Stay south of thunderstorm paths, says Purdue scientist

November 17, 2003

Note to Journalists: A QuickTime illustration of a moving storm front is available at

WEST LAFAYETTE, Ind. - Damaging winds can occur in previously overlooked places within a thunderstorm, according to a Purdue University earth scientist. The finding could help meteorologists save lives and reduce injuries by issuing more accurate storm warnings.

Based on new data on the behavior of winds in developing storms, Purdue's Robert J. "Jeff" Trapp has found that the north side of a storm front can host cyclonic winds that are more intense than those at the storm's "apex," or leading point, which is generally thought to usher in the strongest winds. These newly found whirlpools of wind can be miles wide and create gusts reaching 100 miles per hour.

"On average, whatever lies in the path of the apex suffers wind damage," said Trapp, who is an associate professor of earth science in Purdue's School of Science. "However, it's not the whole story. Meteorologists should be aware of these other vortices in order to present the full picture of a storm front."

The study appears in this month's Monthly Weather Review. It was co-authored by Morris Weisman of the National Center for Atmospheric Research (NCAR) in Boulder, Colo., where the team conducted computer simulations that contributed to their research.

Using a supercomputer at NCAR, the team initially set out to look at the tornadoes that can form along a front's leading edge, often called the squall line. These tornadoes are particularly dangerous because of how difficult they are to predict. But what the researchers found in their simulations were much larger vortices that can form at the squall line north of the apex.

"If you've watched a weather program's time-lapse animation of a storm's development, you've seen a squall line as a long, generally north-to-south bank of precipitation," Trapp said. "While the edges of these fronts can resemble straight lines at first, as storms grow in strength a front can look more like a boomerang, with the storm's apex forming the 'point.'"

Trapp said it is north of this "point" that the vortices generally develop.

"These strong, spinning winds can do great damage over large areas," Trapp said. "They are not tornadoes themselves, but tornadoes can develop from them. We plan to research how this happens as well."

The vortices form only on the north side of the apex because of the spinning of the earth, which tends to deter vortices from forming on the south side.

"The effect of this force, called the Coriolis force, is usually neglected in discussions of thunderstorms," Trapp said. "But out work shows that it is critical to the formation of the damaging vortices in squall lines."

After seeing the vortices form in the simulation, Trapp, Weisman and numerous colleagues across the country observed them in many storms in the Midwest during a recent field program called BAMEX. Trapp said he thinks the reason these vortices have been overlooked in the past is because tracks of storm damage are seldom related back to weather radar images, particularly Doppler radar images, which can indicate the presence of vortices. Special data collected during the BAMEX program will provide Trapp and his colleagues the opportunity to do just that.

Trapp said existing technology could be modified to predict this newly found danger.

"The Doppler radars in use around the U.S., known as 'Nexrads,' can be used to detect these vortices," he said. "It's just a matter of adapting the computer software that sorts through the Nexrad data to this problem."

The next step for Trapp and Weisman is to head back to the simulator and attempt to create more sophisticated computer models of the vortices.

"We still have a lot to learn," he said. "Our explanations need to be modified to take into account all the possible real-world factors that we neglected in our initial models. Until we have more specific answers, the most useful thing we can do is simply make meteorologists aware of what could happen and tell them to be on the lookout for it."
This study was funded in part by a grant from the National Science Foundation and by the National Severe Storms Laboratory.

Writer: Chad Boutin, (765) 494-2081,
Source: Robert "Jeff" Trapp, (765) 496-6661,

ABSTRACT Low-level Mesovortices Within Squall and Bow Echoes.
Part II: Their Genesis and Implications
Robert J. Trapp and Morris L. Weisman
This two-part study proposes a fundamental explanation of the genesis, structure and implications of low-level, meso-scale vortices within quasi-linear convective systems (QLCSs) like squall lines and bow echoes. Such "mesovortices" are observed frequently, at times in association with tornadoes. Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs especially when the environmental vertical wind shear is above a minimum threshold, and, when the Coriolis forcing is non-zero. As illustrated by a QLCS simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative, and more notably, planetary vorticity is stretched vertically; hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line. In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with "straight-line" surface winds found at the bow-echo apex and additionally in association with, and in fact induced by, the low-level mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or "upscale" vortex growth. In weaker environmental shear - in which significant low-level mesovortices tend not to form - damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.

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