UCAR > Communications > Staff Notes > June 1995 Search

Into the VORTEX with CLASS

Editor's note: In a previous life as a meteorology student at the University of Oklahoma, I became enamored with storm chasing. It's been part of my life ever since then, so the opportunity to join a VORTEX expedition was impossible to resist. On Wednesday, 3 May, I hit the road with ATD's mobile CLASS unit. With me were MMM scientist Morris Weisman and CLASS technician Larry Murphy.

From where I sat--sidesaddle in the back of the CLASS van, facing a monitor and a raft of dials--it didn't look promising for storms. It was 10:00 a.m. We were cruising down I-35 in a cool, steady southern-Oklahoma rain. Wind profiles were forecast to become highly favorable for supercells, the long-lasting thunderstorms that produce the most violent of tornadoes. But for supercells to form, the atmosphere would have to heat up substantially.

At noon we were to take the day's first sounding (weather-balloon measurement) west of Gainesville, Texas. Along with supporting VORTEX, our job was to sample the air that would flow into supercells. Morris had put forth two of the 20 or so hypotheses being studied in VORTEX. One held that the air at a certain distance from a supercell would be more representative of the surrounding environment than of the storm itself. His other hypothesis was that variations in the warm-sector air feeding a storm were not large enough to contaminate computer-modeling experiments. To test both hypotheses, we needed to take measurements far and wide across a storm's inflow region.

"So are we ready for 20 soundings today?" asked Morris with a teasing grin. Larry let the comment pass with a smile. I soon found out why. Each sounding involves far more than simply launching a balloon. The better part of an hour is required to ensure a steady data stream from the radiosonde as it travels from ground level up to 100 millibars (about 15 kilometers).

This was the first day out in VORTEX for the CLASS van, which Larry had just brought to Norman from Boulder. "I've driven 1,200 miles in three days," he noted with inexplicable energy. The day after traveling from Boulder to Norman, Larry had paid a visit to fixed CLASS sites in the Oklahoma towns of Ardmore and Altus. The goal was to make sure that equipment there was working well and that the locals recruited to launch the soundings were comfortable with their duties.

While Morris and Larry recalled adventures from the 1994 VORTEX season, we sailed into ever-greener terrain, finally crossing the Red River. By 12:30 p.m., we'd gotten into dry, mild air and found a high spot just off a Texas state highway near Bowie. Free of telephone lines and trees, it was a good spot from which to launch. Larry started the process by hitting a few keys on the user-friendly CLASS computer. He then went to the van's tailgate, opened a sealed packet containing a transmitter and prepared the instrument parcel, and began inflating the balloon. Morris stayed inside, feeding responses to queries from the CLASS software that was guiding the process.

Once the balloon was fully inflated and the radiosonde ready to go, Larry ripped open the Velcro-secured sheet of tough plastic that protected the paper-thin balloon and its 1.2 cubic meters of helium. When Larry set the ballon free, it bobbed skyward at four meters per second while the radiosonde package swung beneath it. "It's looking good," said Larry, adding, "A lot of things can go wrong in a launch."

Now came the real uncertainty: would the signal stay clear as the balloon rose? It immediately came in strong-40 to 50 decibels-and stayed there. Every ten seconds, the CLASS software displayed the pressure, temperature, dew point, and winds as just measured by the radiosonde. After only a few minutes, Morris was able to call up a hodograph, a plot that shows the curve of wind trajectories as they change with height.

This first hodograph of the day was encouraging: a smooth and broad sweep from southeast winds at the surface to south and southwest winds through the lowest kilometer of the atmosphere. But the temperature data revealed a cap: a warm layer about one kilometer above the surface. This would inhibit ascent of the surface-based moisture-and thus any storm development-until the surface air warmed further. By the time the sounding was complete, at 1:30 p.m., it was still overcast and not yet 20°ree;C. "It's still cool out here," said Morris as we piled back into the van. "I'm glad I wore long pants today."

Not long after the launch, we received instructions from the VORTEX operations center to head southwest, toward the mesquite-dotted terrain near Olney, for another launch. By the time we began that one, it was 4:00 p.m. and we'd broken just west of the stratus overcast into muggy sunshine with a persistent southeast breeze. The full VORTEX armada, which didn't leave Norman until noon, had by now made it into our radio range. Crackling exchanges among the VORTEX vehicles and the two aircraft punctuated the silent anticipation in the air. But no storms were on the horizon.

The day's second sounding revealed why: a cap was still putting the lid on convection. We needed to go still further south and west, deeper into Texas, where higher terrain and fuller sunshine would provide the only hope of surface warmth strong enough to break the cap. But the cloud edge, still visible to our east, snaked its way back into our path. Coming into Mineral Wells, we quickly found ourselves a few degrees cooler, which forced us west yet again.

By 6:45 p.m., storm prospects were fading fast. We did the day's final launch near Palo Pinto in full sun and a stiff but too-cool wind. A few desultory cumulus straggled upward to our west, but they quickly fell apart against the cap. As the wind reports came in, Morris couldn't help exclaiming, "That's a great hodograph!" Alas, the heating needed for storms was simply not there.

Photo of Murphy, Weisman with CLASS van and apparatus

As cumulus clouds build on the western horizon, Morris Weisman sets up the surface measurement apparatus while Larry Murphy (left) prepares the day's final CLASS launch. (Photo by Bob Henson.)

Just then, the NOAA aircraft and National Weather Service radar (as monitored and reported from the mobile VORTEX operations center) both reported a strong storm near Frederick, Oklahoma-more than 100 kilometers to our northwest and firmly within the surface cool zone, or so we thought. In fact, a narrow pocket of just-warm-enough air had filtered into the southwest corner of Oklahoma. We charged northward, hoping to intercept the cell before sunset, but darkness intervened. The storm ended up producing baseball-sized hail near Lawton, Oklahoma.

Before rolling into Norman near midnight, we met the other VORTEX folks for dinner in Lawton, where-tired and chastened-we reflected on how difficult it was to anticipate when and where storms would develop, even with VORTEX's full array of models and instruments. Once again, Morris, Larry, and I, along with the rest of VORTEX, found ourselves reminded of something that nearly every storm chase has taught me: it's not at all easy to meet up with a tornado. --BH


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Edited by Bob Henson, bhenson@ucar.edu
Last revised: Wed Mar 29 16:34:08 MST 2000