Tornadogenesis: Our Current Understanding

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The structure and evolution of vortex lines in
supercell thunderstorms
Paul Markowski Yvette Richardson Pennsylvania
State University
Acknowledgments D. Dowell, R. Davies-Jones, H.
Murphey, H. Cai, E. Rasmussen, J. Straka, J.
Trapp, R. Wakimoto
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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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Evolution of vortex lines in a simulated
supercell
t 20 min
t 40 min
t 80 min
arches
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What do the vortex line arches tell us?
Markowski et al. (2008)
Straka et al. (2007)
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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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Rotunno and Klemp (1985)
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What do the vortex line arches tell us?
baroclinic mechanism (Straka et al. 2007
Markowski et al. 2008)
barotropic mechanism (Davies-Jones 2000, 2007
Markowski et al. 2003, 2008)
negative buoyancy

• vortex lines are frozen in the fluid and move as
  material lines (Helmholtz Theorem)
• counter-rotating vortices straddle downdraft

• leads to arching vortex lines and
  counter-rotating vortices
• in actual storms, both environmental and
  baroclinic vorticity are likely important

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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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photo by Jim Marquis
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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 seem to be a robust trait of
  supercell low-level mesocyclone regions
• 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 (i) baroclinic vorticity
  generation within the hook echo and associated
  rear-flank downdraft region of the supercells
  (ii) 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
• Not necessarily new dynamics being proposed, but
  rather a new way of looking at things that (i)
  suggests dynamical similarity to larger-scale
  convective systems and (ii) provides possible
  insight into why low LCLs and strong low-level
  shear is a favorable combination for tornadic
  supercells