No one is sure exactly how a multiple-vortex tornado forms, because it doesn’t last very long and is very difficult to see clearly during its life. Researchers have a number of theories about it that include vortex breakdown or possibly unstable horizontal wind shear affecting the main vortex. Today, it takes familiarity with higher mathematics to understand it all, but back in the 1970s, University of Chicago meteorologist Tetsuya Theodore “Ted” Fujita was the first to identify these complex structures.
♦ How a tornado forms
We don’t often think of it this way, but our atmosphere is indeed a fluid and follows the laws of fluid mechanics. This is very nice because its lets scientists use well-known mathematical models to understand weather events, like the formation of tornadoes, that they would never be able to study in the lab.
According to the Weather World 2010 Project at the University of Illinois, instability in the atmosphere determines how strong the updrafts and downdrafts are in any thunderstorm. Ordinary storms have weak updrafts and downdrafts, while very strong updrafts and downdrafts are the signs of a supercell, the most intense and dangerous type of thunderstorm.
Supercells are notable because their main updraft rotates. This rotation comes about (PDF file) when sheared winds make surface air rotate horizontally. When a supercell then passes over and its updraft pulls that horizontally rotating air up into the storm, it tilts, forming a vertically rotating vortex called a mesocyclone. (“Meso” refers to the size, and “cyclone” is used because the rotation is usually counterclockwise, or cyclonic.)
Meteorologists will sometimes issue a tornado warning when they see a mesocyclone signature on the Doppler radar image of a supercell. Only about 30% of supercell mesocyclones develop tornadoes, but it happens quickly when it comes, and forecasters want to give the public the maximum amount of warning time possible.
A tornado may indeed form if that supercell’s vigorous rotation and the surface low pressure underneath it cause a wave to form on the storm’s leading edge of cool, falling air—called the gust front—when the supercell’s rear flank downdraft (RFD) meets its forward flank downdraft. The colder air from the RFD is wrapped into this area, but so is warm, moist inflowing air, and things start getting extreme at the surface. This also weakens the lower part of the updraft but not its upper part; consequently, the whole rotating vortex stretches out.
A simplified model (PDF file) shows three theories of how a tornado can from from this stretched vortex. In the “dynamic pipe” theory, the rising and rotating air must constrict as it enters the stretched-out vortex. This constricts inflow at lower and lower levels, until finally a tornado funnel forms. The “bottom-up” theory is a lot like the model for formation of the mesocyclone, with a buoyancy difference causing horizontally rotating surface air to rise and begin rotating vertically as it enters the mesocyclone. The third theory, vortex breakdown, takes note of the lower pressure near the surface inside the mesocyclone. When the pressure is low enough, a downdraft starts to descend inside the mesocyclone and forms a tornado when it reaches the ground.
♦ Suction vortices
The vortex breakdown theory also shows how mini-twisters, or multiple smaller vortices, could form on the edge of the main tornado, through a “breakdown bubble.” Another explanation is that horizontal shearing instability “causes the vertical sheet of vorticity at a wind-shift line to roll up into individual vortices, which are stretched vertically by convective updrafts if overhead” (Davies-Jones, R., and Brooks, H. (1993) “Mesocyclogenesis from a Theoretical Perspective”; page 113, in in “The Tornado: its structure, dynamics, prediction, and hazard.” Edited by Christopher R. Church).
A detailed look at all this quickly gets into vector calculus and fluid dynamics theory. While the scientists debate, the rest of us can ponder the fact, that up until the 1970s, no one even knew that tornadoes could have more than one funnel.
♦ Ted Fujita, “Mr. Tornado”
Author of the most widely used scale of measuring tornado intensity, Dr. Ted Fujita was also a pioneer in other forms of tornado research. By all accounts, he was a remarkable man, able to intuitively visualize weather processes that other scientists needed to express in terms of math symbols and, eventually, computers. Reportedly, Fujita didn’t care much for computers, saying that they didn’t “understand” weather.
He was the first to research tornadoes in the field by doing overflights and damage surveys, taking hundreds of photographs, and later, using the information to draw precise maps of tornado tracks. While studying the tracks of the Palm Sunday tornadoes of April 11, 1965, and the Barrington, Illinois, tornado of 1967, Fujita noticed peculiarities in the shape of some debris that led him to suggest the possibility of a smaller “suction vortex” within the larger tornado (Fujita, T. T. and Smith, B. E. (1993). “Aerial Survey and Photography of Tornado and Microburst Damage,” page 479, in “The Tornado: its structure, dynamics, prediction, and hazard.” Edited by Christopher R. Church).
Although “Mr. Tornado,” as the public called him, was world famous in 1971, when his theory about suction vortices was first published, the idea wasn’t accepted. That seems strange to us today, but multiple-vortex tornadoes are fairly rare, and the smaller vortices are usually hidden by debris, rain and swirling clouds. Scientists in 1971 just didn’t have the video and photographic images that we have today.
What is remarkable is that Fujita lacked these as well; in fact he had never seen a tornado at that point in his career. He had seen vortices in a dust devil once while returning from a survey, but his theory was built only on interpretation of damage patterns. There was no direct proof for it until the Super Outbreak of April 3-4, 1974, the worst tornado outbreak in United States history, with 148 confirmed tornadoes in 13 states.
It was also one of the first widely photographed weather events, and many of those photographs and home videos showed tornadoes with multiple, smaller vortices in them. According to Nancy Mathis, Fujita noted afterwards, “Some meteorologists did not believe my story until vortex pictures became available.” (Mathis, Nancy [2007]. Storm Warning [p. 105]. Touchstone. Kindle Edition).
Multiple vortices inside a tornado have higher wind speeds and are the reason why damage can be so intense in one place and light to nonexistent just a few yards away. Ted Fujita was the first person to recognize that multiple-vortex tornadoes exist. While much still needs to be learned about these unique storms, scientists today suggest that the multiple vortices form either as a result of vortex breakdown as a downdraft descends inside the main tornado or possibly because of horizontal wind shear instability affecting its funnel.
