From: Andy Soos, ENN
Published April 21, 2011 12:31 PM

Hurricane Intensity

Coastal residents and oil-rig workers may soon have longer warning when a storm headed in their direction is becoming a hurricane, thanks to a University of Illinois study demonstrating how to use existing satellites to monitor tropical storm dynamics and predict sudden surges in strength. Meteorologists have seen large advances in forecasting technology to track the potential path of tropical storms and hurricanes, but they've had little success in predicting storm intensity. One of the biggest forecast problems facing the tropical meteorology community is determining rapid intensification, when storms suddenly transform into much stronger cyclones or hurricanes.

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Hurricane damage comes not only from wind, but also from rain, tornadoes, floods, and the effects of very low air pressure. So a system that would rank hurricanes by wind force alone would not tell the whole tale.

In the 1970s the Saffir-Simpson hurricane intensity category system was developed to characterize the destructive potential of hurricanes. In addition to maximum sustained wind speed and central pressure, the Saffir-Simpson hurricane categorization includes storm-surge height and coastal destruction potential.

On average, there are about 10 named tropical storms off the east coast of the United States each year. Of these, 6 are likely to develop into hurricanes, but only 2 to 3 are likely to reach Saffir-Simpson category 3 or greater intensity.

"Rapid intensification means a moderate-strength tropical storm, something that may affect a region but not have a severe impact, blowing up in less than 24 hours to a category 2 or 3 hurricane," Harnos said. "This big, strong storm appears that wasn't anticipated, and the effects are going to be very negative. If you don't have the evacuations in place, people can't prepare for something of the magnitude that's going to come ashore."

For example, Hurricane Charlie, which hit southern Florida in 2004, was initially forecast as a category 1 storm. However, when it made landfall less than 24 hours later, it had strengthened to a category 4, causing major damage.

Rapid intensification is so hard to predict in part because it's driven by internal processes within the storm system, rather than the better-predicted, large-scale winds that determine the direction of the storms. The satellite imagery most commonly used for meteorology only looks at the clouds at the top of the storms, giving little insight as to what's going on inside the system.

Harnos and Nesbitt focused their study on passive microwave satellite imagery. Such satellites are used commonly for estimating precipitation, surface temperature and other data. The Illinois researchers were the first to use them systematically to observe hurricane structure and intensity changes.

"What makes it ideal for what we are doing is that it's transparent to clouds. It senses the amount of ice within the clouds, which tells us the strength of convection or the overturn of the atmosphere within the hurricane," Nesbitt said. "It's somewhat like trying to diagnose somebody with a broken arm by taking a picture of the arm, versus being able to X-ray it."

The researchers scoured data from passive microwave satellites from 1987 to 2008 to see how hurricanes behaved in the 24 hours before a storm underwent rapid intensification. Such a big-picture approach, in contrast to the case studies atmospheric scientists often perform, revealed clear patterns in storm dynamics. They found that, consistently, low-shear storm systems formed a symmetrical ring of thunderstorms around the center of the system about six hours before intensification began. As the system strengthened into a hurricane, the thunderstorms deepened and the ring became even more well-defined.

The study also looked at high-shear storms, a less common phenomenon involving atmospheric winds hanging with height.

Such storms showed a different structure when intensifying: They form a large, bull's-eye thunderstorm in the center of the system, rather than a ring around the center.

Since passive microwave satellites orbit every three to six hours, meteorologists can use them to track tropical storms and watch for the telltale rings to give forecasters about a 30-hour window before a storm hits its maximum strength.

"The satellite gives up as snapshot of what's taking place," Harnos said. "We know what's going on, but not how those changes are occurring to end up in the pattern that we're seeing. So what we're working on now is some computer modeling of hurricanes."

For further information: http://news.illinois.edu/news/11/0420forecasting_StephenNesbitt.html

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