Archive of Spotlight Feature Articles

Scientists to explore massive thunderstorm complexes

by Susan Cobb

"We came to realize that we'd gone about as far as we could with the idealized simulations. We needed to get good data."

You know it as a long line of thunderstorms that can split an entire state and last all night. This hazardous weather system, known to take on a life of its own, is called a Mesoscale Convective System (MCS), and scientists are in hot pursuit this spring and summer. By land and by air, researchers from several organizations, including the NOAA National Severe Storms Laboratory, will examine two features of intense MCS's responsible for most of the extreme destruction: bow echoes, the cause of large swaths of damaging straight-line surface winds, and Mesoscale Convective Vortices (MCV), producer of convective storms that can result in widespread heavy rainfall and flash flooding.

The Bow Echo and MCV Experiment (BAMEX) is scheduled from May 20 to July 6, 2003. Scientists hope to come away with some answers about how these storms develop and what forecasters need to know to better predict their trails of devastation. According to NSSL scientist Dave Jorgensen, “The overall objective of BAMEX is to improve predictions of bow-echo systems so warning lead time of strong surface winds can be extended.”

The BAMEX study area encompasses most of the Midwest. Field operations, including three aircraft and a forecast center, are based at MidAmerica Airport, just east of St. Louis, Mo. BAMEX, organized by scientists from NSSL and the National Center for Atmospheric Research (NCAR), also involves participants from the NOAA National Weather Service and various universities.

Mesoscale convective systems are not like the garden-variety summer storms that develop and decay in an hour or two. MCS's are often much larger, intense, and long-lasting. They usually develop in the warmth of the late afternoon. As they grow, a downdraft of high winds from rain-cooled air can push it into a bow-like shape, known as a bow echo on radar. Weak tornadoes can form along the bow or at either end, but “the severest of bow-echoes are sometimes referred to as derechoes and can produce hurricane force straight-line winds at the surface” warned Jorgensen. He added, “As an example of such a strong system, three years ago a bow-echo system raced through Kansas City packing winds of 74 mph. It ripped off roofs, downed trees, and cut electric power to about 100,000 homes and businesses. These storms are much larger than a typical tornadic thunderstorm, often spanning 50-100 miles in length and moving up to 60 miles per hour.”

Once an MCS dissipates, its vortex may persist as a focus for new storms the next day, making the vortex a potential forecasting tool. Several days of storms can be triggered as an MCV makes its way across the central and eastern U.S. Heavy rain is a particular threat with multi-day MCV episodes. One MCV that formed in South Dakota in July 1977 ended up producing a catastrophic flood in Johnstown, Penn., that killed 78 people.

Researchers have made progress in simulating MCS's with computer models. Two of BAMEX's principal investigators, Jeff Trapp from NSSL and Morris Weisman from NCAR, have identified a new mechanism for damaging winds at the ground based on the computer-simulated bow echoes. But Weisman said, “We came to realize that we'd gone about as far as we could with the idealized simulations. We needed to get good data.”

Results from the experiment will help bring new knowledge into the forecast office, Trapp said. “Special data collected during BAMEX will allow us to confirm our simulated results, which can be applied immediately to help improve severe weather warnings issued by the National Weather Service,” he explained.

During BAMEX, three research aircraft will track developing bow echoes and MCV's as they move east across the Midwest from South Dakota, Nebraska, and Kansas to the Ohio Valley. Two of these aircraft have Doppler radar on board. A third will release dropsondes—instrument packages that sample the atmosphere and transmit weather data as they descend via parachute.

“We'll be zipping around the Midwest after midnight looking for the biggest, meanest lumps in the soup,” said co-PI Brad Smull from NSSL, alluding to the nocturnal and intense nature of typical bow-echo systems.

Crews in the field will intercept the storms in mobile weather laboratories, deploying weather balloons and using atmospheric profilers and other instruments to sample the storm environment. Some of the other ground-based observing systems to be used in BAMEX include a mobile profiling system operated by the University of Alabama at Huntsville, two mobile GPS sounding systems operated by NCAR, and a mobile mesonet vehicle.

Forecasters from the National Weather Service offices throughout the Midwest will take turns sharing their expertise in St. Louis, examining computer models and diagnosing storm behavior to help coordinate focus areas for BAMEX operations.

“BAMEX will certainly provide unique coordination challenges,” said co-PI and NWS host Ron Przybylinski, who added, “forecasting bow-echo occurrence 24 hours in advance with sufficient accuracy to move the ground teams into place will really tax our capabilities.” But Przybylinski also said NWS forecasters in the region are eager to participate in BAMEX, which is the area's biggest thunderstorm-related study since the 1970's. “This is a once-in-a-lifetime experience. The more knowledge we have about the evolution of these systems, the better we can predict them.”

Additional links:

• Bow Echo and MCV Experiment Web site
• BAMEX: Air and ground-based strategies
• Damaging winds associated with low-altitude mesovortices and bow echoes
• NSSL scientists to explore "bow echoes" in BAMEX

[5/19/03]

CLIMATE · OCEANS, GREAT LAKES, and COASTS · WEATHER and AIR QUALITY
ABOUT US · RESEARCH PROGRAMS · EDUCATION · HOME

Contact Us
Privacy Policy