AN OVERVIEW OF
TORNADOGENESIS AND
VORTEX STRUCTURE
by
Ernest M. Agee
Purdue University
West Lafayette, Indiana
An aerial view of a classic supercell thunderstorm above southern
Maryland on 29 April 2002. At the time this photograph was taken this
storm was producing twin tornadoes.
A Doppler radar image of a supercell thunderstorm near Oklahoma City
on 3 May 1999. The image on the left shows a nice example of a hook
echo radar feature that sometimes accompanies the mesocyclone. The
image on the right shows the indication of the TVS, identified by ▼.
Three supercell storms near Dallas, TX on 5 April 2003. These cells
are separated but are located in a straight line.
Solid line of thunderstorms near Louisville, KY on 24 October 2001.
An example of a BOW echo near San Angelo, TX on 7 April 2002.
The conceptual model of the evolution of the BOW echo (after
Fujita, 1978).
A BOW echo and associated comma head over central Illinois on
4 June 2002. A weak tornado (F0) was reported by the Illinois
State Police.
Left: Radar image from the Quad Cities containing two supercells with
well defined hook echoes (5:00 pm; 4/30/03).
Right: Radar image from Tulsa, OK containing a squall line with
embedded supercells ( 6:25 pm; 5/6/03).
Colby, Kansas, 21 July 1996
First radar tracking of a tornado’s hook echo.
Two mini-tornado cyclones and associated wall clouds in
Fort Cobb, Oklahoma.
Two wall clouds within a mesocyclone near Canadian, TX
(see Bluestein 1999). Each wall cloud is producing a
tornado, resulting in a parallel-mode tornado family.
Multiple vortex columns in the formative stages of the Jarrell,
Texas tornado of 27 May 1997.
Multiple vortex tornado, Tm, near Friendship, Oklahoma (May 1982).
Processes Affecting Tornadogenesis
 CAPE, SRH and SLU
 Rear-Flank Downdraft (RFD)
 Baroclinic Boundaries (BB)
 Advection of Shear Vortices (ASV)
 Rear Inflow Jets (RIJ)
 Book-End Vortex (BEV)
 Dynamic Pipe Effect (DPE)
 Friction (aka the Rer)
Scatter diagram (after
Johns et al. 1993)
showing combinations
of CAPE (J kg-1) and 02-km AGL helicity (m2 s2) utilizing the 20R85 /
30R75 storm motion
assumptions for 242
strong and violent
tornadoes of JDL
dataset. All triangles
(open and solid)
represent cases in
which the assumed
storm motion is 30R75,
while the assumed
storm motion for the
remainder of the cases
is 20R85. The open
circles and open
triangles represent
violent tornadoes (F4F5). The crosses
represent cases
associated with tropical
cyclones.
Horizontal flow relative to moving storm at 6.4 km above ground.
Lengths of arrows are proportional to relative wind speed.
Example of locally produced shear-driven vortex or "gustnado"
along the gust front boundary of thunderstorm outflow.
Critical swirl ratio as a
function of radial
Reynolds number, for
the transitions between
various vortex types.
L  T indicates the
transition between a
laminar and a turbulent
core at the height of the
updraft hole (mean
position of breakdown in
the plane of the hole).
The single turbulent core
usually evolves into a
single helical mode.
1  2 then represents
the transition from the
single spiraling roll
vortex to a configuration
containing two
interlocking spirals.
Similarly, 2  3
represents the transition
from two to three
subsidiary vortices.
SR
Rer (X10-5)
An idealized schematic of the
flow structure in a laminar one
cell convective vortex.
Laboratory simulation of laminar
one cell convective vortex.
An idealized schematic of the
flow structure during vortex
breakdown in a transition vortex.
The red dot represents the
stagnation point.
Laboratory simulation of a transition
vortex.
Single cell laminar vortex with
transition bubble.
Laboratory simulation of a single
cell laminar vortex with transition
bubble.
Transition vortex, depicting Vortex
Breakdown with “bubble” in the
axially erupting boundary layer jet,
and downstream stagnation point
with transition from laminar flow to
turbulent flow.
Laboratory simulation of a transition
vortex, depicting VB with “bubble” in
the axially erupting BL jet, and
downstream stagnation point with
transition from laminar flow to
turbulent flow.
Initial weak background Vz
with no vortex spin up.
Weak axially erupting
boundary layer jet at initial
spin up of subcritical vortex.
Increased background swirl (S) enhances the
erupting axial jet to supercritical values (Vz 
3Vzout). Continuity argument is applied to this
schematic to explain bubble formation.
An idealized schematic of the
flow structure in a two cell
turbulent vortex.
Laboratory simulation of a two
cell turbulent vortex.
Tangential velocity shear profiles for (a) one cell convective
vortex, (b) transition vortex, (c) two cell turbulent vortex, and
(d) transition into multiple vortex structures. The gray shaded
bar represents the region of critical shear for vortex (d).
Laboratory simulation of four vortices.