The Severe Weather Institute and Radar & Lightning Laboratories (SWIRLL) building in Huntsville, Alabama.
Where do scientists studying southern twisters meet up? The SWIRLL building at the University of Alabama in Huntsville. The home of the Severe Weather Institute and Radar & Lightning Laboratories, or SWIRLL, not only sounds tornado-ish, it looks it, too. The brick building wraps around a giant vortex-like spiraling staircase enclosed in a cone of glass.
Two dozen or so scientists crowd inside SWIRLL’s impressive main control room for a late morning briefing. Out the windows is a partly sunny March sky. Some in the room grumble about the less-than-severe weather.
“It’s the shear that’s the problem,” says a young man wearing a Mississippi State T-shirt.
The Severe Weather Institute and Radar & Lightning Laboratories (SWIRLL) building in Huntsville, Alabama.
VORTEX-SE team members plan their attack.
Robin and the other VORTEX-SE team members check in on the weather.
“There’s just not enough CAPE,” a middle-aged woman adds. She’s talking about convective available potential energy—CAPE—the stuff that fuels storms. Others chime in, complaining of weak low-level flow, disappointing dew points, and poor cellular convection.
“It’s only 10:30,” someone chimes in with a cheerful Texas twang. “Storms can still cook up.”
VORTEX-SE team members set their deployment area for the day.
Sunny skies are a bummer for this group. Everyone is hoping for bad weather—severe weather, actually. The kind that twists up tornadoes. It’s what they’ve come from as far as Colorado and Massachusetts to study.
All eyes are on the massive nine-paneled screen that covers most of a wall. It shows a map of where Mississippi, Alabama, and Tennessee come together. There are state borders marked in black, counties outlined in red, and highways drawn in yellow. There’s also a fruit salad of differently sized icons, symbols, and numbers.
The map is difficult to read, much less understand. “It’s complicated,” agrees Robin Tanamachi. “That’s why you go to college to learn this stuff.”
Robin and the rest of VORTEX-SE are getting ready to set their tornado trap. All the map’s symbols and numbers make up the knots of the net being laid. Once the teams of scientists drive off in their dozen or so different vehicles, this map will track where everyone goes and keep them updated. It’s called SASSI, the Situational Awareness for Severe Storm Intercept.
Robin discusses a close-up look of the rain on the radar map during the briefing.
SASSI is a computer program that shows all the collected weather information on a map. “In real time,” notes Robin. Team leaders have SASSI on their laptops. This means that when they’re out in the field they can watch the weather measurements coming in. And see where everyone else is.
Robin is in charge of one of three mobile radars during VORTEX-SE. Her team is using the X-Pol truck, the one whose radar looks like a giant metal mushroom. Its specialty? “Getting exceptionally high-resolution scans,” says Robin. A detailed and up-close look inside a storm.
It’s time to head out.
VORTEX-SE’s weather maps include wind speeds, wind directions, and lines of barometric pressure.
VORTEX-SE uses a computer program called SASSI to communicate real-time weather information and show where teams are collecting data on a map.
The radar truck exits the highway. It goes left at the top of the ramp, turning onto the overpass bridge. Down below, four lanes worth of traffic speed by on wet pavement.
Robin watches the radar data being collected in the truck cab.
By the time the radar truck is parked on the gravel shoulder, the sky looks more promising. Dark clouds clump and swell to the south. The rain has picked up and a stiff wind is pushing it sideways a bit. The lunchtime temperature is dropping, too.
Excellent. All are good signs of bad weather on the way.
Inside the cozy truck cab everyone’s eyes are on screens.
Robin goes back and forth between watching her laptop screen of SASSI and X-Pol’s incoming radar scans.
In the back, a meteorology student, Jessica Bozell, is sandwiched in next to a floor-to-roof stack of computer components, watching a mounted monitor.
Joe Waldinger, the radar’s engineer from the University of Massachusetts, is keeping dry in the driver’s seat. “I’m seeing some weird velocity things,” he says, looking at radar scans. A gust of wind rocks the truck as if in agreement.
“Yeah, I see that,” Robin answers, setting her laptop aside. “Let me get out and look at what we’re dealing with here.” As soon as she cracks open the truck door, wind and cold rain invade the cab.
Robin and her team hunker down in the truck, waiting for the storm to arrive.
Outside the sky is dark and the rain is like stinging pellets. Looking south, a thick bolt of blue-white lightning flashes. Unimpressed, Robin slams the door after climbing back in. Back to letting the radar look around.
“Now that’s interesting,” Robin comments after a bit, pointing to the radar scan on her laptop.
Robin points out some stormy weather on the way.
What’s interesting? “This little comma shape.” It’s sort of a sloppy half swirl, like the last bit of bath water disappearing down the drain. “That’s what we call a mesoscale convective vortex.” Vortex? As in a rotating vortex of wind—a tornado?
Nope, says Robin. A mesoscale convective vortex is simply a storm that pulls wind into a swirling shape. “It’s a big weak rotation, rather than a nice tight rotation, like we would want to see with a tornado.”
Switching to SASSI, Robin checks in to see where everyone else is. An icon that looks like a raindrop slides toward the comma-shaped storm. It’s the vehicle Robin’s husband, Dan Dawson, is in.
Robin gets ready to start scanning the skies.
“Dan’s racing, trying to catch up with this thing,” she says. Robin hopes he’s not the one driving. Raindrops hitting a windshield are a big distraction for Dan, she says. That’s because raindrop shapes and sizes are what he studies.
If you want to measure raindrops, a disdrometer is your tool. It sends a laser beam across a gap to a light sensor.
“When a raindrop, hailstone, or snowflake falls through there, it breaks the beam up a little bit,” says Dan, pointing to the instrument. Like a marble dropped in front of a flashlight beam, the instant of darkness created by a falling water droplet is recorded by the disdrometer.
By measuring the amount of laser light lost and for how long, the disdrometer figures out the kind and size of precipitation falling. Every ten seconds, the instrument records its findings. This way meteorologists like Dan can track the rate of rain over time. He’s studying the disdrometer information from VORTEX-SE to see how it’s different from raindrops in Tornado Alley.
Dan Dawson sets up a PIPS, or Portable in Situ Precipitation Station.
“We want to compare them with storms on the Plains with higher CAPE,” explained Dan. Raindrop size matters. Small drops evaporate faster than big ones. How much rain is falling and how many of those raindrops are changing into water vapor around storms makes a difference. Evaporation is cooling, like sweat leaving your skin. How many and how quickly raindrops evaporate during a storm affects whether it grows or dies. A lot of cooling robs a storm of heat, its fuel.
Dan checks on the PIPS’s data recorder. After the storm, he’ll download the information collected onto his laptop.
Dan’s disdrometer is mounted on what looks like a big heavy cube made of metal pipes. It’s called PIPS—Portable in Situ Precipitation Station. Each PIPS has instruments to measure winds, temperature, atmospheric pressure, and humidity—as well as the laser disdrometer to measure the size of the raindrops.
Weather balloons are another tool VORTEX-SE uses to collect information. Each launched balloon carries a radiosonde more than 15 miles (24 km) up into the atmosphere. The radiosonde is a small package of weather instruments attached to a radio transmitter. It sends back measurements during its two-hour journey through the sky.
Dan gets the PIPS ready to record weather data. The disdrometer is the cylindrical stainless-steel instrument on the left. It measures the size of raindrops that fall in between its two halves.
The radiosonde-collected temperature, pressure, humidity, and wind measurements are combined into a chart called a sounding. It shows meteorologists what the atmosphere is like at different heights from the ground up. Soundings are a big part of how CAPE is measured, and so very important for tornado scientists. Soundings are another part of the information net that goes into SASSI.
Back in Alabama, the storms are done for the day. The somewhat disappointed VORTEX-SE teams head for their hotels. Another meeting at SWIRLL is set for the next morning. They hope more bad weather will brew up overnight. If not, the teams will go home to Mississippi and Massachusetts, Tennessee and Texas, returning when the next round of severe weather starts up.
VORTEX-SE scientists launch a weather balloon into the sky. The radiosonde is in the hand of the man on the left.
Meteorologists continue to study the information VORTEX-SE teams collected in 2016 and 2017. “The project is ongoing,” said Erik Rasmussen in late 2017. Scientists will go back to Huntsville to collect more data if the US Congress continues to fund the project. Science research costs money.
Thanks to VORTEX-SE, meteorologists are learning how southern tornadoes are different. “They are a lot less predictable than Great Plains tornadoes,” says Erik. The storms that spin them out are harder to forecast, evolve more quickly, and move fast. They don’t seem to need as much fuel, or CAPE, to get started, either.
The same facts that make Dixie Alley tornadoes different—and so deadly—make them hard to study. Surveying storms in VORTEX-SE turned out to be tough. Twisty country roads through woods and hollows make running down southern storms a challenge.
StickNets are meteorological probes made for easy transport and setup. Each one measures and records barometric pressure, relative humidity, temperature, wind speed, and wind direction.
Laying a net of weather instruments that will catch a passing storm is a good plan. But it only works if you’ve picked the right trapping spot—at the right time. “We probably need to figure out how to cast a larger net,” says Erik. Forecasting where the tornado-making storms will happen isn’t easy. There’s often more than one show going on at the same time. How to pick the winning one is a puzzle. “Even in northern Alabama, tornadoes are relatively rare,” explains Erik. “We have to wait too long to sample enough storms to learn what we are after.”
PIPS includes the laser disdrometer , as well as a wind-speed-measuring anemometer , temperature-tracking thermometer , humidity-recording hygrometer , and a barometer for assessing atmospheric pressure.
Robin agrees that Dixie Alley’s tornadoes haven’t given up their secrets yet. Collecting enough information to understand what’s going on and draw conclusions will take time.
“We need at least five years,” she says. Fortunately, she’s got plenty of other work to do in the meantime.