Tornadogenesis: Our Current Understanding

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Vortex lines observed within the low-level
mesocyclones of supercells and what they might
tell us about tornadogenesis
Paul Markowski Pennsylvania State University
Yvette Richardson Jerry Straka Erik
Rasmussen Bob Davies-Jones Jeff Trapp
Penn State U of Oklahoma
Rasmussen Systems NSSL
Purdue
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Why vortex lines?

• Although the vertical component of vorticity
  tends to be emphasized in supercell thunderstorm
  and tornado studies, there is some merit in
  systematically inspecting the distribution and
  orientation of three-dimensional vortex lines in
  observed and simulated storms.
• The three-dimensional perspective provided by
  vortex lines can expose dynamics that may not be
  as apparent in inspections of only one vorticity
  component at a time.

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Why vortex lines?

• The presence of horizontal buoyancy gradients can
  complicate vortex line analyses in phenomena like
  thunderstorms due to the virtually unavoidable
  baroclinic generation of vorticity by the
  horizontal buoyancy gradients that accompany the
  precipitation regions and vertical drafts of
  thunderstorms.
• In the presence of significant baroclinic
  vorticity generation, vortex lines may not even
  closely approximate material lines.
• Nonetheless, vortex line analyses still can be
  enlightening in that they can suggest plausible
  methods of vorticity generation and reorientation
  (e.g., observations of vortex rings might lead
  one to surmise that a local buoyancy extremum is
  present and responsible for the generation of the
  rings).

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How to make a tornado

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

Eric Nguyen
Wicker and Wilhelmson (1995)
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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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Data and analysis technique

• Pseudo-dual-Doppler ELDORA observations of six
  supercells observed during VORTEX (3 tornadic, 3
  nontornadic)
• 3D wind synthesis obtained via Gamache (1997)
  technique
• Vortex lines computed through the center of the
  low-level mesocyclone (the vorticity maximum at 1
  km) and nearby surrounding points

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3D wind syntheses obtained via Gamache (1997)
technique using ELDORA pseudo-dual-Doppler
observations
Markowski, P. M., J. M. Straka, E. N. Rasmussen,
R. P. Davies-Jones, Y. Richardson, and J. Trapp,
2008 Vortex lines within low-level mesocyclones
obtained from pseudo-dual-Doppler radar
observations. Mon. Wea. Rev., 136, 3513-3535.
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Markowski, P. M., J. M. Straka, E. N. Rasmussen,
R. P. Davies-Jones, Y. Richardson, and J. Trapp,
2008 Vortex lines within low-level mesocyclones
obtained from pseudo-dual-Doppler radar
observations. Mon. Wea. Rev., 136, 3513-3535.
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The fact that such vorticity couplets have been
observed in essentially every dual-Doppler and
numerical modeling study of supercells might lead
one to wonder whether vortex line arches are as
common to supercell RFD regions as are these
vorticity couplets, i.e., it is tempting to
wonder whether vortex line arches are a
ubiquitous trait of supercell thunderstorms.
Rotunno and Klemp (1985)
Wakimoto and Fujita (1981)
C
C
A
A
Brandes (1978)
Friona, TX, supercell, 2 June 1995 (DOW1)
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Vortex line arches suggest an important role for
baroclinic vorticity generation.
purely baroclinic process
purely barotropic process
Straka, J. M., E. N. Rasmussen, R. P.
Davies-Jones, and P. M. Markowski, 2007 An
observational and idealized numerical examination
of low-level counter-rotating vortices toward the
rear flank of supercells. E. J. Severe Storms
Met., 2(8), 1-22.
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max downdraft
Markowski, P. M., J. M. Straka, E. N. Rasmussen,
R. P. Davies-Jones, Y. Richardson, and J. Trapp,
2008 Vortex lines within low-level mesocyclones
obtained from pseudo-dual-Doppler radar
observations. Mon. Wea. Rev., 136, 3513-3535.
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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 bookend vortices
to near-ground mesocyclones to tornadoes?
Weisman and Davis (1998)
Fujita (1979)
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Why do tornadic supercells lack strong cold pools?

• Mobile mesonet observations from Markowski et
  al. (2002), Shabbott and Markowski (2006), Grzych
  et al. (2007), Hirth et al. (2008)
• Climatological studies show that tornadic
  supercells are favored when boundary layer
  relative humidity is large.

tornadic
nontornadic
Markowski, P. M., J. M. Straka, and E. N.
Rasmussen, 2002 Direct surface thermodynamic
observations within the rear-flank downdrafts of
nontornadic and tornadic supercells. Mon. Wea.
Rev., 130, 1692-1721.
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Why do tornadic supercells lack strong cold pools?

• Perhaps supercell baroclinity is another
  Goldilocks problem whereby at least some
  baroclinity is crucial (all thunderstorms have at
  least some baroclinity), but too much, especially
  near the ground, is detrimental in that large
  near-ground baroclinity would imply very cold air
  near the ground and thus rapid gust front motion
  relative to the main updraft, which might
  undercut it (Brooks et al. 2003) or inhibit the
  vorticity stretching required by tornadogenesis
  (Leslie and Smith 1978 Markowski et al. 2003).
• If the downdraft air containing the vortex rings
  is too negatively buoyant, then perhaps the end
  result is something resembling Fujita's
  microburst model rather than significant lifting
  of the leading edge of the vortex rings to
  produce vertical vorticity.

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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, M., P. Markowski, Y. Richardson, and J.
Wurman, 2006 A dual-Doppler analysis of a
nontornadic supercell observed on 12 June 2004
using ground-based mobile radars. Preprints, 23rd
Conf. on Severe Local Storms, St. Louis, MO,
Amer. Meteor. Soc.
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strong shear
Tornadic storms likely
weak shear
low RH
high RH
Tornadic storms unlikely
courtesy of Harold Brooks
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Why is the combination of large low-level shear
and high boundary layer RH so favorable for
tornadoes?

• Strong low-level shear promotes stronger
  low-level dynamic lifting of baroclinic vortex
  lines?
• Low LCLs promote weaker cold pools?

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Summary

• Vortex line arches are a robust trait of the
  sample of supercells studied herein
• The arching of the vortex lines and the
  orientation of the vorticity vector along the
  vortex line arches, compared to the orientation
  of the ambient (barotropic) vorticity, are
  strongly suggestive of
• baroclinic vorticity generation within the hook
  echo and associated rear-flank downdraft region
  of the supercells
• subsequent lifting of the baroclinically altered
  vortex lines by an updraft, rather than ambient
  vortex lines alone being tilted by either an
  updraft or downdraft to produce a low-level
  vertical vorticity maximum