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 thunderstormscalled 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
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. Its being overseen by science coordinator
Christopher Davis and his NCAR colleague Morris Weisman, with participation
from NOAAs 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 isnt 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
NOAAs 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
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 thats 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 forecasters 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 its clear an
MCS will form, he adds, its not always obvious where the worst
effects will materialize. The vortices themselves you can identify
pretty well. Models are pretty good at predicting when theyll
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
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, theyll be without a home base for the entire
The ground-based observing platforms include the Mobile Integrated
Profiling System (University of Alabama at Huntsville); two sounding
units from NCARs 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 hasnt seen much action since. BAMEX will
virtually have the place to itself, which simplifies the aircraft operations
enormously. At least on paper, its probably the ideal setup
for this kind of experiment, says Weisman.
UCARs 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. Its
a living, breathing thing on the Web, says JOSSs 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 experimentfunded
primarily by NSFmay compile as many as 15 case studies. With any
luck, that will include two or three especially intense cases. Its
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.