Tornadoes
QT Movie
Mesoscale
M. D. Eastin
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
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
Significant Tornado Events
The Super Outbreak:
• Occurred on 3 April 1974
• Total of 148 tornadoes in 13 states
(Most on one day in U.S. 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
Significant Tornado Events
3 May 1999 Oklahoma Outbreak
48 confirmed fatalities
$1.5 billion in damages
Mesoscale
M. D. Eastin
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
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
F0
F1
F2
F3
F4
F5
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…
Mesoscale
M. D. Eastin
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: http://www.spc.noaa.gov/faq/tornado/ef-scale.html
Mesoscale
M. D. Eastin
Tornado Climatology
Tornado Alley:
From Brooks et al. (2003)
Mesoscale
M. D. Eastin
Tornado Climatology
Annual Cycle:
From Brooks et al. (2003)
Mesoscale
M. D. Eastin
Tornado Climatology
Seasonality:
From Brooks et al. (2003)
Also see: http://www.nssl.noaa.gov/hazard/tanim/torw8099.html
Mesoscale
M. D. Eastin
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
Tornado Climatology
Death:
• Annual total number of deaths has steadily decreased over the last 50 years
• During the period 1950-1999:
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 1950-1995 (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
Mesoscale Observations
Multiple Scales of Rotation:
Mesocyclone:
2000-7000 m in diameter – Most often detected by NWS Doppler radar
Tornado:
100-1000 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
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
Mesoscale
Mesocyclones
detected by the
automated algorithm
M. D. Eastin
Mesoscale Observations
Multiple Scales of Rotation: Low-level Mesocyclones
• Altitude = 1-2 km AGL
 Associated with hook echo
 Wall cloud
Storm-relative winds
Radar reflectivity
Storm-relative winds
Radar reflectivity
Doppler radial velocity
• 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
Radar
beams
Storm-relative winds
Vertical vorticity
Vertical motion
Airborne
Doppler
Synthesis
(800m AGL)
From Wakimoto et al. (2003)
Mesoscale
M. D. Eastin
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%)
Mesocyclone
TVS
(possible tornado)
Not all storms with a radar-identified TVS
produce a tornado (~80%)
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
3 May 1999, Oklahoma City Tornado
Mesoscale
M. D. Eastin
Tornado Damage Patterns
From Wakimoto and Atkins (1996)
Mesoscale
M. D. Eastin
Tornado Damage Patterns
From Wakimoto
and Atkins (1996)
Mesoscale
M. D. Eastin
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
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
Tornado Core Observations
Hardened In-Situ Tornado Pressure Recorder (HITPR):
• Developed and deployed by the annual
TWISTEX Project since 2003
(http://en.wikipedia.org/wiki/TWISTEX)
• 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
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
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”
(http://www.cswr.org/)
From
Wurman
(2002)
Mesoscale
M. D. Eastin
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):
S 
ro  vt

2Q w
 Tornadoes form in
vortex chambers
when the swirl ratio
is large
From Gallus et al. (2005)
Mesoscale
M. D. Eastin
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
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)
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
Core Region (II):
Inside the maximum
winds, including the
funnel cloud, dust,
and debris.
(cyclostrophic balance)
v
1 p

r  r
2
Mesoscale
Pressure profile:
Assumes an idealized vortex
structure in order to relate the
flow field to the radial pressure
gradient: Rankine Vortex
Burgers-Rott Vortex
2
pmin   vmax
Rotating Updraft (V):
Parent updraft and
mesocyclone
M. D. Eastin
Supercell Tornadogenesis
Not well understood!
 Two current theories have considerable observational and numerical modeling support
 Both 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
FFD
Mid-level
Flow
Inflow
Mesoscale
Horizontal
Vorticity
Vectors
Inflow along
the gust front
acquires
streamwise
vortcity
M. D. Eastin
Supercell Tornadogenesis
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
Supercell Tornadogenesis
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:

B
 
t
x
B-
 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
Mesoscale
M. D. Eastin
Supercell Tornadogenesis
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
Supercell Tornadogenesis
Ample Vertical Vorticity at the Surface: NO Downdraft Required
DOW Radial Velocity Observations
Numerical Simulation
Mesocyclone
Center
Mesocyclone
Gust Front
Vortices
Gust Front
Vortices
Mesoscale
M. D. Eastin
Supercell Tornadogenesis
Numerical Simulation Movie #1
(A Top View)
Numerical Simulation Movie #2
(A Surface Observer View)
Mesoscale
M. D. Eastin
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
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
Tornado Forecasting
Surface Density Potential Temperature Perturbations
(observed by mobile mesonets during VORTEX)
Weak
Cold Pools
Moderate
Cold Pools
Strong
Cold Pools
Mesoscale
M. D. Eastin
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
References
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, 552-563.
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,
2224-2242.
Bluestein, H. B., 1980: The University of Oklahoma Severe Storms Intercept Project – 1979. Bull. Amer. Meteor. Soc., 61,
560-567.
Bluestein, H. B., 1983: Surface meteorological observations in severe thunderstorms. Part II: Field experiments with TOTO.
J. Climate Applied Meteor., 22, 919-930.
Bluestein, H. B., 1999: A history of severe storms intercept field programs. Wea. Forecasting, 14, 558-577.
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, 626-641.
Burgess, D. W., and L. R. Lemon, 1990: Severe thunderstorm detection by radar. Radar in Meteorology. D. Atlas, Ed., Amer.
Meteor. Soc., 619-647.
Davies-Jones, R., 1986: Tornado dynamics. Thunderstorm Morphology and Dynamics, 2nd ed, E. Kessler, Ed., University of
Oklahoma Press, 197-236.
Fujita, T.T., 1981: Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.
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, 162-163.
Klemp, J. B., 1987: Dynamics of tornadic thunderstorms. Ann. Rev. Fluid Mech., 19, 369-402
Mesoscale
M. D. Eastin
References
Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci.,
40, 359-377.
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, 2387-2415.
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, 852-859.
Rotunno, R., 1986: Tornadoes and tornadogenesis. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed.,
Amer. Meteor. Soc., 414-436.
Trapp R. J., and R. Davies-Jones., 1997: Tornadogenesis with and without a dynamic pipe effect. J. Atmos. Sci., 54, 113-133.
Wakimoto, R. M. and N. T. Atkins, 1996: Observations on the origins of rotation: The Newcastle tornado during VORTEX-94.
Mon. Wea. Rev., 124, 384-407.
Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev., 117, 1113-1140.
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, 372-392.
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, 2197-2221.
Wicker, L. J., and R. B. Wilhelmson, 1995: Simulation and analysis of tornado development and decay within a threedimensional supercell thunderstorm. J. Atmos. Sci, 52, 2675-2703.
Wurman, J., 2002: The multiple-vortex structure of a tornado. Wea. Forecasting, 17, 473-505.
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, 1502-1512.
Mesoscale
M. D. Eastin