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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
Paul M. Markowski
Department of Meteorology, The Pennsylvania State University,
503 Walker Building, University Park, PA 16802, USA
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
1. R. Tao, Int. J. Mod. Phys. B 28(22), 1450175 (2014).2. C. Doswell III, in Severe Convective Storms (American Meteorological Society, 2001),
pp. 1–26.3. D. Schultz, P. Schumacher and C. Doswell III,Mon. Weather Rev. 28(12), 4143 (2000).4. C. Doswell III, Weather Forecast. 2(1), 3 (1987).5. R. Johns and C. Doswell III, Weather Forecast. 7(4), 588 (1992).6. A. Moller, in Severe Convective Storms (American Meteorological Society, 2001),
pp. 433–480.7. R. Thompson and R. Edwards, Weather Forecast. 15(6), 682 (2000).8. D. Schultz et al., Bulletin Amer. Meteorol. Soc. 95(11), 1704 (2014).9. K. Browning and F. Ludlam, Q. R. J. Meteorol. Soc. 88, 117 (1962).
10. K. Browning, J. Atmos. Sci. 21(6), 634 (1964).11. C. Doswell III and D. Burgess, in The Tornado: Its Structure, Dynamics, Prediction
and Hazards (AGU, 1993), pp. 161–172.12. R. Rotunno and J. Klemp, Mon. Weather Rev., 110(2), 136 (1982).13. R. Davies-Jones, J. Atmos. Sci. 41(20), 2991 (1984).14. D. Lilly, J. Atmos. Sci. 42(2), 113 (1986).15. R. Rotunno and J. Klemp, J. Atmos. Sci. 42(3), 271 (1985).16. L. Wicker and R. Wilhelmson, J. Atmos. Sci. 52(15), 2675 (1995).17. R. Wilhelmson and L. Wicker, in Severe Convective Storms (American Meteorological
Society, 2001), pp. 123–16618. R. Davies-Jones, in Intense Atmospheric Vortices (Springer, 1982), pp. 175–189.19. R. Davies-Jones and H. Brooks, in The Tornado: Its Structure, Dynamics, Prediction
and Hazards (AGU, 1993), pp. 105–114.20. R. Walko, in The Tornado: Its Structure, Dynamics, Prediction and Hazards (AGU,
1993), pp. 89–95.21. R. Davies-Jones, R. Trapp and H. Bluestein, in Severe Convective Storms (American
Meteorological Society, 2001), pp. 167–221.22. R. Davies-Jones and P. Markowski, J. Atmos. Sci. 70(4), 1204 (2013).
1475004-7
Int.
J. M
od. P
hys.
B D
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oade
d fr
om w
ww
.wor
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ient
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IVE
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11/2
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. For
per
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l use
onl
y.
November 11, 2014 14:5 IJMPB S0217979214750046 page 8
J. M. L. Dahl & P. M. Markowski
23. R. Davies-Jones, Atmos. Res. doi:10.1016/j.atmosres.2014.04.007, (2014).24. M. Parker and J. Dahl, in Proceedings 15th Conference on Mesoscale Processes, AMS
6-9 August 2013, Portland, OR (2013).25. J. Dahl, M. Parker and L. Wicker, J. Atmos. Sci. 71(8), 3027 (2014).26. A. Schenkman, M. Xue and M. Hu, J. Atmos. Sci. 71(1), 130 (2014).27. J. Straka et al., Electron. J. Severe Storms Meteor. 2(8), 1 (2007).28. P. Markowski et al., Mon. Weather Rev. 136(9), 3513 (2008).29. P. Markowski et al., Mon. Weather Rev. 140(9), 2887 (2012).30. P. Markowski et al., Mon. Weather Rev. 140(9), 2916 (2012).31. P. Markowski and Y. Richardson, J. Atmos. Sci. 71(1), 243 (2014).32. P. Markowski and Y. Richardson, Phys. Today 67(9), 26 (2014).33. J. Marquis et al., Mon. Weather Rev. 136(12), 5017 (2008).34. K. Kosiba et al., Mon. Weather Rev. 141(4), 1157 (2013).35. C. Doswell and D. Burgess, Mon. Weather Rev. 116(2), 495 (1988).36. S. Verbout et al., Weather Forecast. 21(1), 86 (2006).37. J. Rauhala, H. Brooks and D. Schultz, Mon. Weather Rev. 140(5), 1446 (2012).38. R. Schneider and A. Dean, in Proceedings 24th Conference on Severe Local Storms,
AMS 27-31 October 2008, Savanna, GA (2008).39. R. Smith, J. Atmo. Sci. 45(24), 3889 (1988).40. B. Coffer, Electron. J. Severe Storms Meteor. 9(4), 1 (2014).
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