Tracking the Birth of a Hurricane

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Hurricane season is approaching, and determining when a storm cloud is getting up the gumption to start really doing some damage may be easier this year. A historical track record of microwave satellite images has revealed two new patterns meteorologists can now watch for.

Atmospheric scientists at the University of Illinois studied the patterns of satellite microwave images of storm clouds as they intensified into hurricanes. They looked at records from 1987 to 2008 to understand how storms changed in the 24 hours before becoming hurricanes.

Storms can switch to hurricanes in either high or low wind shear conditions — the difference between wind speed and direction.

For storms with high wind shear, a less common occurrence, a bull’s-eye thunderstorm formed in the center of the system before it turned to a hurricane.

In storm systems with low amounts of wind shear, the researchers found a ring of thunderstorms that formed in the center of the storm system just 6 hours short of strengthening to a hurricane.

The bull’s-eye and ring patterns will be especially helpful for meteorologists tracking storms that are close to coastlines and teetering on the edge of becoming full-blown hurricanes.

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“It’s a really critical piece of information that’s really going to help society in coastal areas, not only in the U.S., but also globally,” said University of Illinois atmospheric sciences professor Stephen Nesbitt, co-author of the study published in the journal Geophysical Research Letters, in a press release.

“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,” added University of Illinois graduate student Daniel Harnos and lead author of the study.

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One of the challenges to predicting hurricanes is that much of what causes a storm to intensify happens inside the clouds. Those changes can’t be observed in visual satellite images. And they can happen quickly.

 ”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.

Hurricane Charlie, for example, went from a category 1 storm to a category 4 hurricane in less that 24 hours as it hit Florida in 2004.

Every minute counted during Hurricane Charlie, as Floridians tried to batten down their homes or escape the path of the intense winds and rain.

“Now we have an observational tool that uses existing data that can set off a red flag for forecasters, so that when they see this convective ring feature, there’s a high probability that a storm may undergo rapid intensification,” Nesbitt said. “This is really the first way that we can do this in real time rather than guessing with models or statistical predictions.”

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The microwave image satellites forecasters can use to look for these tell-tale rings and bull’s-eyes orbit the Earth once every three to six hours. That frequency means the images can be used help meteorologists watch changes and predict peaks in storm intensity about 30 hours ahead of time.

“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.”

That limitation led to the next step in the research.

“So what we’re working on now is some computer modeling of hurricanes, both real storms and idealized storms, to see dynamically, structurally, what’s taking place and what changes are occurring to produce these patterns that we see in the satellite data,” Harnos said.

IMAGE 1: Cyclone Catarina, as seen from the International Space Station, March 26 2004, near Brazil. (Wikimedia Commons)

IMAGE 2: University of Illinois atmospheric sciences professor Stephen Nesbitt, left, and graduate student Daniel Harnos analyzed passive microwave satellite data to identify telltale structural rings in tropical storms that are about to intensify into hurricanes. (L. Brian Stauffer)

IMAGE 3: Port Sulphur, Louisiana after Hurricane Katrina. (Wikimedia Commons).

IMAGE 4: Damage from Hurricane Charlie, Wachula, FL, April 27, 2005. (FEMA Photo Library)

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