By Will Dunham WASHINGTON, March 1 (Reuters) - Scientists using data from planes flying into a large hurricane have documented changes inside these swirling storms that can quickly alter their intensity, and say these insights can improve forecasting. In a study appearing in the journal Science on Thursday, the researchers described how changes in a hurricane's "eyewall" -- the ring of clouds around the storm's calm center -- can dictate changes the storm's strength. Scientists in recent decades have sharpened their ability to forecast the path a hurricane will take -- its track -- but have not made similar strides in predicting the strength of its winds -- its intensity -- when it hits land. As a result, coastal residents trying to decide whether to seek shelter further inland may be in the dark on the danger posed by an approaching hurricane. The researchers used data collected by three planes equipped with sophisticated radar equipment that flew inside hurricanes during the historic 2005 Atlantic hurricane season. They were hurricanes Katrina, which mauled the U.S. Gulf Coast and caused floods that devastated New Orleans, Rita, which struck Texas and Louisiana just three weeks later, and Ophelia, which brushed the U.S. East Coast. Greater understanding of the behavior of a hurricane's eyewall is vital for predicting a storm's intensity because that is where its strongest winds are located. Data from Hurricane Rita showed for the first time that rapid intensity changes can occur when clouds outside the wall of the eye of the hurricane coalesce to create a new eyewall. They saw a band of dry air develop around the existing eyewall as the hurricane swirled into a tighter spin -- similar to a moat around a castle. A new ring of thunderstorms then formed around the existing eyewall, and the new outer eyewall contracted and strangled the inner eyewall, leading to a weakening of the storm. But as the new eyewall contracts, the storm may re-intensify quickly. In less than a day, Rita grew from Category One, the least intense ranking, to a Category Five, the most intense. 'EYEWALL REPLACEMENT' "We comprehensively documented the processes that were taking place in the storm during the eyewall replacement," Robert Houze, a University of Washington atmospheric sciences professor and the study's lead author, said in a telephone interview. "And we did this by flying the aircraft in a unique pattern, a circular pattern, with the plane flying between the two eyewalls and looking out either side with its onboard radar." The researchers also developed a conceptual model to assist computer simulations to predict hurricane intensity changes. Houze said these storms undergo intensity changes in a span of about 12 hours with an eyewall replacement cycle. If a hurricane is within half a day of making landfall, it would be valuable to know how intense it will be when it comes ashore to guide authorities in their emergency management decisions such as evacuations, the researchers said. Hurricane expert Hugh Willoughby of Florida International University, who was not involved in the research, said the findings represent a key milestone on the road to more skillful intensity forecasting. He said current intensity forecasts are not much better than simple statistical extrapolations. The U.S. National Oceanic and Atmospheric Administration provided two research aircraft and the U.S. Navy provided a third, the researchers said. It marked the first time that three Doppler radar-equipped aircraft flew simultaneously in and around these parts of a hurricane.
