Lecture 24: Thunder & Tornadoes (Ch 11)

• general statements about tornados

• soundings associated with severe thunderstorms & tornadoes

• July 1987 Edmonton F4 tornado

http://www.iassistdata.org/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

TornadoesTornadoes

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

pV

R

p

RV

2

2

1

(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 NebraskaWall 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

TTTdTd

Fig 11-25

Is vortex tilting the mechanism to produce the meso-cyclone?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 tornadoEdmonton 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 settingSynoptic setting

250 mb (a) 12Z 31st (b) 00Z 1st

(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 Td

• weak low in SW Ab

Conceptual model of a “cap”Conceptual model of a “cap”

Unusually high boundary-layer humidityUnusually high boundary-layer humidity

Fig. 3 from Smith & Yau (1993; Monthly Weather Review, Vol. 121)

• afternoon dewpoints near 20oC, a near record• colliding sfc winds (convergence)

12 Z Fri July 31, 198712 Z Fri July 31, 1987

moistmoistadiabatadiabat

drydryadiabatadiabat

TT

TTdd

Forecasts:Forecasts:

• afternoon temperature 25oC• large CAPE• note capping abs. stable layer at

about 750 mb

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)

12Z Fri 31 Jul12Z Fri 31 Jul

00Z Sat 1 Aug00Z Sat 1 Aug

• rapidly developing low

00Z 1st

12Z 31st

from the Edmonton Sunfrom the Edmonton Sun

• whole troposphere moist• very stable below 750 mb

Sounding, 12Z Mon 6 Nov., 2006Sounding, 12Z Mon 6 Nov., 2006