How Climate Change can Make Hurricanes Explode

September 20, 2017

Washington Post:

“Maria is developing the dreaded pinhole eye,” wrote National Hurricane Center forecaster Jack Beven on Monday evening, as the storm reached Category 4 intensity.

That inward contraction of a hurricane’s eye can be one telltale indicator of what hurricane gurus technically call “rapid intensification,” although a more evocative word might simply be “explosion.” Whatever you call it, it’s something we keep seeing this year. Harvey, Irma, Jose and now Maria have rapidly strengthened — and all too often, have done it just before striking land.

It’s a dangerous and scary phenomenon that scientists and forecasters are still trying to understand.

“It’s not a common event. Typically, that occurs in maybe 5 percent of our forecasts,” said Mark DeMaria, acting deputy director of the National Hurricane Center.

But DeMaria said that this season is seeing more rapid intensification events than usual and that Maria, in particular, appears to have set a key record for hurricane rapid intensification in the Atlantic.

“Looking back through the records, Maria went from a tropical depression to a Category 5 hurricane in just two and a half days,” he said. “I couldn’t find any other tropical cyclones in our historical record that went that quickly from a depression to a Category 5 hurricane.”

That’s a big problem, because rapid intensification sets the stage for worst-case scenarios. Sadly, that’s what happened to the Caribbean island of Dominica on Monday night, hit by Maria at full Category 5 strength.

There’s little chance to warn people or for them to prepare if rapid intensification occurs, so forecasters naturally want to be able to have a handle on it — but it’s a struggle.

“One of the key issues is that it remains quite difficult to predict on a day-to-day basis. And of course, it’s something we would very much like to be able to predict, especially when an intensifying storm is near land,” said Gabriel Vecchi, a hurricane expert at Princeton University.

The National Hurricane Center technically defines rapid intensification as a wind speed increase of at least 35 miles per hour in 24 hours. All four of the most intense Atlantic storms in 2017 beat that easily:

  • On the evening of Aug. 24, a day before landfall, Harvey was a Category 1 hurricane with 85-mile-per hour winds. Twenty-four hours later, at landfall in Texas, the storm was a Category 4 with 130-mile-per-hour winds.
  • At 11 a.m. on Monday, Sept. 4, Hurricane Irma was already a strong Category 3 storm with 120-mile-per-hour winds. But Irma then radically strengthened further, becoming a superpowered upper-end Category 5 storm with 180-mile-per-hour winds in just 24 hours.
  • Following behind Irma in the middle of the day on Sept. 7, Hurricane Jose was a Category 1 storm with 90-mile-per-hour winds. Twenty-four hours later, it was rated a high-end Category 4 with 150-mile-per-hour winds.
  • Beven’s “pinhole eye” language came as Hurricane Maria reached Category 4 intensity, despite having been a Category 1 just 12 hours earlier. But Maria wasn’t done. The storm would leap further to Category 5 strength, ultimately increasing in intensity by 65 miles per hour in 24 hours.

While scientists don’t fully understand rapid intensification, they do know that it has something to do with hurricanes being in a highly favorable environment for intensification in general.

Rapid strengthening tends to happen when waters are warm, when that warm water is deep, when the atmosphere is moist and when there’s little adverse wind flow that could disrupt the storm, according to research papers on the topic and interviews with experts.

And broadly speaking, what we appear to be seeing this year — similar to the catastrophic Atlantic hurricane season of 2005 — is that the environment is extremely hurricane friendly. Storms simply rev their engines and find that the fuel is of the highest grade, and there’s a deep well of it. Then they take off, and there’s nothing to disrupt them.

One key scientific concept that helps explain how such an environment creates the conditions for rapid hurricane intensification is the idea of “potential intensity” — defined as the maximum strength that a hurricane can theoretically achieve in a given environment. That doesn’t mean the storm will actually get there, but when potential intensities are high, you have to worry about what storms are capable of doing — including spinning up very fast.

“Rapid intensification likes to occur when the potential intensity is far from the actual intensity,” said Jim Kossin, a hurricane scientist with NOAA and the University of Wisconsin at Madison. “Rapid intensification likes a lot of head room. Those warm waters have been creating some very high potential intensity, which increases the head room.”

But none of this is, necessarily, enough. Scientists don’t understand every single spark necessary to create a rapid intensifier, which is what makes solving this atmospheric problem so difficult.

“What makes it so mysterious is that there’s a lot of what we call necessary conditions, but they’re not always sufficient,” said Kieran Bhatia, a postdoctoral research associate at Princeton University who is studying the subject.

“We know that certain thresholds need to be met for rapid intensification to occur, but it doesn’t mean that as soon as these switches are turned on, rapid intensification will initiate.”

One of the most striking things about rapid intensification is that according to recent research, it seems to effectively separate out the storms that reach high intensities from those that do not. In a 2015 study, Chia-Ying Lee of Columbia University and colleagues found that hurricanes around the world tend to come in two big bunches — the ones that reach a relatively low intensity, and the ones that get quite strong. And the study found that 79 percent of the latter storms, the strong ones, undergo rapid intensification.

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