Storm center: a detailed look inside the core of a hurricane
Thursday, September 13th, 2007
Flying an aircraft through a hurricane is risky business, even if the plane is specially equipped for the job. In the hurricane’s eye, skies are clear and calm prevails, but in the ring of intense storms surrounding the eye–the eyewall–rain falls in thick sheets and winds gust to 300 kilometers per hour.
In 2005, despite those perils, the pilots of three “hurricane-hunter” planes flew repeated missions into the cores of the monster storms Katrina and Rita as well as the much tamer Ophelia. During the missions–collectively dubbed the Rainband and Intensity Change Experiment, or RAINEX–scientists” on board the instrument-laden aircraft collected unprecedented data on the structure, configuration, and interaction of clouds within the massive hurricanes. Probes dropped from the planes garnered additional information.
In one case, the aircraft were the first ever to encounter and directly observe a ring of intense thunderstorms just outside the storm’s eyewall. Such secondary eyewalls, which appear to have significant effects on hurricanes’ strengths, had often been detected by satellites and radar but had never been seen in the fine detail achieved during RAINEX.
Analyses of data from that encounter may enable researchers to identify the features within a hurricane that most affect the storm’s intensity. With that information in hand, meteorologists could do a far better job of forecasting wind speed and ocean surge as a storm approached land. Also, scientists say, the new techniques that RAINEX researchers employed on shore to coordinate hurricane hunters’ flights could transform how such missions are flown.
INQUIRING MINDS Whenever meteorologists announce a new tropical storm or hurricane, two questions immediately arise: Where’s the storm headed? and How strong will it be when it gets there?
“The first question is by far the easier of the two” says Hugh E. Willoughby, an atmospheric scientist at Florida International University in Miami. The path that a hurricane takes depends largely on prevailing weather patterns throughout the surrounding region, including factors such as the strength, configuration, and movement of high- and low-pressure areas. Recent improvements in forecasting hurricane paths stem primarily from enhancements in the computer models used to predict weather in general, he says.
Meteorologists gauge the accuracy of path predictions by their “track error”–a measure of how far off its predicted line a hurricane’s eye wanders, explains James Franklin, a forecaster at the National Hurricane Center in Miami. In the 1970s, the average track error in the 3-day forecasts for hurricanes and tropical storms was 700 km. So far this decade, 3-day forecasts have missed the mark only by 300 km on average, he notes.
Predictions of hurricane intensity haven’t improved nearly as much. In the past 2 decades, errors in the National Hurricane Center’s 2- and 3-day forecasts for wind speeds within hurricanes and tropical storms have dropped only a couple of kilometers per hour. That’s because computer models that aim to represent hurricanes must pack data points close together to accurately simulate the small-scale, rapidly evolving features that swirl around the core of a storm. If a computer model has weather-data points spaced no closer than 5 km apart, for example, the theoretical storms it portrays turn out to be “larger, weaker cartoons of their counterparts in nature,” says Willoughby.
“It’s critical for forecasters to get a hurricane’s track right, but it’s an even bigger challenge to predict the strength of its winds,” says Bradley F. Smull, a research meteorologist at the University of Washington in Seattle.
Accurate wind forecasts are vital for several reasons. As well as directly affecting how much dam age a storm inflicts on structures, wind speed dramatically influences the height of a hurricane’s storm surge, the mound of water its winds push ashore. However, hurricanes are notorious for their sudden, and sometimes severe, variations in intensity.
Some of the factors behind such changes are well understood, says Willoughby. For instance, three of the four hurricanes that struck the Gulf Coast in 2005–Dennis, Katrina, and Rita–intensified as they passed over the Gulf of Mexico’s Loop Current, whose warm waters provided a ready source of energy for the storms. Rita strengthened from category-1 status (wind speeds between 121 and 153 km/hr) to category-5 (sustained winds exceeding 250 km/hr) in less than a day.
Then there are murkier influences on storm intensity, such as the interactions between thunderstorms immediately surrounding a hurricane’s eye and those arranged in bands that, seen from space, lend hurricanes a pinwheel appearance. The dearth of information about such interactions led researchers to propose the 2005 RAINEX missions, which ended up differing from previous hurricane-hunter flights in several ways, says Robert A. Houze Jr., an atmospheric scientist at the University of Washington in Seattle.