A quest to decode a hurricane's secret force

It's a fair question to ask hurricane forecasters: How strong will a storm be when it strikes land? So far, however, it's been a tough question to answer.

Now, scientists are undertaking a $3 million, 45-day field project to uncover key processes within a storm that help turbocharge it or sap its strength in a matter of hours. The ultimate goal is to enable forecasters to predict trends in the landfall strength of a hurricane 12 to 24 hours before it reaches shore.

Several of last year's hurricanes displayed a sudden increase in strength. As Katrina neared Florida Thursday, officials couldn't predict its exact strength at landfall. But it's memories of Hurricane Opal that still cause forecasters to shudder.

The 1995 storm suddenly strengthened as it bore down on the Gulf Coast. It intensified the night before landfall, when sleeping residents would have missed additional warnings. Fortunately, it weakened substantially before hitting land.

Current efforts to forecast storm strength at landfall "have almost no skill," says John Cangialosi, a research associate at the University of Miami's marine and atmospheric science and a forecaster for the new research project, dubbed RAINEX. Between wind damage and flooding from storm surges, "it makes a huge difference if the storm comes in as a Category 2 or Category 3."

The project's focus is the interaction of a storm's eye wall with the bands of rain squalls that spiral out from a hurricane's core. Each feature has been studied individually. This project marks the first time scientists have taken to the field to observe how the squalls play off one another to influence storm intensity, researchers say.

If the first half of this year's hurricane season is any indication, they'll have plenty of storms to study.

"This may be one of the most active Atlantic hurricane seasons on record," noted David Johnson, head of the National Weather Service, when the government updated its seasonal forecast earlier this month. Katrina, which at press time was expected to strike South Florida Friday as a weak hurricane, brings the season total to 11 named storms so far. By season's end on Nov. 30, the National Oceanic and Atmospheric Administration anticipates between 18 and 21 tropical storms. Nine to 11 are expected to reach hurricane strength, while five to seven should become major hurricanes.

A similar assessment comes from William Gray, a Colorado State University atmospheric scientist who pioneered these seasonal forecasts. Dr. Gray's team anticipates a season total of 20 named storms, including 10 hurricanes and six intense hurricanes. The group notes that this is the most active seasonal forecast in the 22 years it has been crafting them.

Yet while storm-track forecasts have improved dramatically over this period, intensity forecasts have lagged behind.

One reason: Factors thought to govern intensity changes are often too localized for forecast models to mimic, notes Wen-Chau Lee, a researcher with the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and one of the project's scientists.

Another challenge: scientists lack the right kind of measurements within the storm to feed into their models.

Thus, researchers say, the RAINEX project could help uncover the kinds of measurements that should be taken regularly to improve the accuracy of intensity forecasts.

The research team, which includes scientists from the University of Miami, the University of Washington, and NCAR, is throwing a small air force at this year's storms.

Three research planes house sophisticated weather radar that can track rainfall and wind patterns as small as 1,300 feet across. Two others will drop instruments for determining atmospheric conditions above and around the storms.

The researchers will test the notion that the rain bands represent a key energy source for storms. Strong updrafts within the thick clouds that form the eye wall draw water vapor from the warm sea surface below. As that warm vapor condenses into raindrops, it releases its heat into the surrounding air. This heat "engine" drives the storms.

Similar processes occur in the squalls that pop up in the bands of rain clouds around the storm's eye.

As an energy source, "these rain bands may eventually affect the intensity of the hurricane - in what way, we don't know," Dr. Lee says.

One process researchers want to unravel involves the formation of a second eye wall surrounding the inner eye in a mature storm. The fastest winds circulate around the eye wall. When a second wall forms, storms tend to weaken. But if conditions are right, this second eye wall can take over as "storm central" and the hurricane can regain strength.

Beyond testing pieces of what researchers hope will become a coherent theory about the internal workings of the storms, the project also involves a fair amount of seeing what's on the other side of the rain band, adds Robert Houze, a University of Washington atmospheric scientist and one of two lead investigators of the RAINEX project.

"There's a high degree of exploration to this project," he says. It involves making simultaneous flights along rain bands and through the eye walls to study the bands' effect.

Since no one else has done this, the researchers expect some surprises.