Appalachian Lee Troughs: Their role in initiating deep convection and severe thunderstorms Dan...
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Transcript of Appalachian Lee Troughs: Their role in initiating deep convection and severe thunderstorms Dan...
Appalachian Lee Troughs:Their role in initiating deep convection
and severe thunderstorms
Dan Thompson
ATM 504
Why are lee troughs important?• Sources of low-level
vorticity, convergence and moisture– Force convection– Important when
synoptic flow is weak
• Can cause severe thunderstorms
• Storms can affect densely populated areas– Transportation impacts
http://rst.gsfc.nasa.gov/Sect6/Sect6_3.html
Lee Trough Formation
• Background: Potential Vorticity – PV = g(ζθ + f)(-∂θ/∂p) (in isentropic coordinates)
• Product of absolute vorticity and static stability
– d/dt (PV) = 0• PV is conserved for frictionless, adiabatic flows
Lee Trough Formation: PV Perspective
• PV = g(ζθ + f)(-∂θ/∂p)
• Need component of flow normal to mountain barrier• Flow crossing mountain barrier will subside on lee side
– Advects higher θ downward → warming
– -∂θ/∂p decreases → ζθ must increase → low level circulation
Martin 2006
Lee Trough Formation
• Coincide with thermal ridges– Between cooler
mountains, ocean– Adiabatic warming– Differential heating over
sloping terrain → “Heat Low” effect
– Can cause surface to reach convective temperature Peckham and Wicker 2000
L
(Shapes added by presenter)
Lee Trough Formation• Benjamin 1986: 2-D
model simulations– 2000 km-wide plateau– u = u(z); v=0– Flat heated land: Heat
Low effect– Non-heated plateau:
PV conservation effect– Heated plateau: both
effects
• Sum of effects not enough to explain depth of trough
Lee Trough Formation
• Missing ingredient: horizontal gradient of vertical mixing– u increases with height– Mixing higher over
plateau than further east
– Vertical mixing above plateau entrains lower momentum
– Divergence increased above lee of plateau
Benjamin 1986
Case Study: July 16, 2009• SPC placed severe
threat too far south
Mesoscale surface analysis 1900 UTCBlue = Wind Green = Hail
Black = Hail > 2”
Future Work
• Examine satellite/radar data to identify lee trough days that produced organized convection
• Classify by synoptic flow regime– Composite and case study analyses
• Develop conceptual model– Provide guidance to forecasters