Physics Today

Physics Today What we know and don’t know about tornado formation Paul Markowski and Yvette Richardson Citation: Physics Today 67(9), 26 (2014); doi: 10.1063/PT.3.2514 View online: http://dx.doi.org/10.1063/PT.3.2514 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/67/9?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 67.248.153.79 On: Sun, 21 Sep 2014 13:03:13 What we know and don’t know about tornado formation Paul Markowski and Yvette Richardson NOAA LEGACY ARTWORK Forecasters would love to predict violent weather with more accuracy and longer lead times. Researchers are helping them by unraveling the science behind the complex sequence of events that lead to tornadoes. ornadoes and their parent thunder- the time, the two forces are nearly in balance, and storms are among the most intensely vertical accelerations are very small, with air mov- studied hazardous weather phenom- ing predominantly horizontally. But when small- T ena. The vast majority of tornado re- scale pockets of air become cooler and denser or search today is conducted in the US, warmer and less dense than their surroundings, the where tornadoes occur more frequently than any- forces can become out of balance. In fluid dynamics, where else on Earth. Theoretical contributions, com- we call such a pocket of air a parcel: an imaginary puter simulations, and field observations, such as fluid element of arbitrary size, much smaller than those from the 1994–95 Verification of the Origins the characteristic scale of the variability of its envi- of Rotation in Tornadoes Experiment (VORTEX) and ronment but large enough to avoid the complexities subsequent projects, like the recently completed associated with the molecular nature of fluids. VORTEX2, have revealed a great deal.1 (See box 1 The resulting net force on a given parcel of air for an overview of the different approaches to is the familiar buoyancy force, which depends on the studying tornadoes.) In this article we draw on difference between the density of the parcel and that roughly a half century of prior work to summarize of its surroundings, with larger differences result- the latest understanding of how tornadoes form, and ing in larger accelerations. It’s a bit more compli- we discuss where gaps in our understanding remain. cated because the pressure field can also be perturbed and change the force balance. We call such a depar- Deep moist convection ture of pressure from a reference state of hydrostatic Atmospheric convection is the relatively small-scale balance the perturbation pressure. upward and downward movement of air resulting On sunny days convection is ubiquitous in the from an imbalance between the vertical pressure- atmosphere’s boundary layer (typically the lowest gradient force and the gravitational force. Most of 1–2 km), as any air traveler sensitive to the bumps experienced in low-altitude flight can attest. Bound- Paul Markowski and Yvette Richardson are meteorology ary-layer convection is driven by the heating of air professors at the Pennsylvania State University in University as it comes in contact with the warm ground. The Park, where they specialize in severe storms research. They right conditions can trigger so-called deep moist co-organized the second Verification of the Origins of Rota- convection, with large vertical displacements and tion in Tornadoes Experiment and have written a textbook, accelerations of air; in extreme cases the dis - Mesoscale Meteorology in Midlatitudes (Wiley-Blackwell, 2010). placement approaches 20 km and the acceleration This article26 is Septembercopyrighted as2014 indicated Physics in the Todayarticle. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. www.physicstoday.org Downloaded to IP: 67.248.153.79 On: Sun, 21 Sep 2014 13:03:13 0.1 g, where g is the gravitational acceleration. Those numbers hint at the enormous amounts of energy that drive thunderstorms. Box 2 explains how that energy can be quantified. Air parcels ex- pand and cool as they move upward into lower ambient pressure, but the cooling can lead to phase Figure 1. The parts of a supercell thunderstorm shown in (a) an changes for water vapor in the air. In deep moist idealized horizontal cross section. The gray-shaded updraft region is convection, heat release during condensation—and, labeled U, and the downdraft, D. Green shading indicates precipitation; to a lesser extent, freezing—slows the cooling rate the location of most intense precipitation and largest hail is shaded of the parcels as they continue upward. The rate of dark green and marked H. The location of tornado formation, if one temperature decrease in the surrounding environ- develops, is labeled T. The blue barbed line delineates the gust front, ment varies considerably from day to day, from re- which separates warm, humid air in the environment from cool air that gion to region, and in different layers in the vertical has descended to the surface in downdrafts. The blue and red arrows direction. If the ambient temperature decreases rap- indicate the airflow near the surface within the cool and ambient warm idly enough with height, then the rising air parcels air masses, respectively. (Adapted from L. R. Lemon, C. A. Doswell III, will have a higher temperature and lower density Mon. Weather Rev. 107, 1184, 1979.) (b) The same labels are super - than their surroundings, which will give them pos- imposed on a photograph of a tornadic supercell near Bowdle, South itive buoyancy and upward acceleration. Dakota, taken on 22 May 2010 from a vantage point near the purple An atmosphere supporting that kind of vertical star in panel a. (Photo courtesy of Walker Ashley.) air movement is said to be unstable. The rising, buoyant air parcels can be anywhere from a few de- grees warmer than their surroundings to as much as ticity associated with the spin of Earth about its axis. 10–15 °C warmer in an atmosphere with extreme Cyclonic rotation (counterclockwise in the Northern instability. The cauliflower appearance of the mid- Hemisphere, clockwise in the Southern Hemisphere) dle and upper portions of a thunderstorm updraft, is more commonly observed than anticyclonic rota- like the one in figure 1, is a visual manifestation of tion in supercell updrafts. The focus in the remain- tremendous buoyancy. der of the article, therefore, is on cyclonically rotating Air parcels at ground level aren’t necessarily supercell storms and cyclonic tornadoes; however, buoyant to begin with. Oftentimes they must first the general principles apply to anticyclonic torna- rise through a layer in which they have negative does as well. buoyancy before they can experience phase changes Figure 2 summarizes tornado formation in and eventually reach a level where they become supercell thunderstorms. In step 1 of the process, a positively buoyant. Convection initiation is an inter- supercell develops updraft-scale cyclonic rotation— esting and difficult problem. But in a nutshell, deep a mesocyclone—within its updraft high above the moist convection often gets started on elevated ter- ground. The cyclonic rotation comes from the tilting rain features or along boundaries, like cold fronts, of streamwise horizontal vorticity in the storm’s in- warm fronts, and drylines, that separate air masses flow. Streamwise means that the vorticity vector is having different properties. All are places where air aligned with the low-altitude horizontal winds. is forced to rise.2 Box 3 describes just how such tilting occurs. The horizontal vorticity is attributable to vertical wind Supercell thunderstorms shear—the change with height of horizontal winds The overwhelming majority of damaging tornadoes in the storm’s environment (see figure inset in are spawned by what are known as supercell box 2). For example, winds at the surface are often thunderstorms. Figure 1 illustrates the structure of a from the southeast in supercell environments, while supercell and its various parts. The defining charac- winds aloft commonly blow from the southwest at teristic of a supercell storm is its persistent, rotating a much greater speed. updraft. The storm can be visually stunning, with Mesocyclones produced by the upward tilting the rotation often plainly visible to the naked eye. of environmental vorticity are strongest in midalti- The scale of the updraft’s rotation, typically 5–10 km tudes (3–7 km above the ground) and weaker to- wide, is much broader than that of a tornado. ward the ground, because vertical vorticity develops The rotation can be quantified by vorticity, within the updraft only as air parcels rise away from which is the curl of the wind velocity vector. For a the ground. Air parcels do not acquire mesocyclone- supercell updraft, the vertical component of the vor- strength vertical vorticity until they have ascended ticity, or simply the vertical vorticity, is on the order at least several hundred meters, and in many cases of 10–2/s. In midlatitudes, where supercells are most more than a kilometer. So the updraft’s tilting of common, that’s roughly 100 times the vertical vor- the horizontal vorticity can produce neither a near- This article www.physicstoday.orgis copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. September 2014 Physics Today Downloaded 27 to IP: 67.248.153.79 On: Sun, 21 Sep 2014 13:03:13 Tornado formation Box 1. How do we know what we know? The large strides made in understanding tornadoes and their parent storms are the result of observations, numerical simulations, clever applications of theoretical fluid dynamics, and, to a lesser extent, laboratory simulations.

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