Tornadoes
METR 4433: Mesoscale Meteorology
Spring 2016 Semester
Dr. Kelvin K. Droegemeier
School of Meteorology
University of Oklahoma
Formal Definition

A rotating column of air, in contact with the
surface, pendant from a cumuliform cloud, and
often visible as a funnel cloud and/or circulating
debris/dust at the ground.
-- Glossary of the AMS
Types of Tornadoes
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Type I: Supercellular (within a mesocyclone)
Type II: Not associated with a mesocyclone
–
–
–
–
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Non-supercellular
Gustnadoes
Tornadoes from lines
Water spouts (tornadoes over water)
Other small-scale tropospheric vortices
– Dust devils
– Fire whirls
Tornado Characteristics
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Tornadoes occur worldwide but 75% occur in the
US
Average Duration: A few minutes
Average Diameter: 0.4 km
Average Path Length: 6 km
Tornadoes can travel at speeds up to 60 mph
but the average speed is 10-20 mph
99% of all tornadoes in Northern Hemisphere
rotate counterclockwise
Wind speeds vary dramatically
Source: Adapted from R.T. Shindler
Tornado Statistics
Tornado Statistics
Tornado Statistics
Tornado Statistics
Tornado Statistics
Tornado Statistics
Tornado Statistics
Tornado Statistics
Notable Local Event
Notable Local Event
http://www.srh.noaa.gov/images/oun/wxevents
/19990503/radar/travissmith-outbreak-800.gif
First Tornado Forecast
On March, 25 1948, Major
Fawbush and Captain Miller
determined that the conditions
of the atmosphere just west of
Tinker AFB, OK were suitable for
tornado development. The first
tornado forecast ever was
issued. A few hours later, a
tornado arrived causing
significant damage to the base.
However, no deaths and only a
few injuries occurred because
many had been warned by the
tornado forecast.
Source: R.T. Shindler
Original Fujita Scale Concept
Fujita Scale
Fujita Scale
Source: The Tornado Project Online
Fujita Scale
Source: The Tornado Project Online
Tornadic Environments
Source: Markowski and Richardson (2010)
Tornadic Environments
Source: Markowski and Richardson (2010)
Tornadic Environments
Source: Markowski and Richardson (2010)
Life Cycle of Type I (Supercell-Based)
Tornadoes

Stage I: Funnel/Organizing Stage
– Mid-level rotation (possibly a wall cloud)
– Visual evidence of a condensation funnel at cloud base
– Clear slot beginning to develop
Credit: NOAA
Life Cycle of Type I (Supercell-Based)
Tornadoes

Stage II: Mature Stage
– Condensation funnel clearly in contact with ground/debris
– RFD/clear slot wrapping around the circulation
– Updraft decreases somewhat in intensity
Credit: NOAA, Brandon Sullivan
Life Cycle of Type I (Supercell-Based)
Tornadoes

Stage III: Dissipation/Rope Stage
– RFD or rain wraps around tornado, cutting off inflow
– Funnel becomes serpantine in shape
– Inflow may shift to the east if storm cycles
Credit: NOAA, StormChasingUSA.com
Recall Supercell Structure
Forward Flank
Downdraft
Tornado
Rear Flank
Downdraft
Flanking Line/
Gust Front
Mesocyclone
Gustnado
Inflow
© 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations
and Theory of Weather Systems
Transition to Tornadic Phase

Low-level rotation owes its existence to the
tilting of baroclincally-generated vorticity along
the forward flank gust front
Source: Markowski and Richardson (2010)
Origin of Low-Level Mesocyclone
Rotation

Low-level rotation intensifies via stretching,
creating a dynamic pressure force that draws air
down from above (RFD)
Source: Klemp (1987)
Origin of Low-Level Mesocyclone
Rotation

Spreading gust front initiates a new center of
rotation (see later discussion on cyclic
tornadogenesis)
Source: Klemp (1987)
Tornadogenesis
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Multiple theories exist, and considerable research
continues to be performed
Key questions
– How many different ways do tornadoes form?
– Why do nearly identical storms behave differently in
that one produces a tornado and the other does not?
– What is the purpose of a tornado?
– How do large values of vertical vorticity arise at the
ground? Tilting cannot explain it.
Simple Concept “All at Once”
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NEXRAD shows that ~ 50% of tornadoes seem to form
all at once over a depth of 0-2 km
If convergence is constant with height, vortex forms as
shown below
Source: Trapp and Davies-Jones (1997)
Simple Concept “From the Ground Up”

Theoretically, high A.M. air should approach the axis of
rotation faster in the boundary-layer, where convergence
is greater (friction turns air inward)
Later
Ground
Visually, Tornadoes Appear to
Descend: The Dynamic Pipe Effect
Step I
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Maximum convergence is located
aloft and surface inflow is weak
Initial vortex (time = t) is
deformed by inflow/upflow (bold
arrows)
Vortex stretches and contracts,
spinning up
Strongest rotation is located aloft
Source: Trapp and Davies-Jones (1997)
The Dynamic Pipe Effect, Step II
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The vortex aloft is cyclostrophic,
so no flow can come in through
sides
Suction (dynamic vortex) brings
flow in only from bottom, where it
too becomes cyclostrophic
Vortex builds downward,
modifying the flow as it does so
Source: Trapp and Davies-Jones (1997)
Discussion
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These theories may in some cases apply, but for
supercells, are likely overly simplistic
They do not take into account important
observed characteristics such as
– Clear slot
– Hook echo
– Thermodynamic structure across the mesocyclone at
ground-level
– The observation that significant tornadoes do not
occur in the absence of a downdraft
Role of the Downdraft When
Vertical Vorticity is Initially Negligible at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Role of the Downdraft When
Vertical Vorticity is Initially Negligible at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Role of the Downdraft When
Vertical Vorticity is Initially Negligible at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Role of the Downdraft When
Vertical Vorticity is Initially Negligible at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Numerous Observations Show that Significant
Tornadoes Do Not Occur Until a Downdraft is
Present. At Least Some Air In Tornado Passes
Through the Rear Flank Downdraft (RFD)
Source: Markowski and Richardson (2010)
At Least Some Air In Tornado Passes Through
the Rear Flank Downdraft (RFD)
Source: Markowski and Richardson (2010)
Note arches in vortex lines, one the tornado,
across the RFD, not depressed downward as in
the previous schematic. This suggests that
baroclinic processes, not just tilting, are at work
Source: Markowski and Richardson (2010)
Purely baroclinic generation of vertical vorticity
with no environmental tilting. A cold downdraft
contains vortex lines as shown – could be the
RFD or hook echo.
Source: Markowski and Richardson (2010)
Assume ambient flow sweeps vortex rings
forward and as they are descending, they tilt
across the downdraft/updraft boundary
Source: Markowski and Richardson (2010)
The updraft just east of the hook lifts the vortex
lines, creating the arches that straddle the RFD.
Stretching intensifies the rotation on the
cyclonic side near the mesocyclone.
Source: Markowski and Richardson (2010)
Does a purely barotropic mechanism exist? We
argued earlier that an updraft that simply
rearranges vortex lines by tilting (barotropic)
cannot explain vorticity at the ground.
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Axisymmetric model simulating mid-level
mesocyclone in a purely Beltrami flow
Cyclonic updraft surrounded by anticyclonic
downdraft
Initial state is steady – won’t change
Simulate tornadogenesis by introducing
hydrometeors through top in divergent
outflow above the updraft
They do not evaporate (no buoyancy) – only
create a body force
They fall in the updraft/downdraft interface,
simulating a rain curtain (axisymmetric) on
the scale of a hook echo, dragging down air
with substantial angular momentum
Source: Davies-Jones (2008)
Model Initial Conditions (t = 0)
Vr
Vf
W
X
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Note that the downdraft is rotating anticyclonically because the shear
vorticity is negative and stronger than the positive curvature vorticity
Source: Davies-Jones (2008)
Solution at t = 3.5 non-dimensional units
Vr
Vort
W
Rain
Source: Davies-Jones (2008)
Solution at t = 5.25 non-dimensional units
Vr
Vf
W
X
Rain
Source: Davies-Jones (2008)
Solution at t = 6.07 non-dimensional units
Vr
Vf
W
X
Rain
Source: Davies-Jones (2008)
What Happens in the Simulation?
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Precipitation drags down air with high A.M.
Rotating updraft narrows as it is squeezed by
precipitation-induced downdraft
Combined, these cause the mesocyclone to
contract and descend (roughly cyclostrophic)
As the max in tangential velocity hits the ground,
some of it flows radially inward and a new max
forms below and moves inward
The near-ground vortex becomes most intense,
rapidly intensifying into the tornado
Cyclic Tornadogenesis
Burgess et al. 1982
Adlerman and Droegemeier (2000)
Cyclic Tornadogenesis
Adlerman and Droegemeier (2000)
Type II (Non-Supercell) Tornadoes
Photo by Peter Blottman
Role of the Downdraft When
Vertical Vorticity IS Present Initially at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Role of the Downdraft When
Vertical Vorticity IS Present Initially at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Role of the Downdraft When
Vertical Vorticity IS Present Initially at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Role of the Downdraft When
Vertical Vorticity IS Present Initially at Ground
Assume NO Low-Level Baroclinic Generation
Source: Markowski and Richardson (2010)
Life Cycle of Land Spouts Along a NonBaroclinic Convergence Line
Wakimoto and Wilson (1989)
Life Cycle of Land Spouts in a
Baroclinic Zone
Source: Markowski and Richardson (2010)
Gustnado
Source: Markowski and Richardson (2010), Photo by Charles Doswell III
Tornado Structure and Dynamics
Photo by Mike Batchelor
Tornado Structure
Region I: Outer Region. Inward-spiraling air that approximately
conserves its angular momentum, meaning the azimuthal velocity
increases as the air approaches the vortex.
Source: Markowski and Richardson (2010)
Tornado Structure
Region II: Core. From tornado axis outward to radius of maximum
winds. Contains condensation funnel, debris, flow is in cyclostrophic balance and strongly centrifugally stable (little entrainment
into the core)
Source: Markowski and Richardson (2010)
Tornado Structure
Region III: Corner Region. Region in boundary-layer where
the radial flow turns abruptly upward – sometimes called a corner
jet.
Source: Markowski and Richardson (2010)
Tornado Structure
Region IV: Boundary-Layer Flow. The boundary-layer of the
tornado itself with thickness from 10-100 m, turbulent and often
asymmetric. Not cyclostrophic owing to friction, which intensifies
rotation by increasing convergence of angular momentum.
Source: Markowski and Richardson (2010)
Tornado Structure
https://www.youtube.com/watch?v=PygCVIlT4hU
Source: Golden and Purcell (1970)
A “Famous” Tornado
Laboratory Simulation of TornadoLike Vortices
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Pioneered by Neal Ward at
NSSL in the early 1970s
Seminal work done at NSSL
and Purdue University
Fujita was very influential in
laboratory experiments
Later work at OU was the
last major effort at laboratory
simulation
Now large facilities are
simulating vortices
Church et al. 1977
Neal Ward (NSSL) Vortex Chamber
Davies-Jones et al. (2001)
The OU Chamber with Vanes
Rothfusz (1997)
Vortex Structure

Laboratory experiments have shown that the
structure of intense vertical vortices depends
strongly upon a non-dimensional parameter
known as the Swirl Ratio

C = circulation around central axis of radius ro
Q = rate of volume flow through top of chamber

Vortex Structure
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For S < 1, flow is
dominated by updraft
rather than rotation
Above the boundary-layer,
air approaching the axis
slows down and turns
upward
Thus, the pressure
increases along parcel
paths
Davies-Jones et al (2001)
Vortex Structure

As S is increased, the
rotation lowers the
pressure at the center of
the vortex, allowing flow
to penetrate toward the
axis. This is a one-celled
vortex because it has
only a single updraft.
Davies-Jones et al (2001)
Vortex Structure
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As S is increased further, a central downdraft
forms, resulting in a two-celled vortex.
Large (>2) swirl ratios creates a sufficiently large
pressure deficit to induce a downdraft on the axis
of rotation  vortex breakdown
Davies-Jones et al (2001)
Vortex Structure
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For really large S, the central downdraft reaches
the ground.
The interface between the rapidly rotating inflow
and weakly rotating downdraft air at the surface is
unstable, leading to multiple vortices
Davies-Jones et al (2001)
Multi-Vortex Tornadoes are Observed in
Nature and Many Large Tornadoes
Probably Have Them Even Though They
Are Not Obvious
Photo by Amos Magliocco
Multiple Vortices Can be
Simulated Numerically
Credit: Markowski and Richardson (2010) from Dave Lewellen
Multiple Vortices Likely are
Responsible for Erratic Damage
Simple Flow Model: Rankine Vortex
Graphic: Hindawi Publishing Company
Rankine Vortex

For a Rankine Vortex, one can show (see pages
288-289 in Markowski and Richardson) that the
minimum pressure at the vortex center is given
by

For V = 25 m/s, P’ = 6 mb (6 m drop of cld base)
For V = 100 m/s, P’ = 100 mb (1 km drop of cld
base) – probably down to ground level
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Tornado Thermodynamic
Speed Limit
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The tangential wind speed around a tornado is a function
of the pressure deficit at the center – which is related to
latent heating in the updraft and subsidence along vortex
axis
The thermodynamic speed limit is equal to the square root
of 2CAPE (assuming zero CIN)
Simulations have shown that the speed limit can be
exceeded by a factor of 2 for steady vortices and 3 for
transient vortices owing to a supercritical end wall vortex
with axial jet
Accuracy
of Warning Is Just as Critical as Timeliness of Warning
Reasons for High FAR in NEXRAD
Algorithms

NEXRAD cannot sample much of the region where
tornadoes, heavy rain, microbursts occur (ground
up to ~2 km)
Beyond blue circle
460 km
(reflectivity)
230 km
(reflectivity)
110 km
(reflectivity)
(110 km range)
center of lowest
NEXRAD scan is
above 2 km AGL…
76% of scan
CONUS 88-D Network: 1 km AGL
CONUS 88-D Network: 2 km AGL
CONUS 88-D Network: 3 km AGL
Reasons for High FAR in NEXRAD
Algorithms

NEXRAD radars are long-range and spaced
relatively far apart
– Degradation in resolution with range
– Limited overlap in scan coverage, except at long ranges
(where resolution is poor)
(F0) Tornado Events Warned and Unwarned
(F3) Tornado Events Warned and Unwarned
Date
Date
(F1) Tornado Events Warned and Unwarned
(F4) Tornado Events Warned and Unwarned
1990
2002
2000
1998
Date
Date
(F2) Tornado Events Warned and Unwarned
(F5) Tornado Events Warned and Unwarned
Date
2002
2000
2002
2000
1998
1996
1994
1992
0
1998
50
(F5) Tornado Events
Warned
1994
(F2) Tornado Events
Warned
(F5) Tornado Events
Unwarned
1992
100
9
8
7
6
5
4
3
2
1
0
1990
(F2) Tornado Events
Unwarned
Number of Tornadoes
150
1990
Number of Tornadoes
200
1996
1996
1994
1992
1990
0
2002
100
(F4) Tornado Events
Warned
2000
(F1) Tornado Events
Warned
1998
200
(F4) Tornado Events
Unwarned
1996
(F1) Tornado Events
Unwarned
1994
300
45
40
35
30
25
20
15
10
5
0
1992
400
Number of Tornadoes
Number of Tornadoes
500
Date
2002
1990
2002
2000
1998
1996
1994
1992
1990
0
2000
200
(F3) Tornado Events
Warned
1998
(F0) Tornado Events
Warned
400
1996
600
(F3) Tornado Events
Unwarned
1994
(F0) Tornado Events
Unwarned
800
80
70
60
50
40
30
20
10
0
1992
1000
Number of Tornadoes
Number of Tornadoes
1200
The Million Dollar Question:
Will Computer Models Ever
Be Able to Predict
Tornadoes?
24 May 2011 Tornado Outbreak:
Warning on a Numerical Forecast
NWS OUN Graphic
24 May 2011 Tornado Outbreak:
Warning on a Numerical Forecast