Tornadogenesis: Our Current Understanding

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Tornadogenesis Our current understanding and
operational considerations
Paul Markowski Pennsylvania State University
Special thanks to Yvette Richardson, Josh Wurman,
Erik Rasmussen, Jerry Straka, Zack Byko, Jeff
Frame, Mario Majcen, Jim Marquis
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What we know

• Supercells acquire net cyclonic rotation aloft by
  tilting streamwise (horizontal) vorticity

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What we know

• Supercells acquire net cyclonic rotation aloft by
  tilting streamwise (horizontal) vorticity
• Although most significant tornadoes are
  associated with supercell thunderstorms, most
  supercells are not tornadic
• What is troubling is that most supercells contain
  low-level mesocyclones, and perhaps most
  supercells even have circulations that extend to
  the surface.

nontornadic
nontornadic
nontornadic
tornadic
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The most intense mesocyclones are not necessarily
the ones most likely to be associated with
tornadogenesis.
NONTORNADIC
TORNADIC
Superior, NE Mesocyclone 22 June 2003 025200 -
030059 UTC
TORNADIC
Wakimoto et al. (2004)
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What we know

• Supercells acquire net cyclonic rotation aloft by
  tilting streamwise (horizontal) vorticity
• Although most significant tornadoes are
  associated with supercell thunderstorms, most
  supercells are not tornadic

• Tornadogenesis involves rearranging, twisting,
  and stretching vortex lines so that they become
  vertically oriented and packed tightly together
  at the ground

Wicker and Wilhelmson (1995)
Matt Biddle
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How to make a tornado
pre-existing vertical vorticity at the surface
vertical vorticity is initially negligible at the
surface
zeroth order assumption no baroclinity or
viscous effects vortex lines are material lines
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Roberto Giudici
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NSSL archive photo
Courtesy of Dave Blanchard
Courtesy of Dave Blanchard
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What do the vortex lines actually look like? In
a real storm, horizontal density gradients and
baroclinic vorticity generation are unavoidable.
The configuration of the vortex lines tells us
something about the way tornadoes form.
idealized case no baroclinic vorticity
generation vortex lines are frozen in the fluid
vortex lines passing through low-level vorticity
maximum form Us.
observed case vortex lines passing through
low-level vorticity maximum form arches
Markowski et al. (2008)
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purely baroclinic process
purely barotropic process
negative buoyancy
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Markowski et al. (2008)
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squall line case simulation
Is it possible that the same fundamental
dynamical process (baroclinic vortex lines
generated in a cool downdraft and subsequently
lifted by an updraft) can produce vortices that
range in size and intensity from the mesoscale
vortices observed at the ends of convective
systems, to near-ground mesocyclones and even
perhaps tornadoes?
Weisman and Davis (1998)
Fujita (1979)
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What we know

• Supercells acquire net cyclonic rotation aloft by
  tilting streamwise (horizontal) vorticity
• Although most significant tornadoes are
  associated with supercell thunderstorms, most
  supercells are not tornadic

• Tornadogenesis involves rearranging, twisting,
  and stretching vortex lines so that they become
  vertically oriented and packed tightly together
  at the ground

• Tornadogenesis, if it occurs, is associated with
  the development of the rear-flank downdraft (RFD)

at least in environments containing negligible
preexisting vertical vorticity at the
surface
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simulations
Trajectories in both observed tornadoes and
simulated tornadoes can be traced backward
through the RFD.
observations
Wicker and Wilhelmson (1995)
Brandes (1978)
Xue (2004)
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Dual-Doppler analysis showing tornado completely
encircled by downdraft air.
gust fronts
tornado
Marquis et al. (2008)
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Paul Markowski
Jim Marquis
Jeff Beck
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What we know

• Supercells acquire net cyclonic rotation aloft by
  tilting streamwise (horizontal) vorticity
• Although most significant tornadoes are
  associated with supercell thunderstorms, most
  supercells are not tornadic

• Tornadogenesis involves rearranging, twisting,
  and stretching vortex lines so that they become
  vertically oriented and packed tightly together
  at the ground

• Tornadogenesis, if it occurs, is associated with
  the development of the rear-flank downdraft (RFD)
• The temperature of the RFD seems to be important
  to tornadogenesis RFDs that are excessively cold
  apparently are unfavorable for tornadogenesis

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If the vortex line analyses suggest that
baroclinic generation of vorticity is important,
than why do buoyancy observations suggest that
weak baroclinity is more favorable for
tornadogenesis?
Possible answer just because baroclinity might
be important doesnt mean that we necessarily
want baroclinity to be as strong as possible
(i.e., all storms have at least some
baroclinitythe best tornado candidate storms
appear to have weaker cold pools and therefore
less baroclinity near the ground.
Adapted from Markowski et al. (2002)
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What might the vortex line evolution look like in
the case of excessively cold downdraft air?
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dual-Doppler analysis of a nontornadic supercell
on 12 June 2004 near Beatrice, NE
view from southwest
3 km
3 km
Majcen et al. (2006)
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What we know

• Best operational means for discriminating between
  significantly tornadic (i.e., F2 and stronger)
  and nontornadic supercells is to use radar data
  in conjunction with information about the
  near-storm environment
• low-level shear (e.g., 0-1 km shear vector
  magnitude, 0-1 km SRH)
• low-level relative humidity (e.g., cloud base
  height)

Strong low-level shear promotes stronger
low-level dynamic lifting of baroclinic vortex
lines?
High relative humidity promotes weaker cold pools?
Basic idea baroclinically augmented vorticity
(which resides in the outflow) is most likely to
be ingested by an updraft if the outflow is not
too negatively buoyant and if the updraft can
forcibly lift air that is perhaps a little
negatively buoyancy
Nontornadic supercell environments and F0-F1
tornadic supercell environments are
indistinguishable for practical purposes.
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Tornadic storms likely
Tornadic storms unlikely
courtesy of Harold Brooks
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Mesoscale boundaries

• Large-scale conditions occasionally support
  significant tornadoes (outbreak days), but more
  commonly, significant tornadoes are only favored
  in relatively small regions where low-level shear
  and/or moisture is locally enhanced

2 June 1995

• Mesoscale boundaries often enhance low-level
  shear and/or moisture, and many supercells have
  been observed to become tornadic after
  interacting with such boundaries
• But many supercells weaken upon encountering
  mesoscale boundariesclearly not all boundaries
  are favorable