News•May 7, 2013

Scientists Develop New Way of Classifying Hurricanes

For the past 40 years, the Saffir-Simpson Scale has been used to neatly classify all hurricanes into five categories. The scale, created by Herbert Saffir and Robert Simpson in the early 70s, is simple. It measures a storm’s maximum sustained wind speed — no more, no less. Its limitations – the biggest being that it does not account for the size of a storm’s wind field and its capacity to cause coastal inundation at landfall – have spurred scientists to develop a more comprehensive index.

Hurricane Sandy had a massive tropical storm force wind field that at one point spanned the entire East Coast from North Carolina to Massachusetts.
Credit: National Hurricane Center.

Saffir-Simpson’s limitations were brought to the forefront last October by Hurricane Sandy, which briefly reached a maximum intensity of Category 3, and weakened to a Category 1 storm closer to landfall. That minimal hurricane classification may have led some to downplay its destructive potential, a perception that proved dead wrong when the storm made landfall — technically as a post-tropical cyclone at that point — pushing a record-high storm surge onto the Jersey Shore and into New York Harbor, and flooding low-lying areas.

Had people known ahead of time that Sandy’s size and energy were far more powerful than even 2005’s Hurricane Katrina, they might have prepared for the storm differently.

Now, a new study, published in the journal Monthly Weather Review by scientists from Florida State University, proposes a new metric that aims to complement Saffir-Simpson and other recently developed scales by taking into account a storm’s intensity, duration, and size. The metric, known as “Track Integrated Kinetic Energy,” or TIKE, builds from an existing measure of storm integrated kinetic energy (IKE), which was developed in 2007.

This would help augment the Accumulated Cyclone Energy (ACE) index, for example, and the Power Dissipation Index (PDI), both of which seek to describe the amount of energy expended by a tropical storm or hurricane. The biggest benefit of the new TIKE index is that it could raise the alarm for storm surge risk at a time when sea levels are rising and coastal population is soaring. It may also help scientists detect trends in storm size, which may be related to warming sea and air temperatures.

The IKE metric incorporates the ability of a hurricane’s winds to set ocean water into motion, via waves and storm surge, while also measuring how the storm’s winds would affect structures in its path. The TIKE metric expands the IKE concept by adding IKE measurements over the lifespan of a tropical cyclone, as well as over all named tropical cyclones in the hurricane season.

The new study looked at the seasonal and year-to-year variations in Atlantic Ocean tropical cyclone activity during the period from 1990-2011, a period of reliable storm observations, and found that the storms with the highest TIKE values tend to form in the eastern tropical Atlantic. September, which tends to be the busiest month for tropical storms and hurricanes, is also the peak month on the TIKE index, the study found, with the greatest frequency of large storms.

Coastal homes in Texas' Bolivar Peninsula were wiped out by the storm surge from Hurricane Ike in 2008.
Credit: NOAA.

However, during the month of October — which is when Hurricane Sandy struck the U.S. — there tends to be an uptick in the number of particularly large, long-duration storms closer to the western Atlantic, the study found.

Vasu Misra, the lead author of the study and an associate professor of meteorology in the Department of Earth, Ocean and Atmospheric Science at FSU, said recent storms, such as Hurricane Ike in 2008 and Hurricane Sandy, have shown that damages from coastal flooding far exceed the wind damage caused by these storms, and that the size of a storm’s wind field helps determine the height of the storm surge.

“Existing metrics overemphasize the intensity part which is based solely on wind speeds,” Misra said in an interview.

Both Ike and Sandy were exceptionally large hurricanes. In fact, Sandy set a record for having the largest wind field of any hurricane on record, with a diameter of about 900 miles at one point.

“The swath of [Hurricane Sandy’s] very strong winds along the coastline was huge, and it produced a huge amount of inundation along the coast,” Misra said.

According to Misra, Hurricane Sandy had a higher TIKE value than any other tropical storm or hurricane in the North Atlantic Ocean Basin since 1990. Hurricane Lily, which occurred in 1996, was the runner-up, followed by 2010’s Igor, 1995’s Luis, 1997’s Erica, and Hurricane Olga, which occurred in 2001.

Consistent with other research, the study found that La Niña events, which feature cooler-than-average water temperatures in the tropical Pacific Ocean, tend to be associated with hurricane seasons with large TIKE values, as do warmer-than-average conditions in the central and northern Atlantic Ocean.

Misra and his colleagues hope to develop the capability of predicting TIKE values for an entire season, which could be useful for coastal residents and officials, and a welcome addition considering the lessons from Hurricane Sandy.

Editor's Note: This story has been revised to indicate that Hurricane Sandy briefly reached Category 3 intensity, not Category 2 intensity as originally indicated. This is according to a post-event analysis conducted by NOAA.