Transcript Appalachian lee troughs, deep convection and severe

Appalachian Lee Troughs:
Their role in initiating deep convection
and severe thunderstorms
Dan Thompson
ATM 504
Overview
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Importance
Formation
Case study
Future work
Seaman and Michelson 2000
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
L
Peckham and Wicker 2000
(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
Blue = Wind
Black = Hail > 2”
Green = Hail
Mesoscale surface analysis 1900 UTC
Case Study: July 16, 2009
2.5” hail McKownville, NY
Schenectady, NY
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
Thank you
• Questions?