Storms' great unknown

Power: Scientists can predict where a hurricane will hit, but not its strength.

Medicine & Science

August 23, 2004|By Michael Stroh | Michael Stroh,SUN STAFF

As Hurricane Charley lumbered up the Gulf of Mexico toward Florida earlier this month, meteorologists warned that the Category 2 storm might pick up steam as it approached the coast.

What nobody expected was just how much.

In a matter of hours, Charley morphed from a bad storm into a whirling buzzsaw, packing 145-mph winds. By the time it landed, it was Category 4, the second most damaging ranking on the Saffir-Simpson hurricane scale - bad enough to kill at least 20 and cause billions in damage as it ripped across the state.

Although meteorologists can comfortably predict where a hurricane will hit, practically to the county, Charley's last-minute surge was a destructive reminder of one of the shakiest aspects of hurricane science: forecasting how powerful a storm will be when it strikes.

"That's the holy grail of hurricane research right now," says Mark Powell of NOAA's Atlantic Oceanographic Meteorological Laboratory in Miami.

Knowing where a hurricane might wander is obviously crucial. But experts say that a busted intensity forecast can be equally costly.

According to NOAA, Category 1 hurricanes, with sustained winds of 74 to 95 mph, cause an average of $50 million in damage. Category 2 storms are 10 times more destructive. A Category 5 - the most potent hurricane on the scale - is 500 times more expensive Category 1.

Despite its importance, predicting whether a hurricane will strengthen or weaken has proved exceedingly difficult. NOAA estimates that its ability to predict intensity lags 20 years behind its capability to track the storms.

So, in the last few years, researchers have mounted a broad attack on the problem, with efforts ranging from new statistical models to technologies that can penetrate the most savage regions of a storm.

One reason strength forecasts are difficult is that they demand a deeper understanding of what's going on inside, around and in the waters below the storm, says NOAA meteorologist John Kaplan, who is developing a new computer model dubbed SHIPS to predict hurricane intensity.

By contrast, he says, predicting a hurricane's location hinges primarily on understanding the winds flowing around the storm. "Intensity is much, much more complicated," Kaplan says.

But scientists are gradually making headway, especially in unraveling the effects of the storm intensity.

At their essence, hurricanes are massive engines, fueled by heat - most of which is siphoned from the ocean. Scientists, for example, have known for some time that hurricanes spawn only after the ocean surface warms past 78 degrees Fahrenheit.

But now a NOAA team is finding that deep, sub-surface thermal energy also plays a key role and can rev up a passing hurricane.

As a hurricane passes over the ocean, its powerful winds churn the water beneath. Cold water can put the brakes on hurricane growth. But warm water has the opposite effect. In a study of Atlantic tropical cyclones between 1993 and 2000, NOAA researchers Gustavo Goni and Joaquin Trinanes found that 32 of the 36 strongest storms perked up after passing over deep warm water.

To find and study these hurricane fuel depots, NOAA scientists are using satellites to measure subtle height differences in the ocean surface. (Since water expands as it heats, warmer columns of water sit a few inches higher than surrounding regions of cooler water.)

These estimates-known as the tropical cyclone heat potential-are now being folded into prediction models to help fine tune forecasts.

Technology is also helping scientists improve their understanding of the eyewall, a chaotic region near the storm's center which, they believe, plays an important role in intensity.

Hurricanes are composed of spiraling bands of thunderstorms, the innermost of which is the eyewall. Because that's where the strongest winds are found, it has also been one of the most difficult areas to explore.

As a result, models are forced to rely on many assumptions about conditions there, especially near the ocean surface. "If you're off by even a little bit, the resulting intensity can be off by a lot," says Joseph Cione, a researcher with NOAA's Hurricane Research Center in Miami.

Underestimating wind speed or temperature near the eyewall by a single degree, he says, could mean the difference between predicting a Category 2 or a Category 4 storm.

To pull back the curtain on this area, researchers have begun dropping instrument-packed cylinders called sondes from hurricane research aircraft.

The devices beam back measurements of wind speed, temperature and humidity every 15 feet, offering a snapshot of one slice of the storm.

Sonde measurements have already forced researchers to abandon some dearly-held assumptions. For example, in a study published last year in Nature, sonde data revealed that the large waves kicked up by a hurricane had less impact on storm intensity than scientists once thought.

To learn more, NOAA's Cione plans to dispatch a pair of instrument-studded drone aircraft to the eyewall, flying within 500 feet of the ocean's surface - far too low for manned research planes.

The project, a partnership between NOAA and the National Aeronautics and Space Administration, will begin Sept. 1.

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