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International Journal of Modern Physics BVol. 28 (2014) 1475004 (8 pages)c© World Scientific Publishing CompanyDOI: 10.1142/S0217979214750046

Comment on “Eliminating the major tornado threat

in Tornado Alley”

Johannes M. L. Dahl

Department of Geosciences, Texas Tech University,

Box 41053, Lubbock, TX 79409, USA

johannes.dahl@ttu.edu

Paul M. Markowski

Department of Meteorology, The Pennsylvania State University,

503 Walker Building, University Park, PA 16802, USA

pmarkowski@psu.edu

Received 8 July 2014Accepted 21 October 2014

Published 11 November 2014

The authors draw from half a century of meteorological research to expose flaws ina recent proposal to build 300-m-tall tornado-prevention walls across the U.S. GreatPlains. The idea behind the walls is that they would prevent cold and warm air massesfrom clashing and would therefore suppress tornadoes. The problem with this proposal,however, is that atmospheric fronts (“airmass clashes”) are neither a necessary nora sufficient condition for tornadoes and that the proposed walls would not preventthe formation of fronts in the first place. Additional misconceptions about supercellsthunderstorms and tornado formation also are identified.

Keywords: Tornado; supercell; clash; wall.

1. Introduction

In a recent study, Tao1 proposes that the construction of three west-east-oriented

“great walls” across the U.S. Great Plains, each 300 m tall, would mitigate the

tornado threat in this region. Unfortunately, his arguments suffer from fundamen-

tal misconceptions about how the atmosphere works. In the following, we identify

several issues with this study.

2. Clashing of Air Masses and Supercell Thunderstorms

Although it is a popular belief outside the atmospheric physics community that tor-

nadoes are caused by a “clash of air masses,” this notion is a highly oversimplified,

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J. M. L. Dahl & P. M. Markowski

if not inaccurate description of how tornadoes actually form. In this section we first

describe the development of convective storms in relation to frontal boundaries. We

then summarize the current understanding of how storms may acquire rotation and

highlight where this understanding clashes with Tao’s views.

Deep moist convection, which is required for tornadoes, results from the re-

lease of conditional instability.2 Although the ascending air motion required for the

release of this instability may be related to a frontal boundary (which presumably

is what “airmass clash” refers to), the presence of such a boundary does not imply

the presence of instability. Rather, conditional instability requires a relatively large

decrease of temperature with height as well as a sufficient amount of moisture in

the atmosphere.2,3 If there is no instability present while ascent is occurring along

a front, cloud formation and precipitation may still occur, but no convective storms

will develop.4 The three ingredients necessary for convective storms are moisture,

instability and lift.3–5 This concept has been used successfully for many decades to

predict convective storms across the United States,6 and it implies that the presence

of fronts is not sufficient for the development of deep moist convection. Moreover,

the most violent tornado outbreaks, where numerous tornadic storms occur over a

large area, tend to unfold away from frontal boundaries.7 This is discussed in more

detail in Ref. 8.

Virtually all tornadoes rated EF2 and stronger are due to a type of convective

storm known as a supercell thunderstorm. The term “supercell” was originally in-

voked to describe a persistent, intense thunderstorm cell that propagates to the

right of the mean wind.9,10 Today the most common, dynamically equivalent defi-

nition characterizes a supercell as a thunderstorm that contains a deep, persistent

mesocyclone.11 A mesocyclone is a broad storm-scale circulation, in which torna-

does may be embedded. Tao mistakes a presumed horizontal roll vortex at the

location where air masses “clash” as the supercell.

There exists a large body of literature on the dynamics of supercell storms, based

on analytical12–14 and numerical12,15,16 solutions of the Navier–Stokes equations.a

The concepts based on these studies have stood the test of time and are now part

of the scientific consensus. Based on this research mesocyclones above the ground

result from the tilting of horizontal vorticity into the vertical by the storm’s updraft.

The horizontal vorticity in the environment of a supercell is manifest as vertical

wind shear (i.e., variation of the speed and/or direction of horizontal winds in the

vertical). This implies that in addition to the three ingredients mentioned above,

supercells require vertical wind shear (which implies the presence of horizontal

vorticity) for their formation. This horizontal vorticity is not confined to the location

of the front as implied by Tao’s Fig. 4, but it is typically present over a large

region, allowing supercells to form away from frontal boundaries.6,8 This discussion

aThe references refer only to the early publications that uncovered fundamental processes; dozensof subsequent modeling studies have since confirmed these results. An overview of convective stormmodeling is provided in Ref. 17.

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Comment on “Eliminating the major tornado threat in Tornado Alley”

implies that the presence of frontal boundaries is neither a necessary, nor a sufficient

condition for the occurrence of supercells.

3. Tornadogenesis in Supercell Thunderstorms

Tao implies that the tilting of horizontal vorticity in the lower atmosphere into the

vertical by an updraft, and subsequent stretching, directly leads to a tornado. How-

ever, during the past decades, observational, theoretical and modeling studies18–25

have shown that this mechanism cannot explain the development of near-ground ro-

tation because horizontal near-ground vortex lines are lifted away from the ground

as they are tilted at the updraft edge. Instead, a downdraft is needed for the devel-

opment of vertical vorticity next to the ground. This downdraft reorients initially

horizontal vorticity and transports it toward the ground within the precipitation-

ambient

vertical

wind shear

streamwise

vortic ity

c

oo

l

wwaaaa

wwwwrrrrmmrrrr

miiiddlllddd eevveevv ll mmeessooccyycc ccyyy llloooonnnnnnnneeeeeeeeee

downdraft

up

udd

rdd

arrftff

cold downdraft and/or

weak suction

relatively warm downdraft and

strong suction

Step 3Step 3

Step 1

Step 2

(a)

(c)(b)

~3 km

Fig. 1. (Color online) The figure summarizes basic tornado dynamics. Shown are annotatedphotographs of supercell thunderstorms, showing ambient vortex lines (red) contributing to themesocyclone aloft, while baroclinically generated vortex lines (blue) facilitate rotation practicallyat ground level (a). In (b) relatively warm outflow winds (represented by the magenta arrows)allow the near-ground rotation to be concentrated to tornadic strength, while in (c) relativelycool, diverging outflow air effectively prevents tornadogenesis. Adapted from Ref. 32, photos inpanels (a) and (b) courtesy of William T. Reid, photo in panel (c) by Paul Markowski.

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November 11, 2014 14:5 IJMPB S0217979214750046 page 4

J. M. L. Dahl & P. M. Markowski

cooled “outflow” of a supercell.24–26 Moreover, observational and numerical mod-

eling research on the development of near-ground rotation indicates that the hori-

zontal vorticity that is tilted in downdrafts owes its existence mainly to baroclinic

generation (horizontal air density gradients associated with the storm’s precipita-

tion regions generate horizontal vorticity via the torque they exert).12,15,16,19,25–31

That is, the vorticity is generated within the storm. This is inconsistent with Tao’s

Fig. 4, which suggests that a tornado forms by rearranging pre-existing horizontal

vortex lines associated with the environmental vertical wind shear. A schematic

summarizing the current understanding of how rotation develops in supercells is

shown in Fig. 1.

Recent observations and numerical simulations show that the development of

near-ground vertical vorticity proceeds in the form of multiple surges of out-

flow.25,33,34 These surges transport vertical vorticity to the surface as described

above, and into the region beneath the storm’s updraft where the vertical vorticity

may be amplified to tornadic strength via the conservation of angular momen-

tum.31,32 This process works best when the low-altitude vertical wind shear and

relative humidity are particularly large.31,32 Tao ignores all of these processes and

it remains unclear how storm-scale downdrafts and baroclinic vorticity generation,

or any other internal storm dynamical processes, would be affected by 300 m tall

walls spaced some 500 km or more apart.

4. Tornado Climatology and Its Relationship with Topography

Although there certainly are spatial variations of tornado occurrence in China,

Tao asserts without providing a source that there have been relatively frequent

tornadoes in the Jiangsu province in China, and subsequently infers that eastern

China is a “tornado alley.” However, there is typically a large discrepancy between

the observed and actual tornado frequencies and intensities,35,36 and obtaining a

robust tornado climatology for a country without a long history of organized efforts

to document tornadoes is quite challenging.37 Very little is known about the actual

number of tornadoes in China, let alone their spatial variations. Thus, great care

should be taken when working with what are virtually guaranteed to be incomplete

climatologies.

Tao argues that the variations in topography and reported tornado occurrence in

China are causally linked to each other. However, other possibilities (assuming the

tornado reports are perfect) have not been explored. For example, western China

might simply not experience the moisture necessary to support intense convective

storms, regardless of the terrain differences. It is virtually certain that additional

climatological variations of the ingredients required for supercell thunderstorms

and tornadoes would exist across a large country like China even in the absence of

mountains. A more meaningful approach is to consider climatologies of the ingre-

dients for supercells and tornadoes discussed in Secs. 2 and 3, and to analyze why

the occurrence of these ingredients varies spatially. Tao offers no such analysis and

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Comment on “Eliminating the major tornado threat in Tornado Alley”

200

150

100

50

Fig. 2. Frequency distribution of proximity surface wind directions for tornadoes that occurredbetween 2003 and 2013 in Oklahoma, Kansas, Nebraska, South Dakota and North Dakota. Thecardinal directions indicate the direction from which the wind is blowing.

merely claims without any support that the three mountain ranges over western

China prevent tornadoes over this region. As an analogy, consider a dense popula-

tion of some animal species in one part of the world where the terrain is flat, and

a much sparser population of this species where the terrain is hilly. The inference

that the difference in population is attributable to the flatness of the terrain is

unjustified because other factors, such as the availability of prey, are ignored.

5. The Blocking of Winds by the Proposed Tornado-Prevention

Walls

Tao’s proposal hinges on low-altitude winds blowing from the south in synop-

tic conditions that favor tornadoes. However, as revealed by a 10-year climatol-

ogy of tornado-proximity surface winds (obtained using the technique described

in Ref. 38), tornadoes frequently occur when the low-altitude winds are coming

from easterly directions, and occasionally when they come from the southwest or

northwest (Fig. 2). The bigger problem, however, is that 300 m tall walls would not

block the winds on most days, especially not days on which thunderstorms would

be possible. Whether or not a given 2D barrier is able to block the airflow depends,

apart from the barrier-normal flow velocity, on the stratification of the air as shown

in Ref. 39, based on a Bernoulli equation. The potential for Tao’s proposed walls

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J. M. L. Dahl & P. M. Markowski

to block the airflow is well-predicted by the nondimensional mountain height,39

ǫ =NH

U, (1)

where U is the characteristic barrier-normal flow velocity, N is the buoyancy fre-

quency and H is the height of the barrier. The buoyancy frequency is given by

N =

(

g

θ

∂θ

∂z

)1

2

, (2)

where g = 9.81 m s−1 is the acceleration due to gravity and θ is the potential

temperature.b Blocking occurs for ǫ > 1. For U = 10 m s−1, θ = 300 K and

H = 300 m, blocking requires (∂θ/∂z) > 34 K km−1. Thus, for a 2D barrier to block

the flow, as implied in Tao’s Fig. 9, the flow must be strongly stratified (i.e., den-

sity must decrease rapidly with height). Such extremely stable environments are

highly unfavorable for deep convection and tornadoes, however. That a 300-m-tall

barrier would fail to block the motion of air masses on all but the most stable

(nonthunderstorm) days ought to be consistent with the experience of residents of

the northeastern U.S. during the past harsh winter of 2013–2014. The Appalachian

Mountains are generally 400–600 m taller than the upstream flatlands (i.e., taller

than Tao’s proposed walls), yet cold air routinely spills over the mountains. The

weakly stratified air present on thunderstorm days would have even less difficulty.

Not surprisingly, a recent numerical simulation,40 where the 300-m-tall walls were

imposed in the model as proposed by Tao, shows that the walls have no appreciable

effect on the simulation. (Increasing the height of the walls to 2500 m did have an

effect, however. Flooding occurred south of the walls, deserts were produced north

of the walls, and the likely regions of tornado activity were shifted eastward!).

6. Conclusion

Although the idea of preventing tornadoes in the central U.S. is alluring, Tao’s pro-

posal to “eliminate the major tornado threat in Tornado Alley forever” by erecting

300-m-tall walls is problematic. The atmospheric physics community’s understand-

ing of tornadogenesis based on decades of peer-reviewed research is fundamentally

different from Tao’s notion wherein clashing air masses create a horizontal vor-

tex that is subsequently tilted into the vertical to make a tornado. Tornadoes do

bThe potential temperature is defined as

θ = T

(

p0

p

) Rcp

, (3)

where T is the temperature, p is the pressure, p0 is a reference pressure (usually 105 Pa), R isthe specific gas constant of air and cp is the specific heat at constant pressure. The potentialtemperature represents the temperature of a parcel of air with temperature T and pressure p thatis brought dry-adiabatically to the reference pressure.

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Comment on “Eliminating the major tornado threat in Tornado Alley”

not result from the formation of fronts but instead require a specific set of ingredi-

ents, which often materialize away from frontal boundaries. Moreover, the proposed

walls would not block the flow in situations favorable for deep convection in the first

place. In our view, until it becomes practical to pursue geoengineering strategies

that would at least theoretically work, efforts should be focused on mitigating the

effects of tornadoes by developing and enforcing better building codes, reducing the

false alarms of tornado warnings and educating the public how to react properly to

an imminent severe weather threat.

Acknowledgments

We thank Dr. Matthew Parker (North Carolina State University) for insightful

discussions and Mr. Brice Coffer (North Carolina State University) for sharing his

simulation results with us. Mr. Andy Dean (NOAA/Storm Prediction Center) and

Dr. Patrick Marsh (NOAA/Storm Prediction Center) are gratefully acknowledged

for providing the climatological data and analysis software to produce Fig. 2.

References

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