Observations of convection initiation failure from the 12 June 2002 IHOP deployment

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Observations of convection initiation failure
from the 12 June 2002 IHOP deployment

• Paul Markowski and Christina Hannon
• Penn State University
• Erik Rasmussen
• Cooperative Institute for Mesoscale
  Meteorological Studies

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z 100 m
50 km
50 km
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z 1.5 km
50 km
w
4 m s-1
50 km
-4 m s-1
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Towering cumulus development between 2107-2130 UTC
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What was the role of the mesoscale boundary?
w
w
1.5 km
1.5 km
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Next 2 weeks

• Complete photogrammetric cloud mapping
• More accurately relationship between cloud field
  and kinematic fields
• Infer qe dilution below LCL by comparing
  photogrammetrically determined LCL to that
  predicted by undiluted ascent based on soundings
• Hypothesis
• We cant address what processes might have been
  detrimental to CI above the boundary layer, but
  excessive qe dilution and/or excessive CIN appear
  to have contributed to CI failure
• Deepest clouds developed along the outflow
  boundary, but the role of the mesoscale boundary
  in promoting the deepest clouds does not appear
  related to mesoscale moisture upwelling nor
  enhanced vertical velocity there instead, the
  role of the boundary might only have been to
  promote trajectories that were less susceptible
  to qe dilution

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Other observations not directly related to CI
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Nice example of how the influence of gravity
waves can extend into a neutrally stratified,
convective boundary layer.
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z 100 m
z
20 km
0.012 s-1
20 km
-0.007 s-1
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Observations of vorticity extrema

• The vertical vorticity field has remarkable time
  continuity, such that many vorticity extrema
  could be tracked continuously for the entire
  deployment spanning nearly 2 h.
• Vertical vorticity extrema decrease in amplitude
  with height and are tilted by the vertical wind
  shear.
• Periods of vorticity amplification involve the
  superpositioning of an updraft the air parcels
  comprising the vorticity maxima can acquire their
  vertical vorticity from stretching or tilting,
  although the contributions can vary from one
  vorticity maximum to another, and from one
  elevation to another, making it difficult to
  generalize about the dynamical processes
  responsible for the amplification of vorticity.
• The vertical vorticity extrema are associated
  with pressure minima given that the vorticity
  extrema weaken with height, vorticity anomalies
  tend to be associated with a downward-directed
  vertical pressure gradient force.

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Observations of vorticity extrema (con)

• Most of the vertical vorticity maxima weaken
  owing to weakening convergence the demise of
  many strong vorticity maxima is brought about by
  the vortex valve effect.
• The interactions among vertical vorticity extrema
  and between vertical vorticity extrema and the
  vertical velocity field are horribly complex if
  such interactions are later shown to be crucial
  to CI, it may be difficult to develop general
  guidelines for the prediction of CI.
• Not all vorticity maxima are associated with a
  reduction in mixing the degree to which buoyancy
  dilution is inhibited, if at all, by rotation
  within an updraft depends on subtleties of the
  vorticity and velocity fields that vary from one
  vorticity maximum to another

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Observations of vorticity extrema (final remarks)

• One obvious question avoided what is the origin
  of vertical vorticity in the boundary layer?
• Many vorticity extrema persisted for the entire
  data collection period, intensifying and
  weakening depending on the superpositioning of
  boundary layer drafts, to which the vorticity
  anomalies unavoidably feed back.
• Observationally, we cannot find some hypothetical
  time when z0 everywhere.
• Is this provocative question even well-posed in
  the first place?