News•January 26, 2017

Rains From Thunderstorms Rising Rapidly in Europe, Asia

Across a vast swath of Europe and Asia, rain is increasingly falling in the short, localized bursts associated with thunderstorms, seemingly at the expense of events where a steady rain falls over many hours, a new study finds.

Residents sit on makeshift boats during evacuation operations of the Villeneuve-Trillage suburb of Paris on June 3, 2016, after torrential rains caused massive flooding.
Click image to enlarge. Credit: REUTERS/Christian Hartmann

The study, detailed Wednesday in the journal Science Advances, directly links this trend to the warming and moistening of the atmosphere caused by rising greenhouse gas levels.

The results fit with rainfall trends already observed in the U.S., as well as model predictions that massive rains associated with thunderstorms could become both more frequent and more intense in the U.S. as the world continues to heat up.

The shift toward more extreme rains could have implications for water management and flooding because the ground is less able to absorb rainwater when it falls all at once.

“These changes should have pretty big impacts on this region,” Andreas Prein, of the National Center for Atmospheric Research in Boulder, Colo., said.

That a warming atmosphere will lead to more extreme rainfall events is one of the basic predictions of climate science, and is linked to the fact that warming leads to more evaporation, which leads to more water vapor in the atmosphere. That means that when rains occur, there’s more water vapor available to dump as rain.

Extreme downpours have already been increasing in the U.S., most notably in the Northeast, where they have increased by 71 percent since mid-century, according to the 2014 National Climate Assessment.

But rain comes in different types, namely convective and non-convective. Convection is the process that causes thunderstorms, and happens when there is strong warming at the Earth’s surface, creating an unstable atmosphere. That hot surface air rises, cooling as it does so, creating clouds and fueling heavy rain. It’s a short-lived, very local phenomenon that tends to occur most often in the summer. The skies can go from sunny, to downpour, to sunny again within an hour.

Non-convective precipitation, on the other hand, is typically involved with the passing of weather fronts and tends to lead to steady rains falling over several hours or days.

But it is difficult to distinguish how warming will impact these different rain processes, in part because convective rain happens on too small a scale for global climate models to resolve. A recent U.S.-focused study conducted by Prein got around this by using a higher-resolution model looking only at the U.S. to show that extreme rains linked to convective storms would happen both more often and would dump increasingly more rain with each storm.

The new study, led by Hengchun Ye, a climate researcher at California State University, Los Angeles, and director of NASA’s DIRECT-STEM initiative, looked at a wide area of Europe and Asia from the opposite direction, to see what trends might already be happening with convective and non-convective rainfall. The researchers used daily precipitation data and weather observations that distinguished between the two rain types from 152 weather stations spanning the years from 1966 to 2000.

They found that over that period, the yearly total of convective precipitation averaged over the whole study area “has been increasing astonishingly fast,” by about 1.4 inches per decade, with 8.5 more days of convective precipitation each decade.

How convective, non-convective and mixed precipitation averaged across the study area changed over time in terms of precipitation totals, frequency and intensity.
Click image to enlarge. Credit: Ye, et al./Science Advances

Overall annual precipitation doesn’t seem to have changed much, though, which suggests that convective precipitation is increasing at the expense of non-convective rains, a result that Prein, who wasn’t involved with the research, called surprising.

“It’s exciting what they show,” he said.

The shift seems to be happening in particular seasons, with spring and fall becoming more summer-like in their precipitation patterns. For example, non-convective precipitation accounted for the bulk of fall rains prior to the mid-1980s, after which convective rains took over.

The study authors also found that average daily precipitation intensity was increasing, largely because of the increase in convective rains.

The findings make physical sense, Ye said, because “the climate has been warming up and we have more water vapor.”

Surface temperature rise means that there is more energy in the air close to the surface, making that air more buoyant “so you can trigger convection much easier,” Prein said.

The study found that the yearly total of convective precipitation rose by 18.4 percent for every degree Celsius rise in temperature. Since pre-industrial times, or before human activities began warming the planet, global temperatures have risen by about 1°C (1.8°F).

Most of the rise in convective precipitation is due to those events happening more often, with the increase in rain intensity contributing a smaller nudge upward.

As these trends continue with warming, they could have major impacts on water management. The trade-off in non-convective rains for more convective rain means that there will be fewer days that have rain, but more rain falling on days when storms do occur, Ye said.

A shift from more frequent non-convective rains to less frequent downpours could overwhelm the ability of the soil and plants to absorb rainwater. If the ground can’t absorb the water, it runs off into streams, potentially causing floods and changing how cities and countries must think about capturing their water.