Appalachian Lee Troughs:Their role in initiating deep convection

and severe thunderstorms

Dan Thompson

ATM 504

Overview

• 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 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”

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?