1. Tornadoes
QT Movie
Mesoscale
M. D. Eastin

2. Tornadoes Significant Events in U.S. History The Fujita Scale
U.S. Tornado Climatology
Mesoscale Observations
Scales of Motion
WSR-88D look at the 3 May 1999 Oklahoma City tornado
Damage Patterns
Tornado Structure
Core Observations
Conceptual Model of Air Flow
Tornadogenesis
Supercell Tornadoes
Non-supercell Tornadoes
Tornado Forecasting
Mesoscale
M. D. Eastin

3. Significant Tornado Events
The Tri-State Tornado:
Occurred on 18 March 1925
695 confirmed fatalities
(deadliest in US history)
Damage suggests F5 intensity
Over 15,000 homes destroyed
Continuous 219 mile track
Current thought is that it was
actually a family of tornadoes
spawned by the same storm
Griffin, IN
Mesoscale
M. D. Eastin

4. Significant Tornado Events
The Super Outbreak:
Occurred on 3 April 1974
Total of 148 tornadoes in 13 states
(Second most on one day in history)
F5=7 F4=24 F3=35
315 confirmed fatalities
Over 5,000 people injured
Severely damaged over 900 sq miles F4
Parker City, IN (21)
Mesoscale
M. D. Eastin

5. Significant Tornado Events
3 May 1999 Oklahoma Outbreak
48 confirmed fatalities
$1.5 billion in damages
Mesoscale
M. D. Eastin

6. Significant Tornado Events
25-28 April 2011 Outbreak:
358 confirmed tornadoes (TX → NY)
Strongest TOR in MS/AL/GA/TN on 27 April
(EF5 = EF4 = EF3 = 22)
348 deaths
$11 billion in damages (costliest event)
27 April 2011
Mesoscale
M. D. Eastin

7. The Fujita-Scale Estimating Tornado Intensity:
Developed by Dr. Ted Fujita
in 1971
Updated in 2007
Designed to bridge the
gap between the Beaufort
and Mach scales
Note that there are more
than 6 F-scale categories
in the original scale
Mesoscale
M. D. Eastin

8. The Fujita-Scale Damage Examples: To date, no instrument has
proven reliable enough to
accurately (and regularly)
measure the maximum wind
speeds within tornadoes.
The determination (an estimate)
of any tornado’s intensity is
always done via post-storm
damage surveys of the area
Important Note:
Tornado “intensity” is a function
of building construction quality,
whether any buildings were
even damaged, forward motion
of the tornado, as well as many
other factors… F0 F1 F2 F3 F4 F5
Mesoscale
M. D. Eastin

9. The Enhanced Fujita-Scale
A Re-evaluation of Tornado Intensity in the Modern World:
Based on damage to a single family house using traditional construction practices
(i.e. modern quality and design), assuming the house was in compliance with
common building codes and was regularly maintained
In general, lower winds speeds are required to produce the same damage
Officially adopted by the NWS for use beginning February 1, 2007
More information can be found at:
Mesoscale
M. D. Eastin

10. Tornado Climatology Tornado Alley: Mesoscale M. D. Eastin
From Brooks et al. (2003)
Mesoscale
M. D. Eastin

11. Tornado Climatology Annual Cycle: Mesoscale M. D. Eastin
From Brooks et al. (2003)
Mesoscale
M. D. Eastin

12. Tornado Climatology Seasonality:
From Brooks et al. (2003)
Also see:
Mesoscale
M. D. Eastin

13. Tornado Climatology Number of Tornado Reports:
The upwards trends are believed to be artificial
The trend likely reflects:
An increase in population density
Improved reporting procedures
Organized networks of “storm spotters”
From Brooks et al. (2003)
Mesoscale
M. D. Eastin

14. Tornado Climatology Death:
Annual total number of deaths has steadily decreased over the last 50 years
During the period : 4,460 total deaths (average of 89 per year)
40,522 total tornadoes (average of 810 per year)
Damage:
Survey of damage from all tornadoes
in the period (after an
adjustment for wealth and inflation)
Total Damage: $19.3 billion
Annual Average: $0.42 billion
Survey of adjusted damage from only
major (F4-F5) tornadoes during
the same period
Account for 2.3% of all tornadoes
Total Damage: $10.2 billion
Annual Average: $0.22 billion
Mesoscale
M. D. Eastin

15. Mesoscale Observations
Multiple Scales of Rotation:
Mesocyclone: m in diameter – Most often detected by NWS Doppler radar
Tornado: m in diameter – Rarely observed by NWS Doppler radar (TVS)
and never by ASOS (more on this later…)
Suction Vortices: 1-50 m in diameter – Recently observed by high-resolution Doppler radar
Mesoscale
M. D. Eastin

16. Mesoscale Observations
Multiple Scales of Rotation: Mid-level Mesocyclones
Persistent rotation observed in supercells by NWS Doppler radar (automated algorithms)
Typical altitudes → 2-7 km AGL
Less than 25% of radar-detected mid-level mesocyclones produce tornadoes
NEXRAD-88D Radar
Oklahoma City
3 May 1999
Mesocyclone Automated Detection Algorithm:
Looks for quasi-symmetric mesocyclonic circulations (large horizontal shears) that
vertically correlate through a >3.5 km depth
and are persistent for >10 minutes
Mesocyclones
detected by the
automated algorithm
Mesoscale
M. D. Eastin

17. Mesoscale Observations
Multiple Scales of Rotation: Low-level Mesocyclones
Altitude = 1-2 km AGL
Associated with hook echo
Wall cloud
Often difficult to detect by NWS
Doppler radars if more than 50 km
from the radar (due to non-zero
beam elevation angles and Earth’s
curvature)
If detected, probability of a
tornado increases.
Less than 40% of radar-detected
low-level mesoscyclones produce
a tornado
Storm-relative winds
Radar reflectivity
Storm-relative winds
Radar reflectivity
Doppler radial velocity
Radar
beams
Storm-relative winds
Vertical vorticity
Vertical motion
Airborne
Doppler
Synthesis
(800m AGL)
From Wakimoto et al. (2003)
Mesoscale
M. D. Eastin

18. Mesoscale Observations
Multiple Scales of Rotation: Tornado Vortex Signature (TVS):
Historically, a tornado has shown up on
operational NWS radars as a region of
enhanced gate-to-gate (adjacent beams)
horizontal shear
When the horizontal shear exceeds some
criteria, a TVS is identified
Note: The NEXRAD WSR-88D radar can not
resolve a tornado’s circulation.
An identified TVS is highly suggestive
that a tornado is present
Not all reported tornadoes are associated
with a radar-detected TVS (~60%)
Not all storms with a radar-identified TVS
produce a tornado (~80%)
TVS
(possible tornado)
Mesocyclone
Mesoscale
M. D. Eastin

19. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

20. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

21. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

22. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

23. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

24. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

25. 3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin

26. From Wakimoto and Atkins (1996)
Tornado Damage Patterns
From Wakimoto and Atkins (1996)
Mesoscale
M. D. Eastin

27. Tornado Damage Patterns
From Wakimoto
and Atkins (1996)
Mesoscale
M. D. Eastin

28. Tornado Core Observations
Photogrammetric Studies:
Use multiple photographs
to diagnose structure and
air flow patterns
Pioneered by Ted Fujita
in the 1960s
Must know many details as
a function of time:
Camera location
Tornado location
Time of each photo
Camera / film specifics
Assumes visible “features”
move with the local wind
(Is this a good assumption?)
Mesoscale
M. D. Eastin

29. Tornado Core Observations
Totable Tornado Observatory (TOTO):
Developed by Dr. Howard Bluestein (Univ. Oklahoma)
and his graduate students in the early 1980s
Designed to record basic surface observations inside
a tornado vortex
Never successfully deployed
Motivation for a popular movie?
From Bluestein et al. (1983)
Mesoscale
M. D. Eastin

30. Tornado Core Observations
Hardened In-Situ Tornado Pressure Recorder (HITPR):
Developed and deployed by the annual
TWISTEX Project between
lead by renown chaser – Dr. Tim Samarus (
Designed to record basic surface observations
inside a tornado vortex
Successfully deployed in multiple tornadoes
Data from an
F4 Tornado
From Lee et al. (2004)
Mesoscale
M. D. Eastin

31. Tornado Core Observations
Doppler on Wheels (DOWs) :
Vehicle-mounted Doppler
radars can get very close
(sometimes too close) and
resolve the circulation
Multiple radars deployed
each year (most recently
during VORTEX-2)
From Wurman et al. (1997)
Mesoscale
M. D. Eastin

32. Tornado Core Observations
Doppler on Wheels (DOWs) :
Suctions vortices have been observed
(and photographed) by storm chasers
for decades
The DOWs have recently provided the
first direct quantitative observations of
“suction vortices” (
From
Wurman
(2002)
Mesoscale
M. D. Eastin

33. Tornado Core Observations
Tornado Vortex Chambers:
Create artificial tornadoes in laboratories
Primary source of quantitative information before
the DOW radars (pre-1990s)
Two important parameters in a vortex chamber
Γ = Circulation of the flow about the central axis
Q = Rate of air flow through the chamber top
The ratio of Γ to Q is called
the swirl ratio (S):
Tornadoes form in
vortex chambers
when the swirl ratio
is large
From Gallus et al. (2005)
Mesoscale
M. D. Eastin

34. Tornado Core Observations
Tornado Vortex Chambers:
When the swirl ratio is very small (a), no vortex
develops at the surface (notice the descending
motion near the axis of rotation)
As the swirl ratio is increased (b), a vortex develops
at the surface (note the inflow and updraft just
above the surface much like a tornado). This is
called a one-cell vortex (one updraft)
As the swirl ratio further increases (c), a downdraft
develops along the central axis, producing a
cloud–free, or “hollow”, center to the tornado
(which is often observed by storm chasers)
At very large swirl ratios (d), the downdraft
penetrates to the surface and creates a two-celled
vortex (with two updrafts). This results in multiple
suction vortices (e) (as observed in nature)
Mesoscale
M. D. Eastin

35. Conceptual Model of Air Flow
Five Flow Regions and Radial Pressure Profile:
Outer Region (I):
Inward spiraling
air that conserves
angular momentum
(spins faster as it
approaches the
tornado axis)
Core Region (II):
Inside the maximum
winds, including the
funnel cloud, dust,
and debris.
(cyclostrophic balance)
Corner (III):
Region where air
turns upward from
being horizontal
flow to primarily
vertical flow
Boundary Layer (IV):
Flow interacts with
ground and surface
friction enhances
the radial inflow
Rotating Updraft (V):
Parent updraft and
mesocyclone
Pressure profile:
Assumes an idealized vortex
structure in order to relate the
flow field to the radial pressure
gradient: Rankine Vortex
Burgers-Rott Vortex
Mesoscale
M. D. Eastin

36. Supercell Tornadogenesis
Not well understood!
Three current theories have considerable observational and numerical modeling support
All three theories may work in concert
Each assumes the following circulations are present in the parent supercell:
Mid-level mesocyclone generated by tilting and stretching of horizontal vorticity
Low-level mesocyclone generated by tilting and stretching baroclinically-enhanced
streamwise vorticity within the vortical updraft
Mature forward and rear-flank downdrafts and their associated gust fronts
Upper-level
Flow
Primary
Updraft
Updraft
RFD
Horizontal
Vorticity
Vectors
FFD
Mid-level
Flow
Inflow along
the gust front
acquires
streamwise
vortcity
Inflow
Mesoscale
M. D. Eastin

37. Supercell Tornadogenesis
1. Negligible Vertical Vorticity at the Surface: Downdraft Required
Simple tilting of low-level
horizontal vorticity by the
primary updraft cannot
produce vertical vorticity
at the surface since the
air rises away from the
surface during tilting
(top scenario)
However, if an adjacent
downdraft (i.e. the RFD)
is involved in the tilting
process, then vertical
vorticity can be advected
toward the surface (during
titling) and subsequently
stretched into a tornado
(bottom scenario)
Barotropic contribution
Mesoscale
M. D. Eastin

38. Supercell Tornadogenesis
2. Negligible Vertical Vorticity at the Surface: Downdraft Required
The near-surface horizontal vorticity
can be enhanced when the RFD is
driven by negative buoyancy
Recall, horizontal buoyancy gradients
produce horizontal vorticity:
The downward advection of any
such horizontal vorticity will increase
the total available horizontal vorticity
to be tilted toward the surface and
then stretched into a tornado
Baroclinic contribution
Mechanism produces tornadoes
soon after RFD reaches surface B-
Mesoscale
M. D. Eastin

39. Supercell Tornadogenesis
3. Ample Vertical Vorticity at the Surface: NO Downdraft Required
Strong horizontal shear
located along RFD or FFD
gust fronts produce large
near-surface vertical vorticity
Storm-relative inflow slowly
advects any vertical vorticity
“pockets” along the gust
fronts toward the primary
mesocyclonic updraft where
stretching and low-level
convergence increase the
near-surface vertical vorticity,
producing a tornado
Mechanism produces tornadoes
before RFD reaches the surface
or in between RFD “surges”
Mesoscale
M. D. Eastin

40. DOW Radial Velocity Observations
Supercell Tornadogenesis
3. Ample Vertical Vorticity at the Surface: NO Downdraft Required
DOW Radial Velocity Observations
Mesocyclone
Center
Gust Front
Vortices
Numerical Simulation
Mesoscale
M. D. Eastin

41. Supercell Tornadogenesis
Numerical Simulation Movie #1
(A Top View)
Numerical Simulation Movie #2
(A Surface Observer View)
Mesoscale
M. D. Eastin

42. Non-Supercell Tornadogenesis
Even Less Understood!!!
Often occurs along low-level
lines of horizontal shear and
convergence (e.g. gust fronts
and air-mass boundaries)
Large pre-existing low-level
vertical vorticity is stretched
by the updrafts of ordinary
growing cumulus clouds
Produces weak, short-lived
tornadoes (EF0 - EF2)
Mesoscale
M. D. Eastin

43. Tornado Forecasting
Continuous monitoring of ALL available observations:
Use real-time radar data to monitor storm formation and evolution
Use surface observations to monitor storm-relative inflow and cold pool characteristics
Use nearby soundings (rawinsondes and rapid-update numerical models) to monitor
standard forecast parameters (CAPE, SREH, EHI, etc.)
Other useful forecast parameters:
Vertical Shear 0-1 km AGL:
Large values favor tornadoes
Strong shear implies large horizontal
vorticity near the surface that can be
tilted into the vertical by updrafts and
downdrafts (especially the RFD)
Mixed-layer LCL:
Small values favor tornadoes
Moist boundary layers limit negative
buoyancy in downdrafts and prevent
strong cold pools from “under-cutting”
the primary updraft (see next slide…)
Mesoscale
M. D. Eastin

44. Tornado Forecasting Mesoscale M. D. Eastin
Surface Density Potential Temperature Perturbations
(observed by mobile mesonets during VORTEX)
Weak
Cold Pools
Moderate
Strong
Mesoscale
M. D. Eastin

45. Tornadoes
Summary:
Significant Events in U.S. History → Why are they significant?
The Fujita Scale → Basic concept and reason for recent changes
U.S. Tornado Climatology → Basic characteristics and trends
Mesoscale Observations
Scales of Motion → Ability / Methods used to observe each scale
Damage Patterns → Basic structure and reasons for such structure
Tornado Structure
Core Observations → Various methods and laboratory results
Conceptual Model of Air Flow → Basic characteristics of each region
Tornadogenesis
Supercell Tornadoes → Important physical processes (and when)
Non-supercell Tornadoes → Important physical processes
Tornado Forecasting → Methods and additional useful parameters
Mesoscale
M. D. Eastin

46. References Mesoscale M. D. Eastin
Agee, E. M., J. T. Snow, and P. R. Clare, 1976: Multiple vortex features in a tornado cyclone and the occurrence of tornado families. Mon. Wea. Rev., 104,
Atkins, N. T., J. M. Arnott, R. W. Przybylinski, R. A. Wolf, and B. D. Ketchum, 2004: Vortex Structure and Evolution within Bow Echoes. Part I: Single-Doppler and Damage Analysis of the 29 June 1998 Derecho. Mon. Wea. Rev., 132,
Bluestein, H. B., 1980: The University of Oklahoma Severe Storms Intercept Project – Bull. Amer. Meteor. Soc., 61,
Bluestein, H. B., 1983: Surface meteorological observations in severe thunderstorms. Part II: Field experiments with TOTO.
J. Climate Applied Meteor., 22,
Bluestein, H. B., 1999: A history of severe storms intercept field programs. Wea. Forecasting, 14,
Brooks, H. E, C. A. Doswell, and M. P. Kay, 2003: Climatological estimates of local daily tornado probability in the United States. Wea. Forecasting, 18,
Burgess, D. W., and L. R. Lemon, 1990: Severe thunderstorm detection by radar. Radar in Meteorology. D. Atlas, Ed., Amer. Meteor. Soc.,
Davies-Jones, R., 1986: Tornado dynamics. Thunderstorm Morphology and Dynamics, 2nd ed, E. Kessler, Ed., University of Oklahoma Press,
Fujita, T.T., 1981: Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38,
Gallus, W. A., Jr., C. Cervato, C. Cruz-Neira, G. Faidley, and R. Heer, 2005: Learning storm dynamics with a virtual thunderstorm. Bull. Amer. Meteor. Soc., 86,
Klemp, J. B., 1987: Dynamics of tornadic thunderstorms. Ann. Rev. Fluid Mech., 19,
Mesoscale
M. D. Eastin

47. References Mesoscale M. D. Eastin
Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci.,
40,
Lee, B. D., and R. B. Wilhelmson, 1997: The numerical simulation of nonsupercell tornadogenesis. Part II: Evolution of a . family of tornadoes along a weak outflow boundary. J. Atmos. Sci., 54,
Markowski, P. M., E. N. Rasmussen, and J. M. Straka, the occurrence of tornadoes in supercells interacting with boundaries during VORTEX-95. Wea. Forecasting, 13,
Rotunno, R., 1986: Tornadoes and tornadogenesis. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed.,
Amer. Meteor. Soc.,
Trapp R. J., and R. Davies-Jones., 1997: Tornadogenesis with and without a dynamic pipe effect. J. Atmos. Sci., 54,
Wakimoto, R. M. and N. T. Atkins, 1996: Observations on the origins of rotation: The Newcastle tornado during VORTEX-94. Mon. Wea. Rev., 124,
Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev., 117,
Wakimoto, R. M., C. Liu, and H. Cai, 1998: The Garden City, Kansas storm during VORTEX-95. Part I: Overview of storm’s lifecycle and mesocyclogenesis. Mon. Wea. Rev., 126,
Wakimoto, R. M., H. V. Murphey, D. C. Dowell, and H.B. Bluestein, 2003: The Kellerville tornado during VORTEX: Damage survey and Doppler radar analyses. Mon. Wea. Rev., 131,
Wicker, L. J., and R. B. Wilhelmson, 1995: Simulation and analysis of tornado development and decay within a three- dimensional supercell thunderstorm. J. Atmos. Sci, 52,
Wurman, J., 2002: The multiple-vortex structure of a tornado. Wea. Forecasting, 17,
Wurman, J., J. M. Straka, E. N. Rasmussen, M. Randall, and A. Zahari, 1997: Design and deployment of a portable, pencil beam, pulsed, 3-cm Doppler radar. J. Atmos. Oceanic. Technol., 14,
Mesoscale
M. D. Eastin