1. Time-Dependent Mountain Waves and Their Interactions with Large Scales Chen, C.-C., D. Durran and G. Hakim Department of Atmospheric Sciences University of Washington

2. The Goal  Follow mountain-wave evolution in a realistic, but simple time-varying large- scale flow. Avoid artificial initialization Capture the decaying phase of the waves  Examine feedback of the waves on the large-scale flow.

3. The Simulations  Numerical model Boussinesq (“compressible,” nonhydrostatic) f-plane Terrain-following coordinates Gravity-wave absorbing upper boundary Parameterized subgrid-scale mixing Multiply nested grids

4. The Nested Grids  x 1 =  y 1 = 27 km  x 2 =  y 2 = 9 km  x 3 =  y 3 = 3 km  z = 150~500 m half width a = 12.5 km aspect ratio  = 3

5. Initial Condition Ingredients  Square wave in streamfunction plus a mean flow  Doubly periodic  x = y =2700 km

6. Initial Streamfunction  Mean wind 7.5 m/s  0 < local u < 15 m/s  Period = 100 hrs (~4 days)  Constant N (2/3 < Nh/u <  )

7. w & velocity vector @ 5km (x-y plane)

9. w & theta & turbulent mixing coefficient (x-z cross-section)

11. relative vorticity & velocity vector at surface

13. PV @ 295K ~ 2.5km at t = 55 hrs

14. PV @ 295K

16. PV @ 295K, t = 0

17. PV @ 295K, t =100hrs

18. Summary—Near Mountain Flow  Near mountain evolution Flow-around to flow-over to flow-around Low-level blocking Lee vortex generation and shedding Wave breaking Quasi-linear waves  Acceleration phase not mirror image of deceleration phase (period is 4 days)

19. Summary—Scale Interaction  Localized dissipation creates PV dipoles in the lee of the barrier  Adiabatic interactions with the larger scale PV creates significant synoptic-scale anomalies  More analysis is on the way!