The Strength of a Hurricane: Eyewall Replacement

Hurricanes are one of natures most unpredictable forces. They can shift paths easily and strengthen without warning, but being able to predict accurately the strength of one of these storms may be able to save lives in the future. Much emphasis in the past has been put into direction, which is the number one need to know factor of a storm, but the second thing we would like to understand is the intensity of a land falling system to better prepare the public for what is in store for them if they refuse evacuation orders. Another plus to understanding intensity, would be that we as the public would be able to prepare in accordance for the oncoming threat. The most vital way thus far in forecasting the strength of a storm is to look at the center and dive into the eyewall.

Watching for changes in the inner eyewall of a hurricane may help forecasters overcome one of their most perplexing challenges: predicting sudden strengthening or weakening of a tropical cyclone. The ability to predict what path a hurricane will follow has improved dramatically in recent years, but anticipating sudden changes in intensity has remained a problem and these new studies may have provided a solution. Knowing a hurricane's strength is vital to making decisions about evacuating areas when a storm approaches, like what low lying areas will be effected by the storm surge based on intensity, as well as how likely tornados are, flooding issues, and wind speeds that could damage certain homes.

Now, a research team led by Robert A. Houze Jr., a University of Washington professor of atmospheric science, is reporting evidence that clouds around the eyewall of a storm can cause sudden changes in intensity. The findings, in Friday's issue of the journal Science, are based on analyses of data collected in 2005 in storms like Katrina and Rita that devastated New Orleans and portions of Mississippi, Alabama, Louisiana and Texas. Katrina and Rita both shocked forecasters with their rapid organization and development. People literally went to bed thinking a category one storm was going to hit the coast and woke up to a category three or four storm, then by mid day a monstrous beast, category five.

The strongest winds in a hurricane circulate in the cloud wall that surrounds the relatively calm and clear eye of the storm. Taking measurements from aircraft flying into these storms, the researchers led by Houze found that occasionally a "moat" of clear air will form outside the eyewall. Winds funneling toward the center of the storm will then form a new eyewall outside the original one, cutting off the storm center from the incoming flow of energy and eliminating the old eyewall. This is known as "Eyewall regeneration" and all hurricanes, and tropical storms undergo many of these phases throughout their lives

Because the new eyewall is larger than the old one, its winds circulate more slowly -- as an ice skater with arms extended spins more slowly than one with arms held close to the body -- thus reducing the intensity of the storm. But the new eyewall can then begin to contract, spinning faster and faster and increasing the storm's intensity. This process is how tropical storms become hurricanes, they actually start rotating raster around an area of low pressure and then collapse upon themselves. After the cluster of storms collapses the eyewall is formed and the process starts over. Each time a tropical storm or hurricane gets more powerful the eyewall collapses upon itself and is renewed in a stronger form.

He noted that while Hurricanes Rita and Katrina moved along similar paths, Rita experienced eyewall replacement many times, moving over a larger distance and just before landfall experienced one of these cycles. Since it was right before landfall the storm was unable to fully set its new eyewall resulting in a larger storm with weaker wind speeds. Katrina on the other hand hit land soon after a phase, and the result were devastating as we all saw. Rita was a large category three storm and Katrina, while large enough, but small compared to Rita, was a much more devastating storm making landfall as a strong category four.

Now the researchers are delving into the detailed information to see if they can determine what caused the eyewall replacement in one storm and not the other at specific times. Right now, they're not sure what triggers eyewall replacement. Scientist are looking at a couple of ideas but nothing definitive can answer this yet.

One possibility focuses on the rain bands that move away from the center of the storm. There appears to be a critical distance from the center of a hurricane that they cannot go beyond, and some weather experts think an accumulation of rain bands can form the secondary eyewall.

Another possibility, Houze said, relates to the pattern of humidity. If the storm is very moist in its interior but has lower humidity away from the center, the rain bands may be confined within the humid zone. Hugh E. Willoughby of Florida International University, who was not part of Houze's team, said in a commentary on the report, "Earth's atmosphere is still fiendishly unpredictable." Developing a way to predict eyewall replacement is crucial to forecasting, he said.

With time we will develop more and more technology to help us better understand this devastating forces of nature. With the advent of satellite tracking we have already made leaps and bounds in saving lives. In the 1920's and 30's people would only have a few days or even less notice before a deadly storm hit. Their safety relied on shipping reports and looking at a good old fashioned barometer. With these new studies conducted to forecast intensity we will see an increase in survival rates among these deadly storms.