Lecture 24: Thunder & Tornadoes (Ch 11) general statements about tornados soundings associated with...
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![Page 1: Lecture 24: Thunder & Tornadoes (Ch 11) general statements about tornados soundings associated with severe thunderstorms & tornadoes July 1987 Edmonton.](https://reader034.fdocuments.in/reader034/viewer/2022051517/56649d545503460f94a30647/html5/thumbnails/1.jpg)
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)
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• 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
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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)
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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)
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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
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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
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• 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
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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
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• Edmonton in trough exit region• southerly aloft (whereas NE at surface)
Synoptic settingSynoptic setting
250 mb (a) 12Z 31st (b) 00Z 1st
(sharpening trough)
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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”
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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)
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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
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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)
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12Z Fri 31 Jul12Z Fri 31 Jul
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00Z Sat 1 Aug00Z Sat 1 Aug
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• rapidly developing low
00Z 1st
12Z 31st
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from the Edmonton Sunfrom the Edmonton Sun
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• whole troposphere moist• very stable below 750 mb
Sounding, 12Z Mon 6 Nov., 2006Sounding, 12Z Mon 6 Nov., 2006