1. Ch 5. Tornadoes and Tornadic Storms R.Davies-Jones, R.J.Trapp, H.Bluestein Presented by Rebecca Bethke Nov. 13 th, 2007 Ch.5 Tornadoes and Tornadic Storms R. Davies-Jones, R.J. Trapp, H.B. Bluestein Presented by Rebecca Bethke Nov. 13, 2007

2. SUPERCELLS (review) Life span > 1 hour Life span > 1 hour High spatial correlation between Mesocyclone and updraft High spatial correlation between Mesocyclone and updraft Categorized by position and extent of heavy rain to updraft: Categorized by position and extent of heavy rain to updraft: LP, “classic” or HP LP, “classic” or HP

3. Tornadoes: Type I Forms within a mesocyclone ‘parent’ Forms within a mesocyclone ‘parent’ Contains a core, in solid-body rotation, surrounded by a potential vortex Contains a core, in solid-body rotation, surrounded by a potential vortex Parent storm: Parent storm: Isolated supercell Isolated supercell Supercell in a line of thunderstorms Supercell in a line of thunderstorms or Mini supercell with small mesocyclone or Mini supercell with small mesocyclone

4. ISOLATED SUPERCELL: Viewed from southeast Environmental winds veering with height Barbed lines: boundary of cool-air outflow beneath the storm

5. b. Tornadic supercell : Flow field 250 m. above ground Dashed: negative vertical velocity Solid: positive vertical velocity (2m/s interval) Shaded area: radar echo T: vertical vorticity max (tornado)

6. Type II. Not associated with mesocirculation Not associated with mesocirculation Small/weak vortex forms along stationary/slow moving windshift line Small/weak vortex forms along stationary/slow moving windshift line : “Landspout” : “Gust-nado” : Cold-air funnel

8. Mesocyclone observations

9. Looking Northeast: Tornado at Flanking Line/RFD Intersection With Wall Cloud

11. Mesocyclone Identification Criteria Donaldson (1970) Doppler velocity shear ≥ 6 m/s/km Doppler velocity shear ≥ 6 m/s/km Differential velocity ≥ 30 m/s Differential velocity ≥ 30 m/s At base altitude ≤ 5 km above radar At base altitude ≤ 5 km above radar Existing with ≥ 3 km depth Existing with ≥ 3 km depth Lasting > 5-6 minutes Lasting > 5-6 minutes

12. Important Observations Observations that indicate updraft, downdraft and mesocyclone relationships: Observations that indicate updraft, downdraft and mesocyclone relationships: 3-d vertical wind and vortex existence/structure (from Doppler radar observations) 3-d vertical wind and vortex existence/structure (from Doppler radar observations) Mesocyclonic vertical vorticity Mesocyclonic vertical vorticity (computed from above info) (computed from above info)

13. Rotation Near-ground Numerical simulations show: Vertical shear yet no/little vertical vorticity does NOT create rotation near ground Vertical shear yet no/little vertical vorticity does NOT create rotation near ground Streamwise vorticity in air flowing along the ground cannot cause a mesocyclone to make contact with the ground Streamwise vorticity in air flowing along the ground cannot cause a mesocyclone to make contact with the ground

14. Rotation Near-Ground Field observations confirm: Tornadoes do not occur without rainy downdrafts Tornadoes do not occur without rainy downdrafts 1. Advancing buoyant jet 2. Transfer of high momentum air to surface 3. Cool downdraft

15. Baroclinic mechanism (Klemp and Rotunno 1983, Rotunno and Klemp 1985) Air near-ground from forward flank along strong baroclinic zone (warm to left, cool right): Air near-ground from forward flank along strong baroclinic zone (warm to left, cool right): 1. Developed large streamwise vorticity 2. Ingested into updraft and tilted spin-up Produces elevated rotation Similar occurrence in RFD cyclonic outflow near-sfc Similar occurrence in RFD cyclonic outflow near-sfc ( Davies-Jones) Completes cyclonic vortex structure from ground to mid/higher altitudes