The rapid intensification of hurricanes, a fearful development that forecasters still struggle to predict, may be tied to an inner battle between a storm's thunderstorms and winds.
Forecasters typically look at outside environmental conditions — ocean temperatures, moisture in the atmosphere and the flow of upper level winds — to model a hurricane's potential power. Hurricanes in favorable conditions predictably intensify. But sometimes, like Harvey, Irma and Maria did last year, they mutate abruptly, becoming even more dangerous with little warning.
In a paper published in the Journal of the Atmospheric Sciences, scientists now say a look inside the storm might provide forecasters with valuable warning signs.
After examining models from a 2014 hurricane that rapidly intensified, they found that interior thunderstorms were able to overcome the power of upper level winds that held them in place. As the thunderstorms begin swirling around the storm's center, they appeared to increase the storm's circulation, make the hurricane more symmetrical and lessen its tilt, allowing it to spin more furiously.
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"I’m not trying to downplay the role of shear because it’s very important. If you have very large shear, you won't have an [intense] hurricane," said Hua Leighton, the study's lead author and a National Oceanic and Atmospheric Administration scientist at the University of Miami's Cooperative Institute for Marine and Atmospheric Studies. "But when shear is in the middle range, then the distribution of the wind relative to the storm becomes very important."
In recent years, National Hurricane Center forecasters have dramatically improved track predictions. Tracks are 30 to 40 percent more accurate than they were just two decades ago. But gains in intensity have been smaller, with the average error at 72 to 120 hours out still off by one category. Forecasters say that's due to the complexity of balancing broader environmental conditions within the narrower confines of the storm's structure, and they warn ultimately they will reach the limits of predictability.
Forecasting rapid intensification, when wind speeds increase by at least 35 mph in 24 hours, is even more difficult.
Hurricanes are basically coils spun out of tropical water and air. When upper level winds push against the counterclockwise spin of systems that form in the Northern Hemisphere, they can help smother storms. The opposite is true when winds flow in the same direction.
What's not clear is why a storm facing moderate wind shear can intensify quickly, a change that can dramatically increase risks to the public.
For this study, researchers used predictions made by NOAA's Hurricane Weather Research and Forecast model for Edouard, the 2014 storm that developed off the coast of Africa as a tropical depression and rapidly intensified to Category 3 strength. Model runs that accurately predicted a rapid intensification showed thunderstorms normally anchored by upper level winds swept up in the hurricane's spin. Leighton said it appeared the thunderstorms were able to outmuscle the power of the winds.
"You have the strength of the storm and the strength of the inner core. It’s a battle," she said.
It may not be possible to predict which will win, but once the thunderstorms start circulating, they appear to set the stage for the other conditions.
Before the information can be developed into a potential forecast tool, Leighton said scientists need to do more case studies to confirm their findings and rule out other influences. Models that factor environmental assumptions, and not real life conditions, also remain an obstacle, she said.
"Even if we understand the process, it’s not like we’re going to have a perfect forecast because we still rely on a model," she said. "It’s a long way to go for the forecast improvement."