Tornado powerpoint

What’s Left to Learn About Tornadoes?

Erik Rasmussen, Rasmussen SystemsJerry Straka, OUKathy Kanak, CIMMS …and our students!

“ There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don’t know. But there are also unknown unknowns. These are things we do not know we don’t know. ” —United States Secretary of Defense Donald Rumsfeld

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convection

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convection

YOU

ARE

HER

E

tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

tornado—1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

tornado—1. I know one when I see one.

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convection

YOU A

RE H

ERE

YOU ARE HERE

A B

B

A

The Tornado Cyclone is roughly 10 times the diameter of the visible Tornado

Angular momentum M=Vr = tangential, swirling velocity times radius. Dimmitt tornado, 2 June 1995. From single-Doppler analysis. M=14000 contour

Approximate swirling windspeed and angular momentum in the Dimmitt tornado early in mature stage about 200 m above the ground.

This flow would evolve from the previous in < 5 minutes with a modest inflow of (e.g.) about 5 m/s at 500 m radius

As the high-M air penetrates closer to the axis, maximum swirling wind speed ~doubles for each halving of the radius of penetration of the large M region.

Early: In-up

Late: Down-Out

Early: ~307 K, least precip, warmest air near the tornado

Late: ~305 K, most precip,cooler

Thoughts...• Most supercells probably have

tornado cyclones*. Nomenclature isn't so important as understanding that...

• A tornado, whatever wind speed or appearance criteria being used, is the inner portion of a tornado cyclone where enough angular momentum has been transported toward the center to give tornadic windspeeds.

Thoughts...• Tornadogenesis failure is possibly

generally a failure of contraction of the tornado cyclone.

• Strength of the inner portion of the vortex (the tornado) depends partially on angular momentum in the outer portion, and the removal of mass upward through the vortex (and hence convergence below).

Thoughts...• Hence we seek to understand why most

supercells are not conducive to transporting sufficient air upward through the tornado cyclone to increase the vortex to tornado strength.

• Operationally, even if tornado cyclones are close enough to the 88D for detection, the differences between tornadic and non-tornadic TCs may almost always be ~unresolvable.

Thoughts...• Tornado life cycle appears to be related

to the changes of the secondary flow (in-up vs. down-out) in the tornado cyclone.

• Conjecture: long-lived tornadoes occur in tornado cyclones that (for reasons unknown) have a very slow transition from in-up to down-out.

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convection

YOU ARE HERE

So how did the Tornado Cyclone come into existence?

• Does the Rear-Flank Downdraft have a role?

• Here are some historically validated observations about the supercell:

• Updraft acquires horseshoe shape.• Counter-rotation is observed.• A Rear-Flank Downdraft is present in

the interior of the horseshoe pattern.• The tornado cyclone is centered in

strong vertical velocity gradient on the interior left edge of the horseshoe.

• A gust front is present below the rear edge of the updraft.

A simulation...

Horseshoe-shaped updraft

Rear-Flank Downdraft*

Counter-rotating vortices

Vortex straddles up/downdraft

Initial conditions

Mechanism 1: Arching

In a nutshell, vortex rings about the RFD are tugged upward at the leading edge in the low-level updraft, giving rise to vortex line arches and counter-rotating vortices.

See Straka et al. In the Electronic Journal of Severe Storms Meteorology, Vol 2. (EJSSM.org)

Mechanism 2: Tilting/stretching of inflow streamwise vorticity

Mechanism 3: Agglomeration of shear vortices

Real mechanism: All three acting together (others?)

RFD Genesis

• Different forcings for different parts?• One example showing evidence that

precipitation plays a role...

Rear-side precipitation...

0050 UTC

• From airborne Doppler

• 40 dBZ

North

Western tip of forward-flank precipitation

DRC

• From airborne Doppler

• Away• To

North

DRC

Western tip of forward-flank precipitation

Crowther video frame… 0050 UTC (north-northwest)

21 m/s

21 m/s

Gust front

C

A

• 500 m AGL

DRC

CA

Blobette

BWER

North

The Blob

Bobby Eddins video frames ~0053 UTC (N-W)

18 m/s

21 m/s

Gust front

C

A

• 500 m AGL

Blob

Blobette

CANorth

Bobby Eddins video ~0055 UTC (north)

Tornado Forms

Ian Wittmeyer photo ~0106 UTC (northwest)

In summary….• The descent of a DRC

is associated with…• Locally stronger

outflow;• A gust front that

surges;• Counter-rotating

vortices to the ground.

• A locally intense downdraft embedded in the RFD is the key feature, and the DRC is associated with this downdraft.

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convection

YOU ARE HERE

Role of the Mesocyclone and Supercell

• It creates the precipitation structure needed to facilitate all those neato mechanisms of Tornado Cyclone formation (heretical, hyperbolic, etc.)

• And that's all I have to say about that (F. Gump, 1994).

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convectionYOU ARE HERE

The supercell environment

• Issues from being in the field again with VORTEX2....• You don't need no Shtinkin' CAPE• You DO need upper-tropospheric storm-

relative flow• You VERY MUCH need good low-level

shear, low-level humidity, and adequate (?) low-level CAPE.

The supercell environment

• High-CAPE days• Very subjectively:• You DO need upper-trop storm-relative

flow• You VERY MUCH need good low-level

shear, low-level humidity, and adequate (?) low-level CAPE.

Finally, my latest quasi-eccentric perspective change...

• Cloud models initialized with a warm bubble and a supercell sounding love to produce Tornado Cyclones (or tornadoes if they have enough resolution)

Finally, my latest quasi-eccentric perspective change...

• Watching storms in VORTEX2, I felt that many storms would produce tornadoes “if only...”

Finally, my latest quasi-eccentric perspective change...

• Hypothesis: Most supercells would be tornadic if something did not interrupt the series of processes leading to tornado formation.

Tornado

Tornado Cyclone

RFD

Updraft

next to

Supercell

Deep shear,adequate CAPE

Convergence

,tilting of

near-ground

SRHBaroclinic

generatio

n; arching

Vortex

contractionProcessesStates

Development of rear-side

precipitation

Deep convection

30 min 20 min 5-20 min45-120 min

So if...the typical supercell needs to go through a 1.5-3

hour sequence of processes to produce a tornado, and

there are a plethora of ways to interrupt and interfere with these processes,

then warning and research emphases really should be on...identifying the physics and probabilities of these

interfering mechanisms,

not on...identifying the right most-special environments

supportive of supercell tornadoes.

Thank you!

This research is supported by you through the National Science Foundation

erik@rasmsys.comhttp://rasmsys.com