The Evolution of a Convective Squall Line as it Crossed the Upwind Coast of Lake Erie

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The Evolution of a Convective Squall Line as it
Crossed the Upwind Coast of Lake Erie

• Thomas E. Workoff1
• David Kristovich2
• 1Department of Atmospheric Sciences, University
  of Illinois Urbana-Champaign
• 2 Illinois State Water Survey, Institute of
  National Resource Sustainability, UIUC

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Purpose of This Study

• Liang and Fritsch (1997) showed the Great Lakes
  Region is an area of frequent deep convection
• Johns and Hirt (1987) showed that it is also an
  area of high derecho (convective windstorm)
  activity
• How does the Marine Boundary Layer (MBL) alter
  the environment in which the convection is taking
  place?
• Does storm interaction with the MBL
• Alter convective strength or structure?
• Ability to create severe weather?
• Goal To understand how the MBL associated with
  a cooler water surface alters the ambient
  environment and how it effects organized
  convection (squall lines).

Given the enormity of the Great Lakes and the
frequency of MCS (mesoscale convective system)
events in this region, it seems necessary to
investigate the impact of the Great Lakes on
existing convection Graham et al. (2005)
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KCLE 1821-2044Z

• Start by examining a case (26 July 2005) of a
  poorly understood squall line/MBL interaction
• NWS Cleveland, OH

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Characterization of the MBL
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Investigation of the MBL and Effects on Squall
Line

• Use observations (surface, RAOB) to characterize
  the MBL
• WSR-88D radar to observe changes in squall line
  structure and intensity

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Investigative Strategy
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Investigation of the MBL and Effects on Squall
Line

• Use observations (surface, RAOB) to characterize
  the MBL
• WSR-88D radar to observe changes in squall line
  structure and intensity
• Lack of resolution in observation network means
  study needed to lean on theory
• Characterize the MBL and apply to squall line
  theory to examine effect

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Rotunno, Klemp and Weisman (1988) RKW Theory

• Vorticity (?) creation
• Wind shear
• Horizontal changes in buoyancy
• RKW assumes negligible environmental buoyancy
• How does MBL alter this vorticity balance?
• 2D, Boussinesq, inviscid vorticity tendency

Adapted from Weisman (1992)
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Characterization of the MBL

• For the sake of this study, treat as an Internal
  Boundary Layer (IBL)
• IBL wind field unresolved, focus on vorticity
  generation due to IBL buoyancy changes only
• Knowing allows us to examine
  buoyancys effect on the vorticity tendency in
  the IBL

Lake Breeze Circulation was not expected (or
observed) with 9ms-1 background wind.
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Characterization of the MBL

• Smedman et al. (1997a) showed the stability
  profile of the IBL could be estimated by
• In this case, S55 over the middle of Lake Erie
  (treat as statically stable)

F? Coriolis parameter ?? reference (land)
temperature ??? temperature difference between
land/water X? distance from coast Vg? geostrophic
wind speed
where
If S gt 75, IBL can be considered statically
neutral If S lt 75, IBL can be considered
statically stable
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Characterization of the MBL

• Assume IBL has linear buoyancy profile (S55)
• Can estimate surface (minimum) buoyancy
• Garratt (1990) estimated IBL height (H) can be
  estimated by

Using obs from 1800UTC Bsfc -.143ms-2

?v and U ? atmospheric mixed layer properties Tvs
? virtual potential temperature of water
surface X? over-water fetch of advected air
Form used by Angevine et al. (2005)
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Characterization of the MBL
Storm Motion
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Characterization of the MBL
Storm Motion
is negative
is positive
is positive (over land)
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Effects of the MBL
? 305K
? 301K B -.14ms-2
? 295K B -.34ms-2
Lake Erie
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Effects of the MBL
? 307K
? 302K B -.19ms-2
? 295K B -.39ms-2
Lake Erie
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Effects of the MBL
1817UTC Storm Motion 270o _at_ 22ms-1
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Effects of the MBL
1908UTC Storm Motions 270o _at_ 23ms-1
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Conclusions

• 3D shape and profile of the MBL is generally
  unknown
• Theory (Smedman et al. 1997) indicates MBL in
  this case is a statically stable IBL
• SIBL alters the vorticity tendency of the ambient
  environment relative to the squall line
• Narrow region of () vorticity tendency upwind of
  lake
• Broad area of () vorticity tendency over lake
• This change in environmental vorticity production
  potentially alters the cold pool/environmental
  vorticity balance
• Conceptual model
• Radar observations
• Tilting of updraft
• Acceleration of cold pool (outflow boundary)
• Future work is needed