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Doppler To Help Read Hurricanes' Intensity

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By NEIL JOHNSON

The Tampa Tribune

Published: April 18, 2008

Updated: 04/18/2008 12:22 am

TAMPA - Forecasters now can give up to a day of warning when a hurricane headed for land is growing more intense.

A new technique researchers developed uses Doppler radar readings to determine the air pressure in the eye of an approaching storm, a direct indicator of a hurricane's power. Until now, Doppler has been used mainly in spotting potential tornadoes or severe thunderstorms.

Applying it to hurricanes would have provided six hours of additional warning to the change in Hurricane Charley in 2004, said Paul Harasti, a project scientist for the University Corporation for Atmospheric Research in Boulder, Colo., who helped develop the technique.

Charley surprised everyone by quickly growing from a 110 mph storm to a 145 mph Category 4 storm before making landfall near Punta Gorda and remaining at hurricane strength as it moved northeast through Kissimmee and Orlando and across the Florida peninsula.

The National Hurricane Center in Miami will incorporate the intensity changes in the forecasts it issues every six hours, or every two hours once a storm is close to land.

"Charley is an excellent example of how helpful it would be for a storm that's rapidly changing intensity," said Chris Landsea, science officer with the hurricane center.

The method was tested in 2007. Last season, the method accurately tracked Hurricane Humberto's air pressure as it went from a tropical depression to a Category 1 hurricane in less than 19 hours before hitting Texas.

Charley and Humberto illustrate the most serious fear of forecasters - that a storm will rapidly grow just hours before hitting land into a much more powerful hurricane than residents expected to face.

"If the pressure is going down, the wind speed is going up. If we can get reliable pressure, we can see that," Landsea said.

The researchers developed a formula that translates data from coastal Doppler radar stations into pressure inside the storm, as well as into showing the reach of its most powerful winds.

"It's fairly innovative," Landsea said. "It is able to diagnose the central pressure and location of the maximum wind."

More than 20 National Weather Service Doppler radar stations dot the coast from Texas to Maine, meaning an approaching storm usually passes in range of at least one. The weather service has others at inland locations, but those don't reach into the Gulf of Mexico or Atlantic Ocean.

The method offers several benefits to forecasters, mainly providing a nearly constant tracking of pressure changes in the storm's eye.

Hurricane center forecasters usually receive that information from aircraft flying through storms and by dropping packages of instruments from planes. But that method has its limitations.

The aircraft usually can fly through the eye of a storm only every hour or two. Also, the planes don't venture into storms as they get close to land.

Radar can give forecasters pressure and wind data every sweep of the radar, or about every six to 10 minutes.

The technique does not provide a forecast but gives data that can be fed into computerized forecast models. More information fed into the models produces a more accurate forecast.

"It would be one more diagnostic tool for us to analyze what is happening inside a storm," Landsea said.

The technique works only when storms are within range of the coastal radar stations, roughly 100 to 120 miles. But depending on a storm's speed, the method can be used to track a storm for several hours. If the storm is moving parallel to the coast, that time can be stretched up to a day or longer, Harasti said.

"If intensification is happening, it will show up," he said.

In addition to catching changes in a storm's intensity through air pressure, showing the radius of a storm's maximum winds has the potential to improve forecasts of storm surge.

The amount of coastal flooding from a hurricane partly depends on how large an area its winds cover and the hurricane's forward speed.

It works only with data from Doppler radar that can show the direction and speed of objects moving toward or away from the radar.

But it cannot be applied to countries in Central America and the Caribbean that do not have Doppler radar.

At this point, the method cannot be used for storms at sea or out of radar range. That means it will have no impact on the center's three- and five-day forecasts.

Harasti said work is continuing to adapt the technique to Doppler radar mounted on hurricane-hunter aircraft.

"If we could use Doppler data from aircraft, it could improve the three- and five-day forecasts," he said.

Reporter Neil Johnson can be reached at (813) 259-7731 or njohnson@tampatrib .com.

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