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

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 Tornadoes may occur with any type of severe weather system: frontal boundaries, squall lines, MCC’s; most severe associated with supercells

K.D. Hage’s (2003) tornado climatology: since 1879, an average of about 10 tornadoes over Alberta each year (max: 26 in 1982) 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)

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

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

Fig 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

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

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

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

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

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

12 Z Fri July 31, 1987 moistadiabat dryadiabat T TdTdTdTdForecasts: afternoon temperature 25 o C large CAPE note capping abs. stable layer at about 750 mb

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

12Z Fri 31 Jul

00Z Sat 1 Aug

rapidly developing low 00Z 1 st 12Z 31 st

from the Edmonton Sun

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