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 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 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 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 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, T m, 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 Re r )

Scatter diagram (after Johns et al. 1993) showing combinations of CAPE (J kg -1 ) and 0- 2-km AGL helicity (m 2 s -2 ) 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 (F4- F5). 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. Re r (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 V z 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 (V z  3V zout ). 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.