PERiLS at Purdue aims to save lives by studying tornadoes that form along squall lines - Department of Earth, Atmospheric, and Planetary Sciences - Purdue University Skip to main content

PERiLS at Purdue aims to save lives by studying tornadoes that form along squall lines


One of the Portable In-situ Precipitation Stations (PIPS) used in PERiLS can be seen in the foreground with lightning in the background at its deployment spot near Moulton, AL. This photo was taken by undergraduate research assistant Faith Vendl in the pre-dawn hours of Saturday, April 1, 2023, during PERiLS-2023 IOP#4.


In light of recent extreme storms creating havoc across several states and producing multiple tornadoes, it is clear that preparedness and research for tornadoes and storms save lives. Dr. Daniel Dawson, Assistant Professor of the Department of Earth, Atmospheric and Planetary Sciences (EAPS), leads a team of researchers and students at Purdue University as part of a large field program called the Propagation, Evolution and Rotation in Linear Systems (PERiLS) and they are on a mission to study how tornadoes develop within thunderstorms that form along lines, sometimes called "squall lines” or quasi-linear convective systems (QLCSs).

According to Dawson, most tornadoes, especially the strongest ones, form from discrete rotating storms called supercells, and therefore those types of tornadic storms have been studied the most. However, many tornadoes, especially in the southeast United States, form within squall lines.

“While these are often not as strong as their supercell counterparts, tornadoes from linear systems are still very dangerous for several reasons,” he explains. “The fact that they often form during the cooler months of the year when people may not have their guard up, as well as during the night, makes them especially dangerous. It is therefore important to understand the mechanisms by which these types of tornadoes form and the degree to which these mechanisms differ from those that lead to supercell tornadoes. Therefore, the PERiLS project was conceived to fill a large gap in our research in this area. It is important to recognize not only the meteorological aspects of how these tornadoes form, but also the socioeconomic factors that go into making the southeast-U.S. particularly vulnerable. The two go hand-in-hand.

Members of the Purdue University PERiLS team include graduate and undergraduate students working with Dawson: Qin Jiang (PhD student), Kristen Axon (PhD student), Hamid Ali Syed (PhD student), Lauren Kiefer (PhD student co-advised with Dr. Dan Chavas), Jacob Bruss (MS student advised by Dr. Robin Tanamachi), as well as undergraduate students Faith Vendl, Cole Sand and Dominic Gery. The field portion of Purdue PERiLS just completed its second and final year of data collection, ending on May 8th, 2023. The team is now beginning to analyze the massive amount of data collected.

There are many different institutions involved in PERiLS that bring many different instruments to bear on the problem, and the teams all work together to make the most out of their various areas of expertise.

“My particular mission is to deploy six portable instrumented probes that include devices called ‘disdrometers’ that count and measure the sizes and fall speeds of raindrops falling through a sheet of laser light,” says Dawson. “My goal is to understand better how precipitation processes behave in linear storms and how they vary in time and space. The distribution of rain drop sizes in these storms could contain important clues about the vertical profile of the wind field, for example, which is relevant to the tornado-producing potential of these storms. Rain drops also evaporate and cool the air off, which is an important mechanism that contributes to the strength of the storms' outflow, depending on the number and sizes of the rain drops falling in a given area. Research suggests that the strength of this cold air outflow at the surface as compared with the warm moist inflow that feeds the storm can affect tornado potential, and we still don't fully understand the relationship, which may be different for linear storms versus discrete supercells.”

PERiLS is a large-scale collaborative field program which incorporates mechanisms to share data freely with other participants, as well as with other researchers outside of the project and in some cases. Even real-time data during field operations can be shared with the National Weather Service to help them in their operations.

“Last year, during PERiLS-2022 IOP#2 on March 30th, 2022, an EF1 tornado passed just to the north (about 1 kilometer) of two of the PIPS (they were deployed side-by-side for instrument comparison purposes),” says Dawson. “The instruments were unharmed, but a sharp increase in wind and a modest drop in pressure was recorded.”

Dawson says there is a common explanation that floats around the media that tornadoes form from "the clash of airmasses." He says at best, this is oversimplified to the point of being completely incorrect. It is true that horizontal temperature gradients associated with the storm’s own outflow are suspected of playing a role in tornado development, but this has very little to do with this colloquialism in that tornadoes often form in thunderstorms that are very far removed from any airmass boundaries, cold fronts, warm fronts, drylines, and the like. He says the true mechanisms of tornado development are far more complex and have much more to do with vertical​ gradients in temperature, a large drop in temperature with height that is favorable for intense storm updrafts, as well as a strong increase in wind speed and a change in wind direction with height (so-called "vertical wind shear") that favors the development of rotation in the storm updraft.

There is also a common thread in the media that states that so-called “Tornado Alley,” an area of the middle United States that is more prone to having tornadic activity, is shifting east.

“There is increasing evidence that over the past 20 or so years areas of the southeast U.S. and midwest (including possibly Indiana) has seen an increase in tornado activity, with a comparable decrease in overall activity in the Plains states,” says Dawson. “That being said, there are still a great deal of tornadoes in the Plains, so the shift is relatively subtle. But the very phrase "tornado alley" is something of an over-simplification as essentially anywhere east of the Rockies in the U.S. can experience conditions conducive to strong tornadoes, and there is a great deal of year-to-year variability. One year the activity may be centered in the southeast, while the next year in the southern Plains, and the next in the Midwest, or even in different regions during different times of the year. As for the reasons for the shift, this is still very much an active area of research.”

No matter where a person is located, it is important to be vigilant when the National Weather Service (NWS) announces warnings or watches for storms. Dawson has advice for people who think they may possible be in the path of extreme weather possible of producing a tornado, which begins with preparedness.

“First, make sure you have a plan in place ahead of time as to what you will do in a tornado warning. Know the best place to shelter in your home. This can be a basement if you have one, or a small interior room well away from outside-facing walls,” he says. “An interior bathroom works well as the pipes help reinforce the walls. Otherwise, know the location of nearby shelters if you live in a mobile home (a mobile home is never​ a good place to shelter as even weak tornadoes can heavily damage or destroy them).”

“Second,” he continues, “make sure you have a reliable way to get tornado warnings from the NWS. The best is to have a NOAA weather radio on hand with fresh batteries in case the power goes out. Most mobile phones can easily be set to receive alerts as well, although if the cellular network is down this could be unreliable. It's important to understand that tornadoes may not always be very visible. This is especially true at night, but even during the day, many tornadoes are wrapped in rain. On top of this, in many areas of the country, there are too many trees and terrain features in the way that may make it hard to see tornadoes until they are too close to get out of the way."

He says the temptation may be very strong to go outside to look for visual confirmation of the approach of a tornado, but this is extremely risky. Looks can be deceiving.

“If you are caught out on the road with an approaching tornado, the best course of action depends on whether you can see the tornado and how far away it is” he says. “If you can see it, it is at a great distance, and it is moving at a large angle to you, it is best to watch it from a distance and wait until it is well past. If it doesn't appear to moving left or right relative to your line-of-sight, it is very possible that it is in fact moving toward you. It is often hard to tell this is the case until the tornado is very close, so my advice in these situations is to treat it as if it is moving directly toward you, and drive at right angles​ away from the tornado until you are out of its path. In the absolute worst-case scenario where you are caught outdoors and don't have time to get out of the way, you should immediately abandon your vehicle and find a low-lying area, such as a ditch, preferably with something you can grab ahold of to anchor yourselves. Lie down and/or give yourself as low a profile as you can and cover your head.”

Purdue PERiLS project is primarily funded by two NOAA grants: one to support the field observations (NOAA/DOC contract #1305M323PNRMA0093), and the other to support the analysis of the data collected (NOAA/DOC/TTU subaward #21B053-03).


About the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University

The Department of Earth, Atmospheric, and Planetary Sciences (EAPS) combines four of Purdue’s most interdisciplinary programs: Geology & Geophysics, Environmental Sciences, Atmospheric Sciences, and Planetary Sciences. EAPS conducts world-class research, educates undergraduate and graduate students, and provides our college, university, state and country with the information necessary to understand the world and universe around us. Our research is globally recognized, our students are highly valued by graduate schools, employers, and our alumni continue to make significant contributions in academia, industry, and federal and state government.



Writer: Cheryl Pierce, Communications Specialist

Contributors: Dr. Daniel Dawson, Assistant Professor of the Department of Earth, Atmospheric and Planetary Sciences (EAPS)

Photo by: Faith Vendl

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