Lecture 24: Thunder & Tornadoes (Ch 11)

1. http://www.iassistdata.org/tornado/ Bob Charlton responsible for above web report on Edmonton’s 1987 tornado (F4) Lecture 24: Thunder & Tornadoes (Ch 11) • general statements about tornados • soundings associated with severe thunderstorms & tornadoes • July 1987 Edmonton F4 tornado

2. Tornadoes • always in conjunction with Cumulonimbus? • diameter usually order 100 m, may be over a kilometer (record: 4 km) • lifetime minutes to (rarely) hours • translation speed typically of order 50 kph • may occur with any type of severe weather system: frontal boundaries, squall lines, MCC’s; most severe associated with supercells

3. majority rotate cyclonically • pressure depression at axis ( p ) up to 100 mb • centripetal accel’n • pressure gradient force • equating, (tornado’s centreline velocity adds to or subtracts from this rotational speed) K.D. Hage’s (2003) tornado climatology: since 1879, an average of about 10 tornadoes over Alberta each year (max: 26 in 1982)

5. Wall cloud below a supercell cumulonimbus in Nebraska • Supercell storm • single powerful cell • slowly rotating core aloft • (meso-cyclone) Photo by Jeremy Smith. From Houze’s Cloud Atlas, U. Washington Mesoscale Group

6. sounding near a supercell • note wind shear • saturated ascent would produce large temperature excess, ie. much “CAPE” (convectively available potential energy) • “loaded gun” sounding Td T Thomson et al, 2003; Weather & Forecasting, Vol. 18)

7. Is vortex tilting the mechanism to produce the meso-cyclone? • uncertain, speculative explanation • how would this explain preponderance of cyclonically rotating tornadoes? Ambient wind shear typically more complex than this Fig 11-25

8. wall cloud in rotating supercell updraft forms below base of main cloud • is preferred loction for tornado • funnel cloud emerges from base of wall cloud… visible due to dust & debris & condensation

10. contact with ground for 1 hour plus, starting 1500 MDT • moved due N., covered about 37 km • width varied 100 – 1300 m Edmonton tornado

11. Synoptic setting • Edmonton in trough exit region • southerly aloft (whereas NE at surface) • 250 mb • 12Z 31st • 00Z 1st • (sharpening trough)

12. Conceptual model of a “cap” • low-level upslope towards Rockies • Cb at 0600 • high Td • weak low in SW Ab Fig. 2 from Smith & Yau (1993; Monthly Weather Review, Vol. 121). Mountain-plain circulation

13. Unusually high boundary-layer humidity • afternoon dewpoints near 20oC, a near record • colliding sfc winds (convergence) Fig. 3 from Smith & Yau (1993; Monthly Weather Review, Vol. 121)

14. Forecasts: moist adiabat dry adiabat T Td 12 Z Fri July 31, 1987 • afternoon temperature 25oC • large CAPE • note capping abs. stable layer at about 750 mb

15. Edmonton sounding at (a) 12Z 31st Jul 1987, and (b) 00Z 1st Aug 1987 (from "A case study of three severe tornadic storms in Alberta, Canada," by Dupilka & Reuter)

16. 12Z Fri 31 Jul

17. 00Z Sat 1 Aug

18. rapidly developing low 12Z 31st 00Z 1st

19. from the Edmonton Sun

20. Sounding, 12Z Mon 6 Nov., 2006 • whole troposphere moist • very stable below 750 mb