UAH scientist studies factors that might form a tornado - WAFF-TV: News, Weather and Sports for Huntsville, AL

UAH scientist studies local factors that might contribute to forming tornadoes

HUNTSVILLE, AL (WAFF) - A new study at The University of Alabama in Huntsville might help determine whether topography ­ the lay of the land ­ makes some places in the Southeastern U.S. more or less prone to being hit by tornadoes.

 Mountains, lakes, oceans, forests, and concentrations of buildings might all have a role in encouraging or discouraging the formation of tornadoes in some locations, says Dr. Tim Coleman, a research scientist in UAHuntsville's Earth System Science Center. They might also influence whether tornadoes become more or less powerful after they form.

 "There has been little research done on these local factors, in part because most of the research on tornadoes has historically been done in the Great Plains where they don¹t have many mountains or large bodies of water," he said. "Most of this isn't going to matter as much in Oklahoma or Kansas as it does in Alabama, Tennessee or Georgia."

Supported by a grant from NOAA, Coleman will study radar and other records of tornadoes looking for insight into factors that might strengthen or weaken the rotation in a storm or a tornado. These factors include surface roughness or friction that slows low level wind; wind channeling between mountains; and slope.

"We have good tornado data back to the 1950s, so my first step will be to pull that data and plot those storms," he said.

The project began when Coleman "just started noticing patterns," especially in radar scans from hurricane Katrina. Storm cells would form in the front part of the hurricane then dissipate as Katrina¹s rotation carried them along the shore line. Coleman began to wonder if something about air flow over water and land might be affecting the storm cells inside the hurricane.

It was known that air flow is influenced by surface "roughness." Wind blows fastest over water, slower over land, still slower over forests and tall buildings, and slowest over mountainous terrain. If Coleman's theory is correct, wind blowing ashore from a hurricane would be slowed as it shifts from sea to land, while wind still offshore would blow parallel but faster.

The faster wind blowing alongside slower moving air might create spin in the atmosphere.

In the northern edge of a hurricane in the Gulf of Mexico, with the wind blowing east to west, the spin would be clockwise. That is opposite the spin of most tornadoes, which means the rotation of storm cells might be buffered.

To test his theory, Coleman looked at Doppler radar data from a summer field experiment at Wheeler Lake and from Mobile Bay once Katrina was inland. In both cases, wind was blowing faster over the water than over the adjacent land, whether it was a light breeze or the fringes of a powerful hurricane.

The Mobile Bay radar image clearly shows the fastest wind blowing north through the bay, flanked by slower wind in both Mobile and Baldwin counties.

He also studied a storm that spun up a tornado over Panama City Beach, Florida, during Hurricane Ivan. The storm wind was blowing from east to west, along the shoreline. The city¹s condominiums are on a peninsula, so the slower air over the city had faster air over water on both sides. The natural spin or "vorticity" on the bay side would have spun counterclockwise. That counterclockwise spin might have contributed to the formation of the tornado.

The same kind of effect might be created by wind being channeled between mountains or through a valley, just as a mild breeze can accelerate to a buffeting wind if it is pushed between two buildings. The faster air current would create spinning vortices on both sides along the edges of slower air.

The channel effect could be enhanced if the valley has a lake in it. Two of the storms Coleman is studying were tornadoes that hit Guntersville Lake in the Tennessee River valley near the city of Guntersville.

"Two of these theories -- friction and wind channeling -- say the west side of Lake Guntersville would be an ideal place for the formation of tornadoes when winds are from the south or west, and you got two tornadoes that formed there in two years," he said. "If something matches the theory and it matches more than once, you have to look at it."

The third thing to be studied is the influence of slopes on tornado formation and strength. The theory is that a spinning storm is squeezed on a hilltop or mountain, reducing spin.  As the storm passes the summit, it would stretch vertically on the downhill slope, increasing spin, similar to an ice skater stretching their body vertically by putting their hands over their head. Coleman has several case studies of tornadoes that became more powerful and caused more damage on downhill slopes.

Coleman hopes his research will lead to local forecasting models tailored to local terrain, perhaps helping the National Weather Service improve warning decisions. "You might have zones you would want to pay extra attention to if the wind is out of a certain direction," he said.

 

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