Upload
laurennwaggoner
View
3.060
Download
2
Tags:
Embed Size (px)
Citation preview
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
[email protected]://rasmsys.com