A Comprehensive Study of Cool Season Tornadoes in the Southeast United States

1. A Comprehensive Study of Cool Season Tornadoes in the Southeast United States Alicia C. Wasula July 13, 2005

2. Committee Members • Lance Bosart • John Molinari • Dan Keyser • Chris Thorncroft • Russ Schneider • Morris Weisman

3. The Problem • Nocturnal peak in tornado occurrence during cool season along Gulf coast • Difficulty warning public increased injury and loss of life (e.g., Fike 1993) • Difficult to discriminate between true threats and null events in real time

4. Goals • Provide evidence that this phenomenon exists • Provide possible physical explanations via: • Climatological analysis of low-level winds • Composite analysis of southeast US tornado episodes • Case study: 22-23 February 1998 • Relate results to tornadogenesis mechanisms

5. Outline • Historical Background • Review of low-level mesocyclone-genesis mechanisms • Climatology • Composite results • Case Study: 22-23 February 1998 • Conclusions • Future Work

6. Background • Cool season nocturnal tornado peak exists along Gulf coast (e.g. Knupp and Garinger 1993, Brooks et al. 2003) • Peak exists during early morning hours ~ 1200 UTC • Can occur in low CAPE/high shear environments • Return Flow air trajectory effects PBL thermodynamics

7. Background • Cool season nocturnal tornado peak exists along Gulf coast (e.g. Knupp and Garinger 1993, Brooks et al. 2003) • Can occur in low CAPE/high shear environments • Return Flow air trajectory effects PBL thermodynamics

8. CAPE vs. Shear for Cold Season Tornado Cases 0-2 km positive mean shear (x 10-3 s-1) Convective Available Potential Energy (CAPE, J/kg) Johns et al. 1993

9. Background • Cool season nocturnal tornado peak exists along Gulf coast (e.g. Knupp and Garinger 1993, Brooks et al. 2003) • Can occur in low CAPE/high shear environments • Return Flow air trajectory effects PBL thermodynamics

10. Types of Return Flow (Crisp and Lewis 1992)

11. Previous Composite Study: Peninsular Florida Tornado Environments (Hagemeyer and Schmocker 1991) Dry season w’ = ~5°C Wet season w’ = ~1°C Wet Season Dry Season All non-TC tornadoes ± 2 h of 00/12Z 1980-1988 1) Thermodynamics w 

12. Previous Composite Study: Peninsular Florida Tornado Environments (Hagemeyer and Schmocker 1991) Wet Season Dry Season All non-TC tornadoes ± 2 h of 00/12Z 1980-1988 2) Shear u v

13. Question #1: How does buoyancy/shear relationship effect convective mode?

14. Relationship of Buoyancy/Shear to Convective Mode CAPE (J/kg) Shear (m/s) Ordinary Multicell Supercell

15. Question #2: Given that shear is sufficient for supercells, how does hodograph shape determine supercell type?

16. Splitting Storm in Sheared FlowKlemp (1987)

17. Splitting Storm in Sheared FlowKlemp (1987)

18. Straight Hodograph Left & right mover equally dominant Left mover faster Source: A Convective Storm Matrix http://meted.ucar.edu/convectn/csmatrix/

19. Clockwise ¼ turn Hodograph Right mover dominant Left mover dissipates Source: A Convective Storm Matrix http://meted.ucar.edu/convectn/csmatrix/

20. Counterclockwise ¼ turn Hodograph Left mover dominant Right mover dissipates Source: A Convective Storm Matrix http://meted.ucar.edu/convectn/csmatrix/

21. Question #3:How does near-surface hodograph shape effect low-level mesocyclone generation?

22. Source of low-level rotation: baroclinic generation of horizontal vorticity in FFDstretched in updraft FFD Source: A Convective Storm Matrix http://meted.ucar.edu/convectn/csmatrix/

23. Streamwise Vorticity • Maximized when vorticity vector and velocity vector are parallel • 0 when vorticity vector and velocity vector are perpendicular

24. Wicker (1996) Horizontal Vorticity Vector (H) Storm Motion Vector

25. Wicker (1996) Vertical Velocity and Horizontal Winds At 50 m, 110 min Storm Inflow Environmental Winds

26. Wicker (1996) Pert. Temperature and Horizontal Vorticity At 50 m, 110 min  FFD H adds to env. H  H Parallel to inflow at FFD • Large streamwise vorticity X Updraft

27. Previous work has shown that supercells interacting with boundaries can produce long-lived tornadoes e.g., Maddox et al. 1980, Markowski et al. 1998, Rasmussen et al. 2000

28. Maddox et al. 1980 Along outflow boundary (C), streamwise vorticity is maximized

29. Climatology

30. Tornado Climatology Southeast US Domain

31. Storm Data • Tornado Database 1950-2001 • Corrected for duplicate reports • Use only F2 or greater tornadoes for climatology • All tornadoes considered for composites • Cool season tornadoes (November – March)

33.  By month By Hour (UTC) 

34. 1000 m Wind Climatology Pilot Balloon Stations

35. Pilot Ballon (Pibal) Dataset • 1948-1957 • 4x/day (0300, 0900, 1500, and 2100 UTC) • Wind data for lowest ~ 3 km • No thermodynamic data • No winds above cloud base

36. 1000 m Wind Climatology

38. Surface Wind Climatology Surface Stations

39. Surface Observations • Locally archived at University at Albany • Hourly • Cool seasons 1995-96 to 1999-2000 • Subset of stations for which dataset is complete

40. Sample Windroses  Coastal Station Inland Station 

41. CoastalStations (n=16)

42. Inland Stations (n=17)

43. Hypothesis • More cooling / higher pressures over land at night • Less cooling / lower pressures over ocean at night • Easterly geostrophic component at surface

44. MORE COOLING AT NIGHT H Vg LAND OCEAN LESS COOLING AT NIGHT L

45. Eta NARR Dataset • 32 km reanalysis • November – March 1999-2000 • 0900-2100 UTC sea level pressure difference

46. Nov-Mar 1999-2000 09–21 UTC SLP Diff. (hPa) – Eta NARR

47. Summary: Climatology • Cool season tornado peak in southeast US • Nocturnal maximum is strongest near Gulf of Mexico coast • 1000 m nocturnal LLJ is strongest near Gulf coast • Surface winds along coastline back at night due to more (less) cooling over land (water)

48. Composites

49. Tornado Episode Composites • NCEP/NCAR 2.5x 2.5 Reanalysis dataset • All tornado episodes 1950-2001 • ‘Episode-relative’ composite • Grouped by start time: 0000-0600, 0600-1200, 1200-1800, 1800-0000 UTC • Will show only TWO: • DAY: 1800-0000 UTC (n=327) • NIGHT: 0600-1200 UTC (n=130)

50. DAY NIGHT 200 hPa height (m), isotachs (m s-1)