Tornadoes

1. Tornadoes QT Movie M. D. Eastin

2. Tornadoes • Significant Events in U.S. History • The Fujita Scale • U.S. Tornado Climatology • Mesoscale Observations • Scales of Motion • WSR-88D look at the 3 May 1999 Oklahoma City tornado • Damage Patterns • Tornado Structure • Core Observations • Conceptual Model of Air Flow • Tornadogenesis • Supercell Tornadoes • Non-supercell Tornadoes • Tornado Forecasting M. D. Eastin

3. Significant Tornado Events • The Tri-State Tornado: • Occurred on 18 March 1925 • 695 confirmed fatalities • (deadliest in US history) • Damage suggests F5 intensity • Over 15,000 homes destroyed • Continuous 219 mile track • Current thought is that it was • actually a family of tornadoes • spawned by the same storm Griffin, IN M. D. Eastin

4. Significant Tornado Events • The Super Outbreak: • Occurred on 3 April 1974 • Total of 148 tornadoes in 13 states • (Second most on one day in history) • F5=7 F4=24 F3=35 • 315 confirmed fatalities • Over 5,000 people injured • Severely damaged over 900 sq miles F4 Parker City, IN (21) M. D. Eastin

5. Significant Tornado Events 3 May 1999 Oklahoma Outbreak 48 confirmed fatalities $1.5 billion in damages M. D. Eastin

6. Significant Tornado Events 25-28 April 2011 Outbreak: 358 confirmed tornadoes (TX → NY) Strongest TOR in MS/AL/GA/TN on 27 April (EF5 = 4 EF4 = 11 EF3 = 22) 348 deaths $11 billion in damages (costliest event) 27 April 2011 M. D. Eastin

7. The Fujita-Scale • Estimating Tornado Intensity: • Developed by Dr. Ted Fujita • in 1971 • Updated in 2007 • Designed to bridge the • gap between the Beaufort • and Mach scales • Note that there are more • than 6 F-scale categories • in the original scale M. D. Eastin

8. The Fujita-Scale • Damage Examples: • To date, no instrument has • proven reliable enough to • accurately (and regularly) • measure the maximum wind • speeds within tornadoes. • The determination (an estimate) • of any tornado’s intensity is • alwaysdone via post-storm • damage surveys of the area • Important Note: • Tornado “intensity” is a function of building construction quality, whether any buildings were even damaged, forward motion of the tornado, as well as many other factors… F0 F1 F2 F3 F4 F5 M. D. Eastin

9. The Enhanced Fujita-Scale • A Re-evaluation of Tornado Intensity in the Modern World: • Based on damage to a single family house using traditional construction practices • (i.e. modern quality and design), assuming the house was in compliance with • common building codes and was regularly maintained • In general, lower winds speeds are required to produce the same damage • Officially adopted by the NWS for use beginning February 1, 2007 • More information can be found at: http://www.spc.noaa.gov/faq/tornado/ef-scale.html M. D. Eastin

10. Tornado Climatology Tornado Alley: From Brooks et al. (2003) M. D. Eastin

11. Tornado Climatology Annual Cycle: From Brooks et al. (2003) M. D. Eastin

12. Tornado Climatology Seasonality: From Brooks et al. (2003) Also see: http://www.nssl.noaa.gov/hazard/tanim/torw8099.html M. D. Eastin

13. Tornado Climatology • Number of Tornado Reports: • The upwards trends are believed to be artificial • The trend likely reflects: • An increase in population density • Improved reporting procedures • Organized networks of “storm spotters” From Brooks et al. (2003) M. D. Eastin

14. Tornado Climatology • Death: • Annual total number of deaths has steadily decreased over the last 50 years • During the period 1950-1999: 4,460 total deaths (average of 89 per year) • 40,522 total tornadoes (average of 810 per year) • Damage: • Survey of damage from all tornadoes • in the period 1950-1995 (after an • adjustment for wealth and inflation) • Total Damage: $19.3 billion • Annual Average: $0.42 billion • Survey of adjusted damage from only • major (F4-F5) tornadoes during • the same period • Account for 2.3% of all tornadoes • Total Damage: $10.2 billion • Annual Average: $0.22 billion M. D. Eastin

15. Mesoscale Observations Multiple Scales of Rotation: Mesocyclone: 2000-7000 m in diameter – Most often detected by NWS Doppler radar Tornado: 100-1000 m in diameter – Rarely observed by NWS Doppler radar (TVS) and never by ASOS (more on this later…) Suction Vortices: 1-50 m in diameter – Recently observed by high-resolution Doppler radar M. D. Eastin

16. Mesoscale Observations • Multiple Scales of Rotation: Mid-level Mesocyclones • Persistent rotation observed in supercells by NWS Doppler radar (automated algorithms) • Typical altitudes → 2-7 km AGL • Less than 25% of radar-detected mid-level mesocyclones produce tornadoes NEXRAD-88D Radar Oklahoma City 3 May 1999 Mesocyclone Automated Detection Algorithm: Looks for quasi-symmetric mesocyclonic circulations (large horizontal shears) that vertically correlate through a >3.5 km depth and are persistent for >10 minutes Mesocyclones detected by the automated algorithm M. D. Eastin

17. Mesoscale Observations • Multiple Scales of Rotation: Low-level Mesocyclones • Altitude = 1-2 km AGL • Associated with hook echo • Wall cloud • Often difficult to detect by NWS • Doppler radars if more than 50 km • from the radar (due to non-zero • beam elevation angles and Earth’s • curvature) • If detected, probability of a • tornado increases. • Less than 40% of radar-detected • low-level mesoscyclones produce • a tornado Storm-relative winds Radar reflectivity Storm-relative winds Radar reflectivity Doppler radial velocity Radar beams Storm-relative winds Vertical vorticity Vertical motion Airborne Doppler Synthesis (800m AGL) From Wakimoto et al. (2003) M. D. Eastin

18. Mesoscale Observations • Multiple Scales of Rotation: Tornado Vortex Signature (TVS): • Historically, a tornado has shown up on • operational NWS radars as a region of • enhanced gate-to-gate (adjacent beams) • horizontal shear • When the horizontal shear exceeds some • criteria, a TVS is identified • Note: The NEXRAD WSR-88D radar can not • resolve a tornado’s circulation. • An identified TVS is highly suggestive • that a tornado is present • Not all reported tornadoes are associated • with a radar-detected TVS (~60%) • Not all storms with a radar-identified TVS • produce a tornado (~80%) TVS (possible tornado) Mesocyclone M. D. Eastin

19. 3 May 1999, Oklahoma City Tornado M. D. Eastin

20. 3 May 1999, Oklahoma City Tornado M. D. Eastin

21. 3 May 1999, Oklahoma City Tornado M. D. Eastin

22. 3 May 1999, Oklahoma City Tornado M. D. Eastin

23. 3 May 1999, Oklahoma City Tornado M. D. Eastin

24. 3 May 1999, Oklahoma City Tornado M. D. Eastin

25. 3 May 1999, Oklahoma City Tornado M. D. Eastin

26. Tornado Damage Patterns From Wakimoto and Atkins (1996) M. D. Eastin

27. Tornado Damage Patterns From Wakimoto and Atkins (1996) M. D. Eastin

28. Tornado Core Observations • Photogrammetric Studies: • Use multiple photographs • to diagnose structure and • air flow patterns • Pioneered by Ted Fujita • in the 1960s • Must know many details as • a function of time: • Camera location • Tornado location • Time of each photo • Camera / film specifics • Assumes visible “features” • move with the local wind • (Is this a good assumption?) M. D. Eastin

29. Tornado Core Observations • Totable Tornado Observatory (TOTO): • Developed by Dr. Howard Bluestein (Univ. Oklahoma) • and his graduate students in the early 1980s • Designed to record basic surface observations inside • a tornado vortex • Never successfully deployed • Motivation for a popular movie? From Bluestein et al. (1983) M. D. Eastin

30. Tornado Core Observations • Hardened In-Situ Tornado Pressure Recorder (HITPR): • Developed and deployed by the annual • TWISTEX Project since 2003 • (http://en.wikipedia.org/wiki/TWISTEX) • Designed to record basic surface observations • inside a tornado vortex • Successfully deployed in multiple tornadoes Data from an F4 Tornado From Lee et al. (2004) M. D. Eastin

31. Tornado Core Observations • Doppler on Wheels (DOWs) : • Vehicle-mounted Doppler • radars can get very close • (sometimes too close) and • resolve the circulation • Multiple radars deployed • each year (most recently • during VORTEX-2) From Wurman et al. (1997) M. D. Eastin

32. Tornado Core Observations • Doppler on Wheels (DOWs) : • Suctions vortices have been observed • (and photographed) by storm chasers • for decades • The DOWs have recently provided the • first direct quantitative observations of • “suction vortices” • (http://www.cswr.org/) From Wurman (2002) M. D. Eastin

33. Tornado Core Observations • Tornado Vortex Chambers: • Create artificial tornadoes in laboratories • Primary source of quantitative information before • the DOW radars (pre-1990s) • Two important parameters in a vortex chamber • Γ = Circulation of the flow about the central axis • Q = Rate of air flow through the chamber top • The ratio of Γ to Q is called • the swirl ratio(S): • Tornadoes form in vortex chambers when the swirl ratio is large From Gallus et al. (2005) M. D. Eastin

34. Tornado Core Observations • Tornado Vortex Chambers: • When the swirl ratio is very small (a), no vortex • develops at the surface (notice the descending • motion near the axis of rotation) • As the swirl ratio is increased (b), a vortex develops • at the surface (note the inflow and updraft just • above the surface much like a tornado). This is • called a one-cell vortex (one updraft) • As the swirl ratio further increases (c), a downdraft • develops along the central axis, producing a • cloud–free, or “hollow”, center to the tornado • (which is often observed by storm chasers) • At very large swirl ratios (d), the downdraft • penetrates to the surface and creates a two-celled • vortex (with two updrafts). This results in multiple • suction vortices (e) (as observed in nature) M. D. Eastin

35. Conceptual Model of Air Flow Five Flow Regions and Radial Pressure Profile: Outer Region (I): Inward spiraling air that conserves angular momentum (spins faster as it approaches the tornado axis) Corner (III): Region where air turns upward from being horizontal flow to primarily vertical flow Boundary Layer (IV): Flow interacts with ground and surface friction enhances the radial inflow Core Region (II): Inside the maximum winds, including the funnel cloud, dust, and debris. (cyclostrophic balance) Rotating Updraft (V): Parent updraft and mesocyclone Pressure profile: Assumes an idealized vortex structure in order to relate the flow field to the radial pressure gradient: Rankine Vortex Burgers-Rott Vortex M. D. Eastin

36. Supercell Tornadogenesis • Not well understood! • Three current theories have considerable observational and numerical modeling support • All three theories may work in concert • Each assumes the following circulations are present in the parent supercell: • Mid-level mesocyclone generated by tilting and stretching of horizontal vorticity • Low-level mesocyclone generated by tilting and stretching baroclinically-enhanced • streamwise vorticity within the vortical updraft • Mature forward and rear-flank downdrafts and their associated gust fronts Upper-level Flow Primary Updraft Updraft RFD Horizontal Vorticity Vectors FFD Mid-level Flow Inflow along the gust front acquires streamwise vortcity Inflow M. D. Eastin

37. Supercell Tornadogenesis • 1. Negligible Vertical Vorticity at the Surface: Downdraft Required • Simple tilting of low-level • horizontal vorticity by the • primary updraft cannot • produce vertical vorticity • at the surface since the • air rises away from the • surface during tilting • (top scenario) • However, if an adjacent • downdraft (i.e. the RFD) • is involved in the tilting • process, then vertical • vorticity can be advected • toward the surface (during • titling) and subsequently • stretched into a tornado • (bottom scenario) • Barotropic contribution M. D. Eastin

38. Supercell Tornadogenesis • 2. Negligible Vertical Vorticity at the Surface: Downdraft Required • The near-surface horizontal vorticity • can be enhanced when the RFD is • driven by negative buoyancy • Recall, horizontal buoyancy gradients • produce horizontal vorticity: • The downward advection of any • such horizontal vorticity will increase • the total available horizontal vorticity • to be tilted toward the surface and • then stretched into a tornado • Baroclinic contribution • Mechanism produces tornadoes • soon after RFD reaches surface B- M. D. Eastin

39. Supercell Tornadogenesis • 3. Ample Vertical Vorticity at the Surface: NO Downdraft Required • Strong horizontal shear • located along RFD or FFD • gust fronts produce large • near-surface vertical vorticity • Storm-relative inflow slowly • advects any vertical vorticity • “pockets” along the gust • fronts toward the primary • mesocyclonic updraft where • stretching and low-level • convergence increase the • near-surface vertical vorticity, • producing a tornado • Mechanism produces tornadoes • before RFD reaches the surface • or in between RFD “surges” M. D. Eastin

40. Supercell Tornadogenesis 3. Ample Vertical Vorticity at the Surface: NO Downdraft Required DOW Radial Velocity Observations Numerical Simulation Mesocyclone Center Mesocyclone Gust Front Vortices Gust Front Vortices M. D. Eastin

41. Supercell Tornadogenesis Numerical Simulation Movie #1 (A Top View) Numerical Simulation Movie #2 (A Surface Observer View) M. D. Eastin

42. Non-Supercell Tornadogenesis • Even Less Understood!!! • Often occurs along low-level • lines of horizontal shear and • convergence (e.g. gust fronts • and air-mass boundaries) • Large pre-existing low-level • vertical vorticity is stretched • by the updrafts of ordinary • growing cumulus clouds • Produces weak, short-lived • tornadoes (EF0 - EF2) M. D. Eastin

43. Tornado Forecasting • Continuous monitoring of ALL available observations: • Use real-time radar data to monitor storm formation and evolution • Use surface observations to monitor storm-relative inflow and cold pool characteristics • Use nearby soundings (rawinsondes and rapid-update numerical models) to monitor • standard forecast parameters (CAPE, SREH, EHI, etc.) • Other useful forecast parameters: • Vertical Shear 0-1 km AGL: • Large values favor tornadoes • Strong shear implies large horizontal • vorticity near the surface that can be • tilted into the vertical by updrafts and • downdrafts (especially the RFD) • Mixed-layer LCL: • Small values favor tornadoes • Moist boundary layers limit negative • buoyancy in downdrafts and prevent • strong cold pools from “under-cutting” • the primary updraft (see next slide…) M. D. Eastin

44. Tornado Forecasting Surface Density Potential Temperature Perturbations (observed by mobile mesonets during VORTEX) Weak Cold Pools Moderate Cold Pools Strong Cold Pools M. D. Eastin

45. Tornadoes • Summary: • Significant Events in U.S. History → Why are they significant? • The Fujita Scale → Basic concept and reason for recent changes • U.S. Tornado Climatology → Basic characteristics and trends • Mesoscale Observations • Scales of Motion → Ability / Methods used to observe each scale • Damage Patterns → Basic structure and reasons for such structure • Tornado Structure • Core Observations → Various methods and laboratory results • Conceptual Model of Air Flow → Basic characteristics of each region • Tornadogenesis • Supercell Tornadoes → Important physical processes (and when) • Non-supercell Tornadoes → Important physical processes • Tornado Forecasting → Methods and additional useful parameters M. D. Eastin

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47. References Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci., 40, 359-377. Lee, B. D., and R. B. Wilhelmson, 1997: The numerical simulation of nonsupercell tornadogenesis. Part II: Evolution of a . family of tornadoes along a weak outflow boundary. J. Atmos. Sci., 54, 2387-2415. Markowski, P. M., E. N. Rasmussen, and J. M. Straka, the occurrence of tornadoes in supercells interacting with boundaries during VORTEX-95. Wea. Forecasting, 13, 852-859. Rotunno, R., 1986: Tornadoes and tornadogenesis. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed., Amer. Meteor. Soc., 414-436. Trapp R. J., and R. Davies-Jones., 1997: Tornadogenesis with and without a dynamic pipe effect. J. Atmos. Sci., 54, 113-133. Wakimoto, R. M. and N. T. Atkins, 1996: Observations on the origins of rotation: The Newcastle tornado during VORTEX-94. Mon. Wea. Rev., 124, 384-407. Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev., 117, 1113-1140. Wakimoto, R. M., C. Liu, and H. Cai, 1998: The Garden City, Kansas storm during VORTEX-95. Part I: Overview of storm’s lifecycle and mesocyclogenesis. Mon. Wea. Rev., 126, 372-392. Wakimoto, R. M., H. V. Murphey, D. C. Dowell, and H.B. Bluestein, 2003: The Kellerville tornado during VORTEX: Damage survey and Doppler radar analyses. Mon. Wea. Rev., 131, 2197-2221. Wicker, L. J., and R. B. Wilhelmson, 1995: Simulation and analysis of tornado development and decay within a three- dimensional supercell thunderstorm. J. Atmos. Sci, 52, 2675-2703. Wurman, J., 2002: The multiple-vortex structure of a tornado. Wea. Forecasting, 17, 473-505. Wurman, J., J. M. Straka, E. N. Rasmussen, M. Randall, and A. Zahari, 1997: Design and deployment of a portable, pencil beam, pulsed, 3-cm Doppler radar. J. Atmos. Oceanic. Technol., 14, 1502-1512. M. D. Eastin