uncertainty of extreme storm predictions made by standard climate models, researchers said.

Currently, researchers estimate the frequency of local heavy precipitation events mainly by using precipitation information simulated from global climate models.

However, such models typically carry out complex computations to simulate climate processes across hundreds and even thousands of kilometers.

At such coarse resolution, it is extremely difficult for such models to adequately represent small-scale features such as moisture convection and topography, which are essential to making accurate predictions of precipitation.

To get a better picture of how future precipitation events might change region by region, researchers at Massachusetts Institute of Technology (MIT) in the US decided to focus on not simulated precipitation but large-scale atmospheric patterns, which climate models are able to simulate much more reliably.

"We have actually found there is a connection between what climate models do really well, which is to simulate large-scale motions of the atmosphere and local, heavy precipitation events," said Adam Schlosser from MIT's Joint Programme on the Science and Policy of Global Change.

"We can use this association to tell how frequently these events are occurring now, and how they will change locally, like in New England or the West Coast," Schlosser added.

For California, they calculated that, if the world's average temperatures rise by four degrees Celsius by the year 2100, the state will experience three more extreme precipitation events than the current average per year.

On December 11 in 2014, a freight train of a storm steamed through much of California, deluging the San Francisco Bay Area with three inches of rain in just one hour.

By evening, record rainfall had set off mudslides, floods, and power outages across the state.

The storm, which has been called California's "storm of the decade," is among the state's most extreme precipitation events in recent history.

"One of the struggles is, coarse climate models produce a wide range of outcomes. Rainfall can increase or decrease," said Schlosser.

"What our method tells you is, for California, we are very confident that heavy precipitation will increase by the end of the century," said Schlosser.

The study was published in the Journal of Climate.