In both cases, National Weather Service maps that tracked the April 27, 2011, tornadoes that struck the communities show them reaching peak strength adjacent to higher terrain.
Now, University of Alabama at Huntsville researchers have found that topography is among factors that can influence tornadoes’ power.
And their study of twisters may eventually be able to help determine whether the mountains around Chattanooga affect how tornadoes move and the damage they do.
Atmospheric conditions were just right for the massive April 2011 storm system that spawned hundreds of tornadoes from Texas to New York over four days.
But other factors contributed to the deadly storms, too, researchers say.
An ongoing study at the university’s Severe Weather Institute and Radar and Lightning Laboratory has identified three external factors — gravity waves, topography and surface roughness — that appear to influence the formation and strength of tornadoes, study author and researcher Dr. Kevin Knupp said.
The study team believes tornadoes can be stretched and intensified or squeezed and slowed by slopes that fall or rise beneath them. While the study focuses on the topography of Alabama, Knupp said tornadoes passing over the terrain of southeast Tennessee and northwest Georgia probably would behave similarly.
“It’s not too much of a stretch when you think about it,” said Greg Carbin, warning coordination meteorologist at the National Weather Service’s Storm Prediction Center in Oklahoma. “The atmosphere acts very much like a fluid, so fluid dynamics apply. Just like when you look at a stream and how water rushes through gaps and rocks in a stream and then it becomes calm on the other side of the barrier.”
Yet, he said, there’s still a lot of work ahead before data from the study can be used with certainty in forecasting or issuing warnings.
The University of Alabama study builds on a 2005 analysis that indicates storms are affected by the first of the three external controls, “gravity waves” — a wave of energy “pushed by the stability of the atmosphere” — that generate atmospheric ripples similar to those radiating from a pebble tossed into a quiet pond, Knupp said.
When gravity waves intersect a storm that is potentially tornadic, “it sort of pulls the trigger,” Knupp said. Gravity waves “provide a source of rotation that the storm can then tap,” he said.
The storm system that moved across the country from April 25 through April 28, 2011, spun out at least 356 tornadoes from Texas to New York. In the eight counties hit in the tri-state area, a total of 81 people died — 25 in Tennessee, 10 in Georgia and 46 in Alabama.
On April 27, 2011, researchers in Huntsville were watching radar images of the storms raging toward Alabama with gravity waves, the first external control, in mind.
So far, their research has revealed two more factors believed to figure in tornado intensity.
“The second external control is topography,” Knupp said, referring to terrain features such as mountains and valleys.
Knupp first noticed the effect of topography in 1989 when an EF4 tornado formed near Redstone Arsenal in Huntsville, and the same effect appeared later in other nearby storms.
One project researcher “plotted the rotation as a function of height versus time, and has shown a really good correlation between the formation of a tornado west of a ridge over Huntsville, weakening as it went over the ridge, and reformation as it descended into the valley on the downwind side of the ridge,” Knupp said.
“This is a pattern we see on a regular basis,” he said, noting that data from those observations still is being processed.
The third controlling factor documented in the study is “surface roughness.”
“Forested areas have a rougher surface than open agricultural regions. Forested regions over mountains are even rougher because the mountain topography has a certain roughness associated with it,” Knupp said.
One hypothesis is that surface roughness causes tornadoes to be “stronger and wider,” he said. “Tornadoes” spawned in laboratory conditions behaved this way, but that doesn’t guarantee that real-life scenarios would duplicate the effect, he said.
While the National Weather Service hasn’t created maps to coincide with Knupp’s study, it has made maps of damage paths, showing where there was more or less destruction.
Topographical and satellite maps of tornado damage tracks in Jackson County, Ala., and Dade, Walker and Catoosa counties in Georgia show damage waxing and waning as tornadoes sped northeast. In some cases, terrain appears to match changes in the storms’ intensity. In others, not so much.
In Trenton, the storm damage worsened as the northwesterly twister came off of Sand Mountain and into town.
In Ringgold, after killing one person there, the EF3-turned-EF4 twister hopped over White Oak Mountain into Cherokee Valley. There, on the valley’s namesake road, dozens of homes were flattened and seven people were killed within 200 yards of one another.
More work is needed “to get to the bottom of the physics of what is really going on,” Knupp said. “We need to be able to simulate that in a computer model.”
In the next couple of weeks, researchers are applying for grants to fund the next step in research, Knupp said.
Computer modeling can test researchers’ hypotheses and ultimately improve forecasting, he said.
“The recent trend in forecasting severe storms has been something called ‘warn-on-forecasts’ where you’re actually taking data from radar and feeding it into a computer model,” he said.
As it stands, “false alarms” — meaning tornado warnings issued because rotation is detected by Doppler radar but no funnel cloud actually touches down — cause people not to take tornado warnings as seriously as they should, he said.
“About 25 percent of supercell storms produce tornadoes,” he said. “But 90 percent of the supercells in the (April 27, 2011) outbreak area produced tornadoes. That was a remarkably high percentage.”
Ultimately, the study aims at more accuracy when forecasters issue warnings, Knupp said.
But Carbin believes it’s “problematic” to use damage paths to determine the impact of terrain on tornado intensity, “because there’s more than just terrain that’s going to play a role in intensity changes,” he said.
“I think a lot of this work is very, very preliminary,” Carbin said. “There’s the question of: Is it the terrain that you’re actually seeing influence the change in intensity or is the change in intensity due to the way the vortex interacts with different altitudes?
“I’m not quite convinced that terrain is going to be an overriding factor in the formation of tornadoes,” he said. But the ongoing study could lead to a better understanding of the dynamics involved in the highly complex wind fields associated with tornadoes.