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The U.S. has seen a lot of very rainy hurricanes in the last couple years. In September, Florence set flood records in North Carolina. Last year, Irma and Maria drowned parts of Florida and Puerto Rico. And Hurricane Harvey was the largest rainfall event in U.S. recorded history.
Well, two studies published today shed light on why hurricanes are getting wetter. NPR's Rebecca Hersher reports.
REBECCA HERSHER, BYLINE: The biggest single reason hurricanes are getting rainier is climate change - not future climate change, climate change that's already happening. Christina Patricola is an atmospheric scientist at Lawrence Berkeley National Lab and the author of a study published in Nature today.
CHRISTINA PATRICOLA: We're already starting to see that climate change has enhanced tropical cyclone rainfall.
HERSHER: Tropical cyclone rainfall is scientist speak for hurricane rain. Patricola and colleagues looked at three super destructive hurricanes - Katrina, Irma and Maria. They created a model for how much rain would've fallen if there was no climate change and compared it to what actually happened.
PATRICOLA: We're finding that the total storm rainfall has increased by about 5 to 10 percent.
HERSHER: That's 5 to 10 percent more rain in New Orleans, in Florida, in Puerto Rico. The extra water contributed to death and destruction from all three storms.
GABRIEL VECCHI: It was a very nicely done study.
HERSHER: Gabriel Vecchi is a climate scientist at Princeton. He wasn't involved in Patricola's study, but he says it's the latest confirmation that humans are making hurricanes worse. For example, Hurricane Harvey. In 2017, it stalled over Houston for days and caused at least $125 billion in damage.
VECCHI: Yeah. So humans have warmed the climate, which made Harvey wetter. Humans, in addition, built Houston. And in this particular case, that urban topography increased the amount of rain that Houston felt.
HERSHER: Urban topography means buildings. And, yeah, he's saying that the buildings themselves increased the amount of rain that fell during the storm. Imagine Hurricane Harvey starts in the Gulf of Mexico and blows across Texas.
VECCHI: So the way I would think about it is that the buildings stopped the air from being able to move forwards, away from the ocean, through the grasslands that would've been there if Houston hadn't been built, and further into Texas. They sort of stopped the air in that general area. And the air has nowhere to go but either around the buildings or up.
HERSHER: The buildings force extra air up into the colder parts of the atmosphere, and that generates more rain because the air coming from the Gulf of Mexico is hot and moist.
VECCHI: When moist air touches a cold surface or is in contact with cold, it will tend to form droplets, as the water does on the outside of a glass. And so that's how the rain is formed.
HERSHER: So hot, moist air hits buildings. It goes up, condenses into drops and falls as rain.
VECCHI: The more air that you raise up, in general, the more rain that you will have. And that is one of the impacts of these buildings.
HERSHER: And there's nowhere for the rain to go. Cities are full of paved roads and bridges and parking lots - materials that can't soak up water. So all the extra rain turns into flooding.
In a second study published today in Nature, Vecchi and his colleagues found that the physical city of Houston - the buildings, the sprawling paved areas - made flooding from Hurricane Harvey significantly worse. That's on top of increased rain from global warming.
The next step is to study whether the same applies to other coastal cities or other hurricanes, and whether some types of buildings in development produce more rain than others.
Gabriele Villarini of the University of Iowa is Vecchi's co-author on the study.
GABRIELE VILLARINI: There are a lot of unanswered questions.
HERSHER: He says what is clear is that cities need to address rainier hurricanes in multiple ways. Cutting CO2 emissions and building floodwalls are just the start.
VILLARINI: Potentially moving out of the floodplains and also reconverting some of the built areas to more vegetated areas. So I think having more of a multipronged approach would probably be beneficial.
HERSHER: A multipronged approach that takes into account both climate change and how cities themselves can change the weather. Rebecca Hersher, NPR News.
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