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What’s Left to Learn About Tornadoes?

What’s Left to Learn About Tornadoes?. Erik Rasmussen, Rasmussen Systems Jerry Straka, OU Kathy Kanak, CIMMS …and our students!.

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What’s Left to Learn About Tornadoes?

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  1. What’s Left to Learn About Tornadoes? Erik Rasmussen, Rasmussen Systems Jerry Straka, OU Kathy Kanak, CIMMS …and our students!

  2. “ There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don’t know. But there are also unknown unknowns. These are things we do not know we don’t know. ” —United States Secretary of Defense Donald Rumsfeld

  3. Deep convection Development of rear-side precipitation Deep shear, adequate CAPE Supercell Convergence, tilting of near-ground SRH next to RFD Updraft Baroclinic generation; arching Tornado Cyclone States Tornado Vortex contraction Processes

  4. Deep convection Development of rear-side precipitation YOU ARE HERE Deep shear, adequate CAPE Supercell Convergence, tilting of near-ground SRH next to RFD Updraft Baroclinic generation; arching Tornado Cyclone States Tornado Vortex contraction Processes

  5. tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

  6. tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

  7. tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

  8. tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

  9. tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

  10. tornado—1. I know one when I see one.

  11. Deep convection Development of rear-side precipitation YOU ARE HERE YOU ARE HERE Deep shear, adequate CAPE Supercell Convergence, tilting of near-ground SRH next to RFD Updraft Baroclinic generation; arching Tornado Cyclone States Tornado Vortex contraction Processes

  12. The Tornado Cyclone is roughly 10 times the diameter of the visible Tornado B A B A

  13. Angular momentum M=Vr = tangential, swirling velocity times radius. Dimmitt tornado, 2 June 1995. From single-Doppler analysis. M=14000 contour

  14. Approximate swirling windspeed and angular momentum in the Dimmitt tornado early in mature stage about 200 m above the ground.

  15. This flow would evolve from the previous in < 5 minutes with a modest inflow of (e.g.) about 5 m/s at 500 m radius

  16. As the high-M air penetrates closer to the axis, maximum swirling wind speed ~doubles for each halving of the radius of penetration of the large M region.

  17. Early: In-up Late: Down-Out

  18. Early: ~307 K, least precip, warmest air near the tornado Late: ~305 K, most precip, cooler

  19. Thoughts... • Most supercells probably have tornado cyclones*. Nomenclature isn't so important as understanding that... • A tornado, whatever wind speed or appearance criteria being used, is the inner portion of a tornado cyclone where enough angular momentum has been transported toward the center to give tornadic windspeeds.

  20. Thoughts... • Tornadogenesis failure is possibly generally a failure of contraction of the tornado cyclone. • Strength of the inner portion of the vortex (the tornado) depends partially on angular momentum in the outer portion, and the removal of mass upward through the vortex (and hence convergence below).

  21. Thoughts... • Hence we seek to understand why most supercells are not conducive to transporting sufficient air upward through the tornado cyclone to increase the vortex to tornado strength. • Operationally, even if tornado cyclones are close enough to the 88D for detection, the differences between tornadic and non-tornadic TCs may almost always be ~unresolvable.

  22. Thoughts... • Tornado life cycle appears to be related to the changes of the secondary flow (in-up vs. down-out) in the tornado cyclone. • Conjecture: long-lived tornadoes occur in tornado cyclones that (for reasons unknown) have a very slow transition from in-up to down-out.

  23. Deep convection Development of rear-side precipitation YOU ARE HERE Deep shear, adequate CAPE Supercell Convergence, tilting of near-ground SRH next to RFD Updraft Baroclinic generation; arching Tornado Cyclone States Tornado Vortex contraction Processes

  24. So how did the Tornado Cyclone come into existence? • Does the Rear-Flank Downdraft have a role? • Here are some historically validated observations about the supercell:

  25. Updraft acquires horseshoe shape. • Counter-rotation is observed. • A Rear-Flank Downdraft is present in the interior of the horseshoe pattern. • The tornado cyclone is centered in strong vertical velocity gradient on the interior left edge of the horseshoe. • A gust front is present below the rear edge of the updraft.

  26. A simulation... Vortex straddles up/downdraft Horseshoe-shaped updraft Counter-rotating vortices Rear-Flank Downdraft*

  27. Initial conditions

  28. Mechanism 1: Arching In a nutshell, vortex rings about the RFD are tugged upward at the leading edge in the low-level updraft, giving rise to vortex line arches and counter-rotating vortices. See Straka et al. In the Electronic Journal of Severe Storms Meteorology, Vol 2. (EJSSM.org)‏

  29. Mechanism 2: Tilting/stretching of inflow streamwise vorticity

  30. Mechanism 3: Agglomeration of shear vortices

  31. Real mechanism: All three acting together (others?)‏

  32. RFD Genesis • Different forcings for different parts? • One example showing evidence that precipitation plays a role...

  33. Rear-side precipitation...

  34. 0050 UTC • From airborne Doppler • 40 dBZ DRC Western tip of forward-flank precipitation North

  35. Western tip of forward-flank precipitation • From airborne Doppler • Away • To North DRC

  36. Crowther video frame… 0050 UTC (north-northwest)‏

  37. Gust front • 500 m AGL 21 m/s C 21 m/s DRC A

  38. The Blob Blobette BWER C A North

  39. Bobby Eddins video frames ~0053 UTC (N-W)‏

  40. Gust front • 500 m AGL 21 m/s Blobette C 18 m/s Blob A

  41. C A North

  42. Bobby Eddins video ~0055 UTC (north)‏

  43. Tornado Forms

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