time-dependent mountain waves and their interactions with large scales chen, c.-c., d. durran and g....
TRANSCRIPT
Time-Dependent Mountain Waves and Their Interactions with Large Scales
Chen, C.-C., D. Durran and G. Hakim
Department of Atmospheric SciencesUniversity of Washington
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.
The Simulations
Numerical model• Boussinesq (“compressible,” nonhydrostatic)• f-plane• Terrain-following coordinates• Gravity-wave absorbing upper boundary• Parameterized subgrid-scale mixing• Multiply nested grids
The Nested Grids
x1 = y1 = 27 km
x2 = y2 = 9 km
x3 = y3 = 3 km
z = 150~500 m
half width a = 12.5 km
aspect ratio = 3
Initial Condition Ingredients
Square wave in streamfunction plus a mean flow
Doubly periodic
x =y=2700 km
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 < )
w & velocity vector @ 5km (x-y plane)
w & velocity vector @ 5km (x-y plane)
w & theta & turbulent mixing coefficient (x-z cross-section)
w & theta & turbulent mixing coefficient (x-z cross-section)
relative vorticity & velocity vector at surface
relative vorticity & velocity vector at surface
PV @ 295K ~ 2.5km at t = 55 hrs
PV @ 295K
PV @ 295K
PV @ 295K, t = 0
PV @ 295K, t =100hrs
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)
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!