Spring and Summer Bring Stormy Weather

An intense bow echo approaches the operations center for the BAMEX study (Mid-America Airport, east of St. Louis) on the morning of June 10, 2003. (Photo courtesy Nolan Atkins, Lyndon State University.)

As the Northern Hemisphere tilts towards the Sun, the days grow longer and warmer. That extra dollop of solar heating signals more than T-shirts and barbecues. Spring and summer warmth also fuels the high season for severe thunderstorms and tornadoes.

Atmospheric scientists are discovering new techniques for measuring, tracking, and scoping out the workings of warm-weather storms. Hang on to your hat as we explore some warm-and-stormy weather research.

Thunderstorms: solo acts and group gigs

Thunderstorms are convective systems--they form when relatively warm, moist air near the ground rises and encounters cooler air aloft, just as water in a teakettle boils from being heated at its base.

When the right conditions are in place, a fair-weather cumulus cloud can blossom into a dangerous cumulonimbus (thunderstorm cloud) in less than an hour.

Most thunderstorms are not severe. Single-cell storms produced by a single updraft and small multicell storms grow and die quickly, kicking out a little wind, spitting out some lightning, and depositing rains that are often beneficial. Some parts of the world get most of their precipitation from thunderstorms.

However, if temperatures and wind vary greatly with height, then a thunderstorm can become severe. The strongest thunderstorms produce winds above hurricane force, hailstones larger than golf balls, and even tornadoes.

The largest thunderstorm clusters, which can span over 100 miles (160 km), are called mesoscale convective systems. During the summertime, many of these systems develop in the afternoon across the U.S. High Plains. Often they march east into the Midwest during the night and continue into the next morning. NCAR scientists have identified preferred corridors of storm movement that can persist for weeks at a time.

A large field study called BAMEX, led by NCAR in 2003, examined mesoscale convective systems across the Midwest in detail. Because of that expedition, BAMEX researchers have now documented small-scale circulations embedded in thunderstorm squall lines that not only spew destructive straight-line winds, but may spawn up to 20% of all U.S. tornadoes. Meanwhile, the remnant circulations from large thunderstorm clusters can survive for days, triggering new storm cells. Over warm oceans, similar remnant circulations provide seed for hurricane development. In BAMEX, scientists found that upper-level winds tore some of these circulations apart, while daytime heating helped to strengthen them. Scientists expect these and other findings to help improve forecasts of damaging winds and heavy rain.

Computer models have helped forecasters predict spring storms for many years, but they've struggled to depict just what kind of storms to expect. Now a research model being tested at NCAR is giving some of the clearest, sharpest outlooks of storm type ever produced. With a top resolution of 4 kilometers (about 2.5 miles), the Advanced Research version of the Weather Research and Forecasting model (AR-WRF) can portray individual storms rather than broad-brushing a larger area. An even higher resolution of 2 km is being explored in 2005 by the Center for the Analysis and Prediction of Storms. The storm cells predicted in AR-WRF aren't identical to the ones that actually occur. However, they provide an important heads-up on whether to watch for multicells, supercells, or mesoscale convective systems.

The devil in the details: A tornado-packing supercell prowled the southeast corner of Kansas on the evening of April 21 (right image, supercell at bottom; county outlines are in green, with the town of Emporia at upper left and Joplin, MO, at lower right). In its 24-hour forecast, a seven-year-old version of NCAR's MM5 model (left image), with a resolution of 30 km, gave only a general sense of where storms might form. The Advanced Research WRF model, with 4-km resolution (center image), produced a much more detailed picture of the April 21 storms. Despite some errors in location, AR-WRF accurately showed the risk of large supercells to the south and weaker multicells to the north. (Illustrations courtesy Jim Bresch, NCAR/MMM.) (Click on the model images at left and center for larger versions of each.)

Tornado formation: still shrouded in mystery

Each year more than 1,000 twisters touch down in the United States, more than in any other nation by far. Spring is high season, and the hours between noon and sunset are most likely to see one form. However, U.S. death and injury tolls have dropped considerably in recent decades due to better warnings.

What has research revealed about these fast-striking creatures, and what aspects are still puzzling scientists?

Virtually all tornadoes develop out of thunderstorms. Strong thunderstorms tend to form on a boundary between air masses, similar to a small-scale cold front, that pushes surface air upward. Moisture in the warm air adds to the air's potential buoyancy.

Somehow, that air must be given a spin. Thunderstorms often develop weak rotation as strong winds aloft, sometimes racing eastward at 100 miles (160 kilometers) per hour or more, impart a spin to the column of rising air. On one side of a severe storm--usually toward the south end--you can sometimes see clouds moving in a circular fashion. A wall cloud may hang from a larger rain-free cloud base. Large hail and heavy rain may occur near the wall cloud, and winds can be blowing upward at 100 miles per hour.

What finally produces a tornado? As the updrafts in a thunderstorm intensify, the circulation in the storm's lower levels may tighten into a narrow cylinder, elongate, and speed up, much as figure skaters spin faster by pulling in their arms. In some thunderstorms, this is enough by itself to trigger a weak, brief tornado. These are often dubbed landspouts, because their development is similar to the way waterspouts form over lakes and oceans. Landspouts are especially common across the High Plains of Colorado.

In more sustained storms, called supercells, a powerful downdraft often wraps around one side of the storm-scale rotation, which is called a mesocyclone. Somehow, this interaction appears to kick-start tornadoes, especially the more violent ones. However, the critical moments of tornado formation--the focus of research for over a decade--are still poorly understood.

Big strides were made during VORTEX, the Verification of the Origins of Rotation in Tornadoes Experiment, staged during the 1994 and '95 storm seasons across the southern U.S. Great Plains.

With data from aircraft, radar, weather balloons, instrumented chase vehicles, and ground stations, VORTEX scientists made some key discoveries. For instance, the downdrafts associated with tornadic updrafts tended to be warmer and drier than expected. In contrast, nontornadic storms were typically dominated by rain-cooled downdrafts. Yet the scientists still found few reliable markers that forecasters--under the gun, with limited data--can use to distinguish the supercells that produce tornadoes from others that don't.

A follow-up experiment, VORTEX-2, is slated to zero in on this problem in 2007 and '08 with a new generation of observing tools.

What does it take to chase down a storm?

Once in a while, a major field campaign

Storms are powerful, fast moving, and short lived. To get as close to their inner workings as possible, researchers pull together a large array of observing tools, including some developed specifically for storm chasing. Because storms don't sit still, these sensors need to see a tempest evolve, minute by minute, across its length, breadth, and height.

On the ground, some vehicles are outfitted with mobile weather stations, while others are equipped to launch weather balloons. NCAR's Mobile GAUS facility does both. Meanwhile, stationary wind profilers, like NCAR's Integrated Sounding System, chart winds aloft.

Ground-based radar detects rain in the storm and winds close to ground level to produce one cross-section of the action from its fixed point of view. That's good.

By mounting radar on an aircraft, researchers can look at the heart of the storm, taking multiple slices as the plane flies back and forth. That's better.

In 2003 during BAMEX, for example, an airborne radar detected wind shear aloft that exceeded 400 kilometers/hour (about 250 mph) across just a few miles. This shear was so strong that ground-based radar couldn't accurately interpret it. But the Electra Doppler Radar (ELDORA) attached to the tail of a Navy P-3 aircraft caught the dramatic shear, because it was designed to measure high winds in and near storms. Built by NCAR and CRPE, France's center for Earth and planetary physics research, ELDORA has been probing storms for over 10 years.

The shear caught by ELDORA occurred in one of a spectacular duo of storms on June 22 in southern Nebraska. Along with its strong shear, the storm near Superior bore the largest and strongest storm-scale cyclone ever measured. The next cell north, near Aurora, dropped the largest U.S. hailstone ever measured, at 7 inches (nearly 18 centimeters) wide.

New technology hits the road on a budget

Because research aircraft are expensive to operate, their use for storm chasing is far from routine. Ground-based vehicles, however, can afford to head after storms every spring.

The newest equipment to join in the chase are radars mounted on trucks, including Doppler on Wheels. Less costly than aircraft, DOW and other mobile radars go storm chasing every year to capture thunderstorm and tornado behavior at much finer resolution than standard, fixed-point weather radar can.

The latest-generation Rapid-DOW collects samples every 5-10 seconds. The frequent, detailed, three-dimensional measurements have the potential to capture multiple vortices, tornadogenesis (the birth of tornadoes), turbulence, and other rapidly evolving processes. NCAR provided hardware & engineering expertise to build the first Rapid-DOW in 2003. The Center for Severe Weather Research is deploying an upgraded Rapid-DOW from an operations base in Kansas in May and June 2005.

The first full season of deployment for the new rapid-scanning Doppler on Wheels will take place this spring. (Photo by Carlye Calvin.)

Doppler on Wheels caught dozens of tornadoes in 2004, including this one on May 12 near Attica, Kansas. (Center for Severe Weather Research / Herb Stein.)