Ch 5. Tornadoes and Tornadic Storms

R.Davies-Jones, R.J.Trapp, H.Bluestein

Presented by Rebecca Bethke

Nov. 13th, 2007

Ch.5 Tornadoes and Tornadic StormsR. Davies-Jones, R.J. Trapp, H.B.

Bluestein

Presented by Rebecca Bethke

Nov. 13, 2007

SUPERCELLS (review)SUPERCELLS (review)

Life span > 1 hourLife span > 1 hour High spatial correlation between High spatial correlation between

Mesocyclone and updraftMesocyclone and updraft

Categorized by position and extent Categorized by position and extent of heavy rain to updraft: of heavy rain to updraft: LP, “classic” or HPLP, “classic” or HP

Tornadoes:Tornadoes: Type I Type I

Forms within a mesocyclone ‘parent’Forms within a mesocyclone ‘parent’ Contains a core, in solid-body rotation, Contains a core, in solid-body rotation,

surrounded by a potential vortexsurrounded by a potential vortex

Parent storm: Parent storm: Isolated supercellIsolated supercell Supercell in a line of thunderstormsSupercell in a line of thunderstorms or Mini supercell with small mesocycloneor Mini supercell with small mesocyclone

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 velocitySolid: positive vertical velocity (2m/s interval)Shaded area: radar echo

T: vertical vorticity max (tornado)

Type II. Type II.

Not associated with mesocirculationNot associated with mesocirculation

Small/weak vortex forms along Small/weak vortex forms along stationary/slow moving windshift stationary/slow moving windshift lineline

: “Landspout”: “Landspout”

: “Gust-nado”: “Gust-nado”

: Cold-air funnel: Cold-air funnel

Mesocyclone Mesocyclone observationsobservations

Looking Northeast: Tornado at Flanking Line/RFD Intersection With Wall Cloud

Mesocyclone Identification Mesocyclone Identification CriteriaCriteria

Donaldson (1970)Donaldson (1970) Doppler velocity shear ≥ 6 m/s/kmDoppler velocity shear ≥ 6 m/s/km Differential velocity ≥ 30 m/sDifferential velocity ≥ 30 m/s

At base altitude ≤ 5 km above radar At base altitude ≤ 5 km above radar Existing with ≥ 3 km depthExisting with ≥ 3 km depth Lasting > 5-6 minutesLasting > 5-6 minutes

Important ObservationsImportant Observations

Observations that indicate updraft, Observations that indicate updraft, downdraft and mesocyclone downdraft and mesocyclone relationships:relationships:

3-d vertical wind and vortex 3-d vertical wind and vortex existence/structure (from Doppler existence/structure (from Doppler radar observations)radar observations)

Mesocyclonic vertical vorticity Mesocyclonic vertical vorticity

(computed from above info)(computed from above info)

Rotation Near-groundRotation Near-ground

Numerical simulations show:Numerical simulations show: Vertical shear yet no/little vertical Vertical shear yet no/little vertical

vorticity does NOT create rotation vorticity does NOT create rotation near groundnear ground

Streamwise vorticity in air flowing Streamwise vorticity in air flowing along the ground along the ground cannot cause a cannot cause a mesocyclone to make contact with mesocyclone to make contact with the groundthe ground

Rotation Near-GroundRotation Near-Ground

Field observations confirm:Field observations confirm:

Tornadoes do not occur without Tornadoes do not occur without rainy downdrafts rainy downdrafts

1.1. Advancing buoyant jetAdvancing buoyant jet

2.2. Transfer of high momentum air to Transfer of high momentum air to surfacesurface

3.3. Cool downdraftCool downdraft

Baroclinic mechanismBaroclinic mechanism(Klemp and Rotunno 1983, Rotunno and (Klemp and Rotunno 1983, Rotunno and

Klemp 1985)Klemp 1985) Air near-ground from forward flank along Air near-ground from forward flank along

strong baroclinic zone (warm to left, cool strong baroclinic zone (warm to left, cool right):right):

1.1. Developed large streamwise vorticityDeveloped large streamwise vorticity2.2. Ingested into updraft and tilted spin-upIngested into updraft and tilted spin-up

Produces elevated rotationProduces elevated rotation Similar occurrence in RFD cyclonic outflow Similar occurrence in RFD cyclonic outflow

near-sfcnear-sfc((Davies-Jones)Davies-Jones)

Completes cyclonic vortex structure from Completes cyclonic vortex structure from ground to mid/higher altitudesground to mid/higher altitudes