1. Appalachian Lee Troughs: Their role in initiating deep convection and severe thunderstorms Dan Thompson ATM 504

2. Overview Importance Formation Case study Future work Seaman and Michelson 2000

3. 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

4. Lee Trough Formation Background: Potential Vorticity –PV = g(ζ θ + f)(-∂θ/∂p) (in isentropic coordinates) Product of absolute vorticity and static stability –d/dt (PV) = 0 PV is conserved for frictionless, adiabatic flows

5. Lee Trough Formation: PV Perspective PV = g(ζ θ + 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 → ζ θ must increase → low level circulation Martin 2006

6. 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)

7. 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

8. 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

9. Case Study: July 16, 2009 SPC placed severe threat too far south Mesoscale surface analysis 1900 UTC Blue = Wind Green = Hail Black = Hail > 2”

10. Case Study: July 16, 2009 2.5” hail McKownville, NY Schenectady, NY

11. 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

12. Thank you Questions?