Chasing mesoscale monsters

BAMEX goes after bow echoes and thunderstorm vortices

by Bob Henson

Tornadic supercells may steal the headlines, but a different kind of storm stirs up a comparable amount of havoc across the Midwest over the long haul. Giant clusters of thunderstorms—called mesoscale convective systems (MCS)—typically form by evening, rage through the night, and die the next morning. Some of them regenerate the following night or even the next. A storm sequence like this can dump as much rain as a landfalling hurricane and plaster huge areas with hurricane-force wind.

Such is the quarry of BAMEX, the Bow Echo and MCV (Mesoscale Convective Vortex) Experiment. Based just east of St. Louis, the project will run from 20 May to 6 July. It’s being overseen by science coordinator Christopher Davis and his NCAR colleague Morris Weisman, with participation from NOAA’s National Severe Storms Laboratory, the Naval Research Laboratory (NRL), and over a dozen universities.

BAMEX is a storm-chasing experiment “gone upscale,” according to Weisman. The reference isn’t to posh surroundings but to the size of the phenomena at hand. A typical tornadic thunderstorm might span 20 kilometers (12 miles). The agglomerations studied in BAMEX can stretch more than 140 km (90 mi) in width and carve paths more than 800 km (500 mi) long.

For years Weisman and colleagues have simulated various modes of thunderstorm growth and decay in computer models. Three years ago, he and Roger Wakimoto (University of California, Los Angeles) decided it was time to study the largest of these modes where they occur often, in a west-to-east belt across the U.S. heartland.

Morris Weisman and Christopher Davis flank a bow echo. (Photo by Carlye Calvin.)

The two scientists joined forces with Davis, who had been studying mesoscale convective vortices in detail. MCVs were discovered in the 1980s, but Davis and NCAR colleague Stanley Trier were the first to reproduce them in a cloud-resolving computer model. Using data from NOAA’s upgraded observing network, the two scientists found far more MCVs occur per year than previously thought. When an MCV outlives its parent thunderstorms, it can help trigger more storms the following day, making it a potentially useful forecast tool.

Big rain, big wind

Davis marvels at a particular MCV that formed in South Dakota on 16 July 1977. Dumping torrents along its way, it caused a devastating flood in Johnstown, Pennsylvania, that killed 78 people. By 21 July, the vortex had moved into the Atlantic, where it became an unnamed subtropical cyclone.

These systems can produce more than rain. When strong upper-level winds descend through the core of rain-cooled air, they can produce a corridor of damaging wind. The winds may cause the leading edge of the system to bow outward, producing a characteristic radar echo.

A strong bow echo appears on radar as a crimson arc that’s a red flag for National Weather Service forecasters. About 20 NWS staff will work shifts at BAMEX to hone their skills and share knowledge. “The people in the forecast offices all through the upper Midwest are thrilled about this project,” says Weisman.

From a forecaster’s point of view, two similar-looking days may have quite different outcomes, notes Davis. “One might produce very damaging winds and the other might produce a marginal case. How do we distinguish between these?” Even when it’s clear an MCS will form, he adds, it’s not always obvious where the worst effects will materialize. “The vortices themselves you can identify pretty well. Models are pretty good at predicting when they’ll occur. If we can develop a conceptual understanding of where the storms develop within these vortices, then we can do a much better job of forecasting precipitation.”

Nomads with sensors

The BAMEX study area extends from the central Plains to the Ohio Valley, which should keep the odometers of ground crews humming. Traveling as a unit between deployments, the teams will zigzag across the Midwest from one MCS to the next, sometimes working past midnight. Other than nightly hotel rooms, they’ll be without a home base for the entire seven-week project.

The ground-based observing platforms include the Mobile Integrated Profiling System (University of Alabama at Huntsville); two sounding units from NCAR’s Atmospheric Technology Division; and a mobile weather station that will double as scout car, seeking out clearings large enough for the balloon launches and profilers to operate safely.

Three aircraft will probe the storms, including P-3s from NOAA and NRL (both with on-board Doppler radars) and a chartered Lear Jet that will deploy NCAR dropsondes, key to analyzing the convective vortices. All three planes will be based at MidAmerica Airport, located about 40 km (25 mi) east of St. Louis. The terminal was built on speculation in the late 1990s and hasn’t seen much action since. BAMEX will virtually have the place to itself, which simplifies the aircraft operations enormously. “At least on paper, it’s probably the ideal setup for this kind of experiment,” says Weisman.

UCAR’s Joint Office for Science Support will help oversee the airport-based operations center. The office also created the online BAMEX catalog, which has a substantial real-time component. “It’s a living, breathing thing on the Web,” says JOSS’s James Moore. The catalog will ingest vast amounts of data from the BAMEX fleet as well as the standard NWS observing network.

If the storm season comes through, this $4 million experiment—funded primarily by NSF—may compile as many as 15 case studies. With any luck, that will include two or three especially intense cases. It’s about time, says Weisman. The last major study of mesoscale convective systems was in 1985, when the emphasis was on fixed sensors. This time, the behemoths are being chased instead of watched. As Weisman puts it, “You could call this the first storm-following experiment for MCSs.”


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North America's ozone: a closer look

Super-sizing a community data trove

Larry Winter: NCAR's new Deputy director

President’s Corner: University roles in the weather and climate services partnership

UCAR Community Calendar

Web Watch

Governance Update