Transcript RebeccaC

Ch.5 Tornadoes and Tornadic Storms
R. Davies-Jones, R.J. Trapp, H.B. Bluestein
Ch 5. Tornadoes and Tornadic
Storms
R.Davies-Jones, R.J.Trapp, H.Bluestein
Presented by Rebecca Bethke
Presented by Rebecca Bethke
Nov. 13th, 2007
Nov. 13, 2007
SUPERCELLS (review)
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Life span > 1 hour
High spatial correlation between Mesocyclone
and updraft
Categorized by position and extent of heavy rain
to updraft:
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LP, “classic” or HP
Tornadoes: Type I
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Forms within a mesocyclone ‘parent’
Contains a core, in solid-body rotation,
surrounded by a potential vortex
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Parent storm:
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Isolated supercell
 Supercell in a line of thunderstorms
 or Mini supercell with small mesocyclone
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ISOLATED SUPERCELL:
Viewed from southeast
Environmental winds veering with height
Barbed lines: boundary of cool-air outflow beneath the storm
b. Tornadic supercell:
Flow field 250 m. above ground
Dashed: negative vertical velocity
Solid: positive vertical velocity (2m/s interval)
Shaded area: radar echo
T: vertical vorticity max (tornado)
Type II.
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Not associated with mesocirculation
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Small/weak vortex forms along
stationary/slow moving windshift line
: “Landspout”
: “Gust-nado”
: Cold-air funnel
Mesocyclone observations
Looking Northeast: Tornado at Flanking Line/RFD Intersection With Wall Cloud
Mesocyclone Identification Criteria
Donaldson (1970)
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Doppler velocity shear ≥ 6 m/s/km
Differential velocity ≥ 30 m/s
At base altitude ≤ 5 km above radar
 Existing with ≥ 3 km depth
 Lasting > 5-6 minutes
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Important Observations
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Observations that indicate updraft, downdraft
and mesocyclone relationships:
3-d vertical wind and vortex existence/structure
(from Doppler radar observations)
 Mesocyclonic vertical vorticity
(computed from above info)
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Rotation Near-ground
Numerical simulations show:
 Vertical shear yet no/little vertical vorticity does
NOT create rotation near ground
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Streamwise vorticity in air flowing along the
ground cannot cause a mesocyclone to make
contact with the ground
Rotation Near-Ground
Field observations confirm:
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Tornadoes do not occur without rainy
downdrafts
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Advancing buoyant jet
Transfer of high momentum air to surface
Cool downdraft
Baroclinic mechanism
(Klemp and Rotunno 1983, Rotunno and Klemp 1985)
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Air near-ground from forward flank along strong
baroclinic zone (warm to left, cool right):
1.
2.
Developed large streamwise vorticity
Ingested into updraft and tilted spin-up
Produces elevated rotation
 Similar occurrence in RFD cyclonic outflow near-sfc
(Davies-Jones)
Completes cyclonic vortex structure from ground to
mid/higher altitudes