So long Saffir-Simpson? NCAR scientists propose new index for evaluating potential hurricane storm damage

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Hurricane Katrina in the Gulf of Mexico, August 2005 (courtesy of NOAA)

In early July 2005, Hurricane Dennis, a Category 3 storm on the Saffir-Simpson Hurricane Wind Scale, was bearing down on the Gulf Coast. Anyone paying attention would have been forgiven for having a foreboding sense of déjà vu.

Just 10 months earlier, another Category 3 storm, Hurricane Ivan, had followed a strikingly similar track, making landfall just west of Gulf Shores, Alabama. Ivan ravaged the region, ultimately causing an estimated $18.8 billion in damages. But Dennis, despite roaring ashore in practically the same neighborhood, caused only $2.5 billion in damages—less than one-seventh that caused by Ivan.

Infrared satellite imagery of Hurricane Ivan (left) and Hurricane Dennis (right). Both storms were rated Category 3, both made landfall in almost the same area, and yet they caused vastly different amounts of damage (Source: NOAA)

Infrared satellite imagery of Hurricane Ivan (left) and Hurricane Dennis (right). Both storms were rated Category 3, both made landfall in almost the same area, and yet they caused vastly different amounts of damage (Source: NOAA)

The fact that two Category 3 hurricanes making similar landfall less than one year apart had such different impacts illustrates a weakness in the Saffir-Simpson scale, the system most commonly used by weather forecasters to categorize hurricane risk.

Scientists at the National Center for Atmospheric Research (NCAR)—in collaboration with global insurance broker Willis—have developed a new index that they suspect can do a better job of quantifying a hurricane’s ability to cause destruction. The Cyclone Damage Potential index (CDP) rates storms on a scale of 1 to 10, with 10 being the greatest potential for destruction.

A prototype for an app that will allow risk managers to easily use the CDP to identify local damage potential is already available and will be among the first tools included in the beta version of the NCAR-based Global Risk, Resilience, and Impacts Toolbox when it is released later this year.

Moving beyond wind speed

On the frequently used Saffir-Simpson Hurricane Wind Scale, hurricanes are placed in one of five categories, based on their sustained wind speeds. On the low end, Category 1 storms have sustained winds between 74–95 mph and are expected to cause “some damage.” On the high end, Category 5 storms have sustained winds of 157 mph or higher and are expected to cause “catastrophic damage.”

Because the Saffir-Simpson scale relies solely on sustained wind speeds, it does not take into account all the characteristics of a storm linked to its destructive power.

“Hurricane wind damage is driven by more than simply wind speed,” said James Done, one of three NCAR scientists who worked on the CDP. “The hurricane’s size and forward speed also are important. A large, slowly moving hurricane that repeatedly batters an area with high winds can cause greater total damage than a smaller, faster hurricane that blows quickly through a region.”

For example, the critical difference between Ivan and Dennis turned out to be hurricane size, according to a study of the storms by Done and Jeffrey Czajkowski at the University of Pennsylvania’s Wharton Risk Management and Decision Processes Center.

To create the CDP, the scientists incorporated hurricane size and forward speed into their index, along with sustained winds. To determine the relative importance of each, the team used hurricane damage statistics from the hundreds of offshore oil and gas facilities that pepper the northern Gulf of Mexico. Because facilities are spread more-or-less evenly across the region, their exposure to hurricanes is approximately the same. Damage differences from storm to storm can therefore be attributed to differences in the storms themselves.

The CDP does not predict actual damage – which could vary markedly, depending on where (or if) a hurricane makes landfall – but instead predicts the storm’s potential.

When applying the CDP to past hurricanes, the index was able to discern a difference between Ivan, which would have scored 5.0 on the CDP prior to landfall, and the much smaller Dennis, which would have earned a 2.4. The CDP rating for Hurricane Katrina, which ultimately caused more than $80 billion in damages in 2005, would have been 6.6.

“The value of the index is in comparing current storms with storms from the past,” Done said. “For example, if a hurricane is approaching New Orleans, you can compare its CDP with Hurricane Katrina’s CDP and get a fuller picture of how much damage the storm is likely to cause.”

The CDP project was led by NCAR scientist Greg Holland, along with NCAR colleagues Done, Ming Ge, and Willis collaborator Rowan Douglas.