The 2011 tornado season was particularly bad for tornadoes and their impacts on society, with nearly 1700 confirmed tornadoes (second highest total since 1950) and an estimated 550 fatalities (deadliest year since 1936). The severity of that particular tornado season prompted many articles in the popular media that questioned whether climate change was to blame and what we can expect in the future. However, there were surprisingly few, if any, articles that addressed how a tornado forms in the first place.
The goal of this article is not to address long-term trends in tornado activity (the scientific community probably isn't able to do this yet), but simply to present the latest understanding of how tornadoes form and to highlight outstanding questions. Our present understanding is derived from decades of theoretical work, computer simulations, and field observations, such as those from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) and subsequent projects, such as the recently completed VORTEX2, which we co-led with six others in 2009-2010 (see the article by Jack Williams in November/December 2012 issue of Weatherwise).
Step 1: The Development of Rotation Aloft
All thunderstorms, regardless of their tornado potential, require an unstable atmosphere. In an unstable atmosphere, rising air becomes warmer than its surroundings, i.e., it becomes buoyant. Buoyancy causes the air to accelerate upward. The buoyancy is acquired primarily by the effects of condensation, which occurs as air becomes saturated during its ascent; if air ascends to sufficiently high altitudes within the storm's updraft, freezing can contribute additional warmth. A parcel of air ascending through an updraft—we can envision a parcel as being a bubble of air that's small enough to be considered to be fairly homogeneous, but much larger than microscopic—can be anywhere from a few degrees warmer than its surroundings to as much as 10-15°C (18-27°F) warmer in an atmosphere possessing extreme instability.
The vast majority of significant tornadoes (EF2 and stronger), and virtually all violent tornadoes (EF4-EF5), are spawned by supercell thunderstorms. Supercells are storms that have a rotating updraft. Horizontally oriented rotation is present in supercell environments owing to the variation of horizontal winds with height—vertical wind shear. For example, winds at the surface might be from the southeast, with winds aloft from the southwest, and they may have very different speeds. A difference of roughly 50 mph between the surface wind and winds at 18,000 feet is typically sufficient to create supercells. Parcels of air in such a wind field—warm, moist parcels that sustain the supercell's updraft—possess what is known as streamwise vorticity. The term refers to spin (“vorticity”) that is aligned with the direction the air parcels are traveling (“streaming”). Air parcels that enter the supercell's updraft spin like spiraling footballs. The horizontal spin becomes vertical as parcels are ingested into the updraft, and the collective influence of the spinning parcels results in a mesocyclone, or updraft-scale rotation about a vertical axis (see Figure 1). Updrafts with mesocyclones can be visually stunning, with the updraft being striated like the threads of a screw and the rotation being plainly visible to the naked eye.
The mesocyclone that results from the tilting of streamwise vorticity into the vertical direction is typically strongest at 10,000-20,000 feet above the ground. Such midlevel mesocyclones are readily detectable by the National Weather Service WSR-88D radars, given their size (several miles in diameter) and altitude. However, the way by which a midlevel mesocyclone develops differs from how rotation develops next to the ground. When only the updraft is responsible for the tilting of horizontal spin toward the vertical, the horizontally spinning air parcels only develop vertically oriented spin as they rise away from the ground. So the updraft's tilting of the streamwise vorticity that originates in the storm's environment cannot produce a tornado, which is a violently spinning vortex in contact with the ground. The differences in the physical mechanisms for the development of rotation aloft (relatively easy to detect with WSR-88D) versus rotation next to the ground (difficult to detect with WSR-88D except at close range) are one reason for imperfect tornado warnings.
Caption: Figure 1. How a tornado develops in a supercell thunderstorm. The photo shows an actual supercell near Flagler, Colorado, on June 2, 2005. The white arrows show the orientation of the axis of rotation, the green curved arrows indicate the sense of spin, and the red/blue arrows indicate the paths taken by updraft/downdraft air parcels.
Caption: Figure 2. In discriminating between supercell environments that favor strong-to-violent tornadoes (EF2+) versus environments that support only weak tornadoes or nontornadic supercells, forecasters pay considerable attention to the relative humidity and wind shear in roughly the lowest half-mile. As the low-level wind shear and relative humidity increase, strong-to-violent tornadoes become increasingly likely.
Step 2: The Development of Rotation Next to the Ground
The development of rotation next to the ground requires a downdraft. All thunderstorms have downdrafts in addition to their updrafts, and supercells are no different. The air that sinks within a downdraft is usually cooler than its surroundings. This is due to the evaporation of rain and, to a lesser extent, the melting of hail and snow. Once downdraft air reaches the ground, it spreads away from the storm as outflow. The leading edge of the outflow is called the gust front. If you've ever experienced the cool breeze that precedes the arrival of a thunderstorm (typically blowing toward you from the area of heavy rain), then you've experienced the outflow from a downdraft.
Supercell storms typically have an expansive region of downdraft and outflow that extends from northeast (ahead) of the updraft, around the updraft's northern flank, and wraps around the western (rear) flank. Though non-supercell thunderstorms usually only feed off warm air from the environment, the supercell updrafts are strong enough to forcibly lift some of the air parcels from the cool outflow (such parcels are heavy and would not rise on their own if not for the strong sucking action of the supercell's updraft), in addition to the warm parcels from the environment.
Outflow air parcels that are drawn toward the updraft gradually descend as they travel toward the updraft because they are cooler than the environment. Those parcels that travel along the immediate cool side of the gust front experience a horizontal temperature gradient en route, with warm air to the parcel's left and cool air to the parcel's right, with respect to the direction that the parcels are traveling (from right to left in Figure 1). The horizontal temperature gradient generates so-called baroclinic vorticity about a horizontal axis—essentially a torque is being applied to the parcels by virtue of the fact that relatively warm air rises and relatively cool air sinks. For example, the horizontal spin that is produced on the flanks of a small pour of milk into a glass of water is the result of the same dynamics: the horizontal density difference between the (heavier) milk and (lighter) water generates horizontal spin just like the horizontal temperature difference between (heavier) cool air and (lighter) warm air.
The horizontal spin of the parcels that gradually descend within the downdraft and outflow becomes tilted upward as the parcels near the ground. Thus, vertical rotation can be acquired next to the ground within the outflow, i.e., within air parcels that have a prior history of descent and baroclinic vorticity generation. As was brought to our attention by Dr. Johannes Dahl of North Carolina State University, the evolution of the axis of rotation resembles the evolution of the pitch of a landing airplane. As the plane descends along its glide path, its nose lifts upward off of the glide path. (Of course, the plane eventually becomes horizontal once touchdown occurs, whereas the axis of rotation of a downdraft parcel can have a large inclination angle at the ground.) In contrast, recall that air parcels possessing only environmental horizontal vorticity and tilted only by an updraft acquire significant vertical vorticity only after the parcels have risen a considerable distance away from the ground.
Step 3: The Intensification of Near-Ground Rotation
Though the development of near-ground rotation in supercells is a prerequisite for tornadogenesis, in recent years we've learned that most supercells develop near-ground rotation yet are nontornadic, i.e., the rotation fails to reach tornado strength. The vertical vorticity that arises next to the ground in Step 2 is roughly one-hundredth that of a tornado. Tornadogenesis requires a dramatic intensification of the vertical vorticity acquired in Step 2.
The intensification occurs by way of the “figure-skater effect”—referred to by scientists as the conservation of angular momentum or vorticity stretching. A figure skater spins faster as she draws her arms closer to her axis of rotation. The same principle applies to the spin about a vertical axis produced in Step 2. If the spinning air can be converged—drawn inward toward its axis of rotation—it will spin faster.
The convergence of the spinning air depends on the extent to which the spinning air can rise: Air that is accelerated upward is unavoidably required to be associated with the convergence of air below (if not, a vacuum would develop!). Recall that the parcels that have vertical vorticity next to the ground are parcels that previously descended through a downdraft. In other words, these parcels are cooler than the environment. In order for them to be accelerated upward, and in doing so, promote convergence and the rapid intensification of rotation to tornado strength, either the parcels must not be too cold or the supercell's updraft must have unusually strong “suction” just above the ground (to the scientists, it's a strong upward-directed pressure-gradient force). The suction is associated with the rotation in the overlying updraft. More will be said about this in the next section.
One of the principal findings of the VORTEX project was that the outflow of tornadic supercells tends to not be as cold as the outflow of nontornadic supercells. In tornadic supercells, the outflow/downdraft air that bears the rotation that is intensified to tornado strength is sometimes just a few degrees colder than the environment. The “suction” tends to be strong as well. The combination of only slightly cold air and strong suction from above makes it likely that upward accelerations and convergence near the ground will be sufficiently strong to intensify vertical vorticity to tornado strength. The parcels of air spin faster as they near the axis of rotation, and they ascend rapidly as well. Conversely, in nontornadic supercells, the outflow/downdraft air can be up to 5-10°C (9-18°F) colder than the environment, which implies that the air is heavy and resists upward acceleration. The suction acting on the near-ground rotation is often weak as well, either because the rotation is shunted away from the strongest suction by the cold outflow, or because the suction is just weak overall (more on this below). The bottom line is that upward accelerations of the cold, heavy air parcels near the ground are inhibited, and air simply spreads away from the storm along the ground. The lack of a strong figure-skater effect in this case results in the near-ground rotation remaining well below tornado strength.
Tornado Forecasting and Nowcasting
In the past decade, forecasters have become skillful at discriminating between the environments capable of supporting strong-to-violent (EF2+) tornadoes and environments incapable of supporting such tornadoes. For example, large outbreaks are now routinely predicted by the Storm Prediction Center days in advance, “high-risk” outlooks capture most major tornado events, and strong-to-violent tornadoes rarely occur outside of tornado watches. The fact that schools sometimes are dismissed early on tornado outbreak days testifies to the skill and public confidence in today's forecasts.
In discriminating between tornadic and nontornadic supercell environments, considerable attention is paid to the relative humidity and vertical wind shear in roughly the lowest half-mile, both of which are relatively easy to diagnose in real time and are fairly well-predicted by operational numerical weather prediction models (this is what enables skillful outlooks to be made days in advance in some situations). Assuming that conditions will be present to support supercell thunderstorms in general, i.e., that the environment has sufficient wind shear and instability to favor rotating updrafts, tornadogenesis becomes increasingly likely as the low-level wind shear and relative humidity increase (Figure 2). On tornado outbreak days, the lower atmosphere can be so humid that cloud bases are just a couple thousand feet above the ground (the cloud base lowers as the relative humidity increases). The wind shear can be so extreme that winds can vary by 50 mph between the ground and cloud base.
Enhanced low-level wind shear usually implies enhanced low-level streamwise vorticity, and the tilting of the enhanced streamwise vorticity promotes a stronger mesocyclone in the updraft that overlies the near-ground rotation that develops in Step 2. The stronger mesocyclone is associated with lower pressure. (Think of stirring a cup of coffee—the faster you stir, the greater the fluid drop/pressure drop at the center of rotation.) The lower the pressure that overlies the near-ground rotation, the stronger the suction will be. As for the effect of enhanced relative humidity, as the low-level relative humidity increases, the outflow/downdraft air parcels tend to be less cold because evaporation is suppressed. The combination of the strong suction and air parcels that are only slightly colder than the environment strongly favors the intense near-ground upward accelerations and associated convergence required in Step 3.
Though analyzing low-level wind shear and relative humidity has worked well for identifying environments capable of supporting strong-to-violent tornadoes, we as of yet have little ability to discriminate between weak-tornado (EF0-EF1) supercell environments and nontornadic supercell environments. Many weak tornadoes also occur in non-supercell thunderstorms such as squall lines. And waterspouts and landspouts (which also tend to be weak) can develop from otherwise benign cumulus congestus clouds. Though there is some evidence suggesting an enhanced tornado threat in squall lines when the instability and low-level shear are exceptionally large, identifying environments favorable for waterspouts and landspouts has proven even more difficult.
Even if the environment is known to be extremely favorable for supercell tornadoes, we have a limited ability to say when or if a specific storm will produce a tornado. For example, even on tornado outbreak days, not all of the supercells are tornadic, and even the tornadic supercells are not tornadic all of the time. Our ability to say whether a particular radar signature is associated with a tornado is also limited. In some cases, forecasters are vigilant if a supercell appears to be approaching a preexisting air-mass boundary, such as a warm front or outflow boundary from a prior storm. These are regions in which the low-level wind shear and relative humidity often are enhanced, and near-ground rotation sometimes rapidly intensifies within storms upon encountering these regions.
These and other tornadogenesis “triggers,” such as small-scale surges of outflow and descending precipitation shafts on the supercell's rear flank, are being actively investigated by scientists at the present time. VORTEX2 data already have provided some insight into how these small-scale triggers can operate. Scientists are also taking a closer look at the contribution to near-ground vertical rotation from the horizontal spin that's generated by the friction between the ground and the air passing over it, as well as the processes by which tornadoes are maintained once they form. As of now, if a tornado is occurring, forecasters have a limited ability to provide guidance to the public on the tornado's current intensity (spotter reports are about the only source of information), future intensity, or expected duration. Not knowing the optimal tornado warning duration affects the number of people who are potentially unnecessarily warned.
We anticipate that tornado warnings will continue to improve, owing to the continued hard work of forecasters, new discoveries by scientists, and the continued transfer of scientific information from scientists to weather forecasters. Optimism also is warranted by promising new technologies, such as dual-polarization WSR-88Ds (which can detect tornado debris signatures, at least in close-range storms, and perhaps precipitation characteristics favorable for tornadogenesis), gap-filling radars, and the development of extremely short-fuse, high-resolution computer models that are capable of making thunderstorm-specific predictions. We look forward to writing about these new developments in a future article.
Can a Tornado Form Simply by Abruptly Tilting Horizontal Vorticity at a Gust Front?
It is tempting to imagine that a tornado could develop if extreme horizontal vorticity (attributable to extreme variations in wind with height) is simply tilted into the vertical by the thunderstorm's gust front. However, theory and computer simulations reveal that this mechanism fails to produce a tornado because of the deceleration that occurs within an air stream that approaches a gust front. In order for air to rise abruptly at the leading edge of the cold outflow, air must decelerate as the gust front is neared. The deceleration reduces the horizontal spin of the parcels via the same physical mechanism that increases spin in the presence of accelerations (recall that the final step in tornado formation involves upward accelerations that rapidly increase rotation via the “figure-skater effect”). With only weak horizontal vorticity to be tilted upward at the gust front, regardless of how strong the horizontal vorticity is well ahead of the gust front, only weak vertical vorticity is produced next to the ground, and so no tornado is produced.
Caption: Figure 3. A tornado cannot form via the abrupt tilting of extreme horizontal vorticity by a gust front because the horizontal vorticity is weakened considerably during the deceleration that occurs prior to its tilting. The magenta arrows show the orientation of the axis of rotation (the size of the magenta arrows is proportional to the vorticity strength), the black curved arrows indicate the sense of spin, and the gray arrow shows the path of a parcel of air as it approaches and rises over the gust front. The cold air is shaded dark blue, and the gust front is drawn as a cold front.
Tornadoes in Non-Supercells
Though strong-to-violent tornadoes are almost exclusively produced by supercells, tornadoes also occasionally occur in non-supercell thunderstorms, such as squall lines. Some of these situations involve a supercell that is overtaken by a squall line and subsequently becomes embedded within it. In other cases, it is possible that the squall line contains a downdraft that serves a similar role in the development of near-ground rotation as the downdrafts in supercells (as in Step 2 in Figure 1).
Other tornadoes, called landspouts and waterspouts over land and water, respectively, can develop beneath rapidly growing cumulus congestus clouds that eventually become otherwise nonsevere showers or thunderstorms. Sometimes these waterspouts are referred to as fair-weather waterspouts to distinguish them from the tornadoes that occur in conjunction with the mesocyclone of a supercell thunderstorm that just happens to be over water (though the showers and thunderstorms that often closely follow fair-weather waterspout formation should probably not be regarded as “fair weather!”).
Landspouts and waterspouts develop from the amplification of near-ground vertical vorticity that preexists the updrafts associated with the tall cloud development (Figure 4). Such vertical vorticity can be present along wind-shift lines, which also happen to be favorable locations for thunderstorm initiation. The amplification of the vertical vorticity happens via the figure-skater effect, i.e., Step 3 in Figure 1. Steps 1 and 2 are bypassed; because the vertical vorticity already exists at the surface, there is no need for the downdrafts of Step 2. A midlevel mesocyclone (Step 1) is not needed either. In fact, these tornadoes are actually favored in environments containing weak vertical wind shear, unlike supercells. The weak shear tends to reduce the horizontal motion of the updraft relative to the underlying reservoir of near-ground vertical vorticity, thereby increasing the time that the updraft can amplify the vorticity via the figure-skater effect.
Caption: Figure 4. Tornadoes sometimes develop without a preexisting midlevel mesocyclone. Near-ground vertical vorticity preexists the updraft and is simply intensified to tornado strength via the figure-skater effect. A downdraft is unnecessary. The magenta arrows show the orientation of the axis of rotation (the size of the magenta arrows is proportional to the vorticity strength), the black curved arrows indicate the sense of spin, and the gray arrows show the convergence and ascent of air beneath the growing updraft.
PAUL MARKOWSKI meteorology professors at Penn State University, where they specialize in severe storms research. They co-organized the Second Verification of the Origins of Rotation in Tornadoes Experiment and have authored a textbook, Mesoscale Meteorology in Midlatitudes (Wiley-Blackwell, 2010).
YVETTE RICHARDSON meteorology professors at Penn State University, where they specialize in severe storms research. They co-organized the Second Verification of the Origins of Rotation in Tornadoes Experiment and have authored a textbook, Mesoscale Meteorology in Midlatitudes (Wiley-Blackwell, 2010).