When a tropical storm is approaching, its intensity or wind speed often gets the bulk of the attention. But as Tropical Storm Barry bears down on the Gulf Coast in the coming days, it’s the water that the storm will bring with it that has weather watchers worried.
The National Weather Service is calling for roughly 10 to 20 inches of rain to fall from late Thursday night through Saturday. The average rainfall for July in New Orleans, which is in the path of the storm, is just under six inches.
And Tropical Storm Barry, which may become a Category 1 hurricane before making landfall, will drop rain on already saturated land. On Wednesday, the region was hit by severe thunderstorms, which dropped as much as seven inches of rain according to preliminary National Weather Service data.
“Climate change is in general increasing the frequency and intensity of heavy rainfall storms,” said Andreas Prein, a project scientist with the National Center for Atmospheric Research.
This week’s rainfall came after the region experienced an extremely wet spring, causing the region’s rivers to swell, and raising concerns that the upcoming storm may overtop levees in New Orleans. “If you look at the records, mostly it’s the water that kills most people,” Dr. Prein said.
In an email interview, David Gochis, a hydrometerological scientist at the National Center for Atmospheric Research, said that flooding of the Mississippi River had left very little room to accommodate additional water, and that the storm surge would inhibit river water from flowing out to sea.
“The ingredients are there for a real catastrophe if the flood control infrastructure simply gets overwhelmed,” he said.
In recent years, researchers have found that hurricanes have lingered longer, as Barry is expected to do, and dumped more rainfall — a sign of climate change, said Christina Patricola, a research scientist at Lawrence Berkeley National Laboratory, and a co-author of a study that found that climate change is making tropical cyclones wetter. (Tropical cyclones include both hurricanes and tropical storms, which are hurricanes’ less speedier kin.)
Researchers have been studying the effects of climate change on tropical cyclones because those sorts of storms are driven by warm water. Water in the gulf is 0.5 to 2 degrees Celsius warmer, according to Dr. Prein, who said: “This is really increasing the likelihood of a hurricane to form in this basin. And it will increase the intensity of the hurricane as well.”
Though storms can form at any time, the Atlantic hurricane season stretches from June 1 through Nov. 30 because that is typically when the Atlantic Ocean’s waters are warm enough to sustain storms. But the oceans are now warmer than ever: They have absorbed more than 90 percent of the heat caused by human-released greenhouse gas emissions.
“We wanted to understand how climate change so far could have influenced tropical cyclone events,” Dr. Patricola said about her study. “And then the second part is to understand how future warming could influence these events.”
The researchers used climate models to simulate how tropical cyclone intensity, or wind speed, and rainfall would change if hurricanes like Katrina, Irma and Maria had occurred absent climate change and under future climate scenarios. They found that for all three storms, climate change increased rainfall by up to 9 percent.
This study is not the first to find that climate change is causing tropical cyclones to have more rainfall. Studies on Hurricane Harvey found that climate change contributed as much as 38 percent, or 19 inches, of the more than 50 inches of rain that fell in some places. Dr. Patricola’s study broadens the research by using global climate models and analyzing a large number of storms.
“What’s really interesting is that, regardless of the methodology that you use, we’re starting to see more and more evidence that climate change so far has been enhancing the rainfall on some of these recent hurricane events,” she said.
When the researchers looked at the impact on storms under some possible future conditions, they found that under scenarios with higher greenhouse gas emissions there would be more rainfall associated with storms. The largest increases would occur over regions, like the Gulf Coast, that also have the heaviest historical rainfalls.
In other words, the wetter places are just going to get wetter.
And the structure of cities may exacerbate the problem even further, said Gabriele Villarini, an associate professor of civil and environmental engineering at the University of Iowa.
At issue: Dirt absorbs water, but paved surfaces such as roads, sidewalks and even the footprint of building homes that make up cities don’t. The end result is that less water gets absorbed and the excess inevitably has to go somewhere.
Dr. Villarini and his colleagues researched what might have happened in Houston in 2017 during Hurricane Harvey if the area had been cropland. They looked at both the changes in rainfall patterns that cities cause as well as differences in how water behaves based on ground type. They found that the twin effects increased the likelihood of extreme flooding by 21 times, he said.
In addition to factors faced by most cities, New Orleans has some unique geological factors at play. There are degraded wetlands and a complex drainage system that keeps much of the city dry enough for development but has also contributed to roughly half of the city sinking below sea level, making it especially vulnerable.
Dr. Villarini noted that in the case of Hurricane Harvey, even absent the impact of urbanization, there was “a huge amount of rainfall. And I’m struggling to think how you would design a city so that basically you would be able to zero out any effect of flooding.”
Figuring out how to do that is something that researchers are working on, particularly in places that, unlike New Orleans, are subject both to intense rainfalls and intense periods of drought.
In places along the Texas Gulf Coast, for example, “we have too much water during the floods and not enough water during the drought,” said Qian Yang, a research associate in geology at the University of Texas at Austin.
To help balance out that flow, Dr. Yang looked at a concept known as managed aquifer recharge and studied sites in Texas.
The idea is that because aquifers, or large bodies of permeable rock that contain groundwater, get depleted during droughts, cities should work to refill them during times of significant rainfall. The potential benefit would be to avoid the large capital construction, and large geographic footprint, that comes with building new reservoirs because these aquifers already exist.
For the Houston area, the researchers found that under high flow scenarios they could recharge aquifers with roughly the same amount of water as contained in Lake Mead, a reservoir formed by the Hoover Dam.
But in the case of extreme rain events like Hurricane Harvey and what is expected of a potential Hurricane Barry, “you would need some sort of interim storage because the aquifers can’t take the water in that fast,” said Bridget R. Scanlon, a senior research scientist in geoscience at the University of Texas at Austin and co-author on the study.
What many scientists and experts agree on: As climate change increases extreme precipitation, cities will need to adapt.
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