AN OVERVIEW OF AN OVERVIEW OF TORNADOGENESIS AND TORNADOGENESIS AND

VORTEX STRUCTUREVORTEX STRUCTUREbyby

Ernest M. Agee Ernest M. Agee Purdue UniversityPurdue University

West Lafayette, IndianaWest Lafayette, Indiana

An aerial view of a classic supercell thunderstorm above southern An aerial view of a classic supercell thunderstorm above southern Maryland on 29 April 2002. At the time this photograph was taken this Maryland on 29 April 2002. At the time this photograph was taken this storm was producing twin tornadoes. storm was producing twin tornadoes.

A Doppler radar image of a supercell thunderstorm near Oklahoma 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 City on 3 May 1999. The image on the left shows a nice example of a hook echo radar feature that sometimes accompanies the hook echo radar feature that sometimes accompanies the mesocyclone. The image on the right shows the indication of the TVS, mesocyclone. The image on the right shows the indication of the TVS, identified by ▼. identified by ▼.

Three supercell storms near Dallas, TX on 5 April 2003. These cells Three supercell storms near Dallas, TX on 5 April 2003. These cells are separated but are located in a straight line. are separated but are located in a straight line.

Solid line of thunderstorms near Louisville, KY on 24 October 2001.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. 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 The conceptual model of the evolution of the BOW echo (after Fujita, 1978). Fujita, 1978).

A BOW echo and associated comma head over central Illinois A BOW echo and associated comma head over central Illinois on 4 June 2002. A weak tornado (F0) was reported by the on 4 June 2002. A weak tornado (F0) was reported by the Illinois State Police. Illinois State Police.

Left: Radar image from the Quad Cities containing two supercells with Left: Radar image from the Quad Cities containing two supercells with well defined hook echoes (5:00 pm; 4/30/03).well defined hook echoes (5:00 pm; 4/30/03).

Right: Radar image from Tulsa, OK containing a squall line with Right: Radar image from Tulsa, OK containing a squall line with embedded supercells ( 6:25 pm; 5/6/03). embedded supercells ( 6:25 pm; 5/6/03).

Colby, Kansas, 21 July 1996Colby, Kansas, 21 July 1996

First radar tracking of a tornado’s hook echo.First radar tracking of a tornado’s hook echo.

Two mini-tornado cyclones and associated wall clouds inTwo mini-tornado cyclones and associated wall clouds inFort Cobb, Oklahoma.Fort Cobb, Oklahoma.

Two wall clouds within a mesocyclone near Canadian, TX Two wall clouds within a mesocyclone near Canadian, TX (see Bluestein 1999). Each wall cloud is producing a (see Bluestein 1999). Each wall cloud is producing a tornado, resulting in a parallel-mode tornado family.tornado, resulting in a parallel-mode tornado family.

Multiple vortex columns in the formative stages of the Jarrell, Multiple vortex columns in the formative stages of the Jarrell, Texas tornado of 27 May 1997.Texas tornado of 27 May 1997.

Multiple vortex tornado, TMultiple vortex tornado, Tmm, near Friendship, Oklahoma (May 1982)., near Friendship, Oklahoma (May 1982).

Processes Affecting TornadogenesisProcesses Affecting Tornadogenesis

CAPE, SRH and SLUCAPE, SRH and SLURear-Flank Downdraft (RFD)Rear-Flank Downdraft (RFD)Baroclinic Boundaries (BB)Baroclinic Boundaries (BB)Advection of Shear Vortices (ASV)Advection of Shear Vortices (ASV)Rear Inflow Jets (RIJ)Rear Inflow Jets (RIJ)Book-End Vortex (BEV)Book-End Vortex (BEV)Dynamic Pipe Effect (DPE)Dynamic Pipe Effect (DPE)Friction (aka the ReFriction (aka the Rerr))

Scatter diagram (after Scatter diagram (after Johns et al. 1993) Johns et al. 1993) showing combinations showing combinations of CAPE (J kgof CAPE (J kg-1-1) and 0-) and 0-2-km AGL helicity (m2-km AGL helicity (m22 s s--

22) utilizing the 20R85 / ) utilizing the 20R85 / 30R75 storm motion 30R75 storm motion assumptions for 242 assumptions for 242 strong and violent strong and violent tornadoes of JDL tornadoes of JDL dataset. All triangles dataset. All triangles (open and solid) (open and solid) represent cases in represent cases in which the assumed which the assumed storm motion is 30R75, storm motion is 30R75, while the assumed while the assumed storm motion for the storm motion for the remainder of the cases remainder of the cases is 20R85. The open is 20R85. The open circles and open circles and open triangles represent triangles represent violent tornadoes (F4-violent tornadoes (F4-F5). The crosses F5). The crosses represent cases represent cases associated with tropical associated with tropical cyclones.cyclones.

Horizontal flow relative to moving storm at 6.4 km above ground. Horizontal flow relative to moving storm at 6.4 km above ground. Lengths of arrows are proportional to relative wind speed.Lengths of arrows are proportional to relative wind speed.

Example of locally produced shear-driven vortex or "gustnado" Example of locally produced shear-driven vortex or "gustnado" along the gust front boundary of thunderstorm outflow. along the gust front boundary of thunderstorm outflow.

Critical swirl ratio as a Critical swirl ratio as a function of radial function of radial Reynolds number, for Reynolds number, for the transitions between the transitions between various vortex types. various vortex types. L L T indicates the T indicates the transition between a transition between a laminar and a turbulent laminar and a turbulent core at the height of the core at the height of the updraft hole (mean updraft hole (mean position of breakdown in position of breakdown in the plane of the hole). the plane of the hole). The single turbulent core The single turbulent core usually evolves into a usually evolves into a single helical mode. single helical mode. 1 1 2 then represents 2 then represents the transition from the the transition from the single spiraling roll single spiraling roll vortex to a configuration vortex to a configuration containing two containing two interlocking spirals. interlocking spirals. Similarly, 2 Similarly, 2 3 3 represents the transition represents the transition from two to three from two to three subsidiary vortices.subsidiary vortices.

Rer (X10-5)

SR

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.Laboratory simulation of four vortices.