Forecasters get a better fix on fury
By DAVID BALLINGRUD
© St. Petersburg Times,
A hurricane strike on any populated coastline is a terrible thing. But let's face it: It's a much more terrible thing if it happens here.
So the first thing we all want to know about a hurricane is its destination, its track.
Until recently, however, they were less prepared to answer the next key questions: How strong is the storm? How great is the danger?
Today begins the six-month hurricane season. From now until the Thanksgiving leftovers are gone, Floridians must keep an eye on the weather. It's expected to be an "average" season, meaning about 10 named storms, six becoming hurricanes, and two of those becoming Category 3 major storms with winds from 111 to 130 mph.
Since the major storms have vastly more destructive potential than smaller ones, telling the difference is vital to giving proper warnings. But in determining storm intensity, forecasters have lacked good data. Satellites provide wonderful pictures of advancing storms, but they don't reveal much about the chaos inside.
That's now changing.
A new generation of disposable, satellite-guided instruments dropped into approaching hurricanes is giving forecasters a better-than-ever picture of the storms' chaotic, destructive center.
The National Weather Service, which operates the National Hurricane Center in Miami, acknowledged the problem and the progress recently when it gave its highest honor, the Isaac M. Cline Award, to hurricane specialist James Franklin for improving the small instruments, called drop sondes.
"For years there have been debates in the scientific community about how to accurately assess a hurricane's maximum surface wind speed," said weather service director Jack Kelly. "Franklin's research provides a detailed and accurate profile of the inner core of a hurricane -- the most intense and turbulent part."
Sondes have been around for years, but the new ones are linked to the Global Positioning System satellites. As they whirl about near the core, they provide precise wind speed measurements. New sensors are more accurate, and a new transmitter design allows the sondes' signals to be received by nearby aircraft even in the tumult of thunderstorms.
"Electrical activity made it real hard" to receive signals from the old ones, said Franklin.
Each is a 16-inch tube of instruments attached to a small parachute. They are dropped from an aircraft -- sometimes a few, sometimes dozens -- and drift slowly downward through a storm, sending data on barometric pressure, temperature, water vapor and wind speed.
"They allow us to detect important changes in hurricane intensity earlier than we could before," said Franklin. Two years ago, Hurricane Bret was upgraded from a Category 3 to a Category 4 storm on the basis of drop sondes in the eyewall, he said, an increase in intensity that aircraft did not detect until 12 hours later.
Bret made landfall in a sparsely populated stretch of the Texas coastline, but if it had hit a more populous area, the additional 12 hours could have saved lives, Franklin said.
Over the years, the problem has been that forecasters had to project the storm's strength based on readings made in the aircraft.
"We have always known that the wind must increase with height just above the surface," said Franklin. "We just didn't know by how much. Now we do. For example, on average, the wind at the top of a 30-story building will be one . . . category higher than the surface wind.
"The bottom line is that our intensity estimates will be more accurate than they were even just two to three years ago."
There are two deployment strategies.
In the first, designed to help forecast a storm's track, 25 to 50 sondes are dropped "on a more or less regular grid completely surrounding the storm in all directions out to several hundred miles," said Franklin. "The idea is to measure the storm's steering currents."
The second, newer strategy is designed to measure intensity.
"Here, we will drop one or two sondes during each aircraft penetration of the center," Franklin said. "After two penetrations we can have sondes in each quadrant of the eyewall."
Sonde usage is increasing, said Franklin, "as we continue to learn about their benefits."
At a cost of about $500 each, the hurricane center expects to spend about $100,000 on the devices this year.
"Not a big number," said center spokesman Frank LePore, "when you consider the billions in damages caused by landfalling hurricanes."
© 2006 • All Rights Reserved • Tampa Bay Times
490 First Avenue South St. Petersburg, FL 33701 727-893-8111
From the Times state desk
From the state wire