Talk Outline

							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
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 enormity of the Great Lakes and the frequency of MCS
“Given thedoes the Marine Boundary Layer (MBL) alter
(mesoscale convective system) events in this region, it seems necessary
    the environment in which the convection is taking
to investigate the impact of the Great Lakes on existing convection”
    place?
Graham et al. (2005)
 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).
       KCLE 18:21-20:44Z
  Start by examining a
case (26 July 2005) of a
poorly understood squall
line/MBL interaction
    NWS Cleveland, OH
                    Characterization of the
                    MBL

                                       Temperature (C)

           35

                                                                               KCLE
           30
Temp (C)




                                                                               KTOL
                                                                               45005

           25                                                                  45132
                                                                               CWAJ

           20
                8   9   10   11   12   13   14   15   16   17   18   19   20
                                        Time (UTC)
     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
Investigative Strategy




             θ≈305K
     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
              Rotunno, Klemp and Weisman
              (1988)
              RKW Theory
           Vorticity (η) creation:
            – Wind shear
            – Horizontal changes in
              buoyancy
           RKW assumes negligible
            environmental buoyancy
           How does MBL alter this
            vorticity balance?
           2D, Boussinesq, inviscid
            vorticity tendency:
        d            B                 u       w
                        where            
    o   dt  o        x                 z       x
                                                       Adapted from Weisman (1992)
          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
                          d     B
             Knowing dt   x allows us to examine
                             
                               o   o


              buoyancy’s effect on the vorticity tendency
              in the IBL:
                         R Z                R Z
                     
                    t
                          dxdz          (  B ) dz
                         L 0                L 0

*Lake Breeze Circulation was not expected (or observed) with ≈9ms-1 background wind.
  Characterization of the
  MBL
     Smedman et al. (1997a) showed the
      stability profile of the IBL could be
      estimated by:                 F Coriolis parameter
                                  1                      θ reference (land) temperature
                                                 X    Δθ temperature difference
S t
       1/ 2
              f
                  1/ 2
                                     where   t         between land/water
                                                  Vg   X distance from coast
                                                          Vg geostrophic wind speed
If S > 75, IBL can be considered statically neutral
If S < 75, IBL can be considered statically stable
     In this case, S≈55 over the middle of
      Lake Erie (treat as statically stable)
Characterization of the
MBL
   Assume IBL has linear buoyancy profile (S≈55)
                          R
                          Z Z                              R Z
              
    R Z                                                               Z

                min (1 h Z  L  B
      dxdz  t  B dxdz  / )dz(  B )dz min (1  h Z ) dz R
t L 0            L
                  0 0             L 0         0

   Can estimate surface (minimum) buoyancy:
       '                                                          Using obs from 1800UTC:
B  g   . 61 ( q v  q v )  q c  q r 
                                                                  Bsfc ≈-.143ms-2
   Garratt (1990) estimated IBL height (H) can be
    estimated by:
            g  v   vs  
                                 1 / 2
                                                *
h  . 02 U                              x
                                              1/ 2   θv and U  atmospheric mixed layer properties
                 v                                Θvs  virtual potential temperature of water surface
                            
                                                     X over-water fetch of advected air
*Form used by Angevine et al. (2005)
Characterization of the
MBL


 Storm Motion
Characterization of the
MBL


Storm Motion




                     
                     t
                          is negative
                     
                     t
                          is positive
                     
                          is positive
                     t
                          (over land)
        Effects of the MBL




               θ ≈ 305K


θ ≈ 295K                  θ ≈ 301K
B ≈ -.34ms-2              B ≈ -.14ms-2
                               Lake Erie
 Effects of the MBL




                                  θ ≈ 307K
                   θ ≈ 302K
θ ≈ 295K
                   B ≈ -.19ms-2
B ≈ -.39ms-2
               Lake Erie
         Effects of the MBL
18:17UTC
Storm Motion: 270o @ 22ms-1
         Effects of the MBL
19:08UTC
Storm Motions: 270o @ 23ms-1
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