Why Katrina Became a Monster and Rita Fizzled

It's a tale of two hurricanes. Katrina and Rita were both massive storms as they churned along a similar path through the Gulf of Mexico in 2005. But Katrina remained strong as it approached land while Rita faded. Scientists say they're beginning to understand why the two storms behaved so differently.

A hurricane is a bit like a figure skater doing a spin. Skaters revolve slowly when their arms and legs are extended. But pull in those arms and legs, and the skater becomes a spinning blur.

Robert Houze of the University of Washington says hurricanes rotate, too.

"The storm is a circular vortex swirling around very, very fast," Houze says.

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And in a hurricane, he says, the equivalent of a skater's arms and legs is the wall of thunderstorms that surrounds the eye. As this eyewall contracts, the storm spins faster.

Houze says that's what was happening with Katrina as it approached Louisiana and Mississippi. Just hours before landfall, it was still a Category 5 storm with winds of more than 160 miles per hour.

Rita was a different story. It had become a Category 5 storm out in the Gulf. But by the time it neared the coast, forecasters at the National Hurricane Center had downgraded it to a Category 3 storm.

Houze says Rita had done something similar to a skater extending her legs and arms: Rita's eyewall had gotten bigger.

A second ring of clouds formed, surrounding and eventually replacing the inner ring of clouds.

"When that happens, the storm de-intensifies," says Houze. "It loses its punch rather quickly. Although it was still a very powerful storm, it was not as strong."

If Rita had spent another day or two in the Gulf, the new eyewall would have begun to contract, causing the storm to speed up again.

Houze says he and other scientists were able to look at both storms in detail, because during that summer they were studying hurricanes using planes equipped with high-resolution radar. The timing was perfect.

"Katrina and Rita have given us almost a natural controlled experiment because the storms followed very similar paths, yet Rita formed a secondary eyewall and Katrina didn't," Houze says.

And now, scientists have to figure out why this so-called eyewall replacement happens. There are some hints. For example, Rita had higher humidity near its center and lower humidity farther away.

But even without knowing why a new and bigger eyewall might form, it should now be possible to create computer models that include this possibility.

Hugh Willoughby, a researcher at Florida International University, says the limitations of current models mean forecasters still occasionally underestimate a storm, like Hurricane Andrew in 1992.

Andrew went through an eyewall replacement cycle out at sea. At first the new, larger eyewall caused Andrew to slow down. Then, Willoughby says, the new eyewall began to contract.

"It had completed the cycle, and it was reintensifying as it came across the Gulf Stream and as it came on shore. That was what made it such a bad hurricane."

Willoughby says forecasters are getting better at predicting the intensity of storms. And he says investing more in the technology used to study Rita and Katrina should help with future hurricanes.

The new research appears in the journal Science.

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