Rivers in the Sky Fuel Devastating Floods – but May Be More Predictable Than Expected
On December 7, 2022, endless rains besieged the Atlantic coast of Portugal. Flash floods hit Lisbon in just three hours and the event caused at least one death.
A new study published in Weather and climate extremes suggests that the large storms that cause this type of flooding are predictable. The storms likely to cause the most damage are also the ones that can be most easily anticipated.
“The irony is that the most dangerous events are often those that the atmosphere signals most clearly,” the study’s authors, from the Karlsruhe Institute of Technology and the Hebrew University of Jerusalem, said in the statement. “When the large-scale structure is solid and organized, the system becomes more “readable.” »
Heavenly Rivers and Extreme Weather Events
The new study focuses on heavy precipitation events (HPE) in the western Iberian Peninsula. Many of the most extreme weather events come from atmospheric rivers, which are huge concentrations of water vapor transported in long channels of moisture over great distances. These rivers carry moisture across the oceans and to coastal areas.
These atmospheric rivers significantly increase precipitation during subsequent storms. The team found that this increase is linked to a change in wind patterns that bring moisture to the peninsula, rather than more humid air overall.
“It’s not just about how much water the atmosphere holds. It’s also about how efficiently the system delivers that water to the ground,” the study authors said.
Learn more: A landslide causes the evacuation of 1,500 residents of a small Sicilian town
Modeling rivers in the sky
This initial finding is useful for researchers seeking to understand the causes of intense precipitation in HPEs, but the real impact of the paper lies in its modeling work.
The team modeled how these celestial rivers change atmospheric conditions. They examined alterations in the upper and lower atmosphere. The team noticed that the most predictable storms had a distinct signature: deep cyclones forming over the North Atlantic, about 400 kilometers southwest of Ireland.
These systems caused greater pressure changes due to less predictable storms. It was more obvious to see how these cyclones would interact with the jet stream and how they would move through the atmosphere.
Predicting devastating storms
On average, when these predictable storms hit the Iberian Peninsula, they produced rain levels 80% higher than those created by unpredictable storms.
The authors showed that the devastating storm that hit Portugal in 2022 began when an atmospheric river collided with a cyclone and a jet stream. Future storms of this type could be more easily predicted. Similar storms affecting coastal regions around the world could also be predicted using the same techniques, including on the western coasts of North and South America, as well as those of southern Africa.
“Improving the ability to quantify the predictability of such events has clear value for impact mitigation and risk management, particularly in the context of climate change,” the authors write in their paper.
Learn more: An underwater landslide may have contributed to this devastating 1957 tsunami
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