TimeDependent Mountain Waves and Their Interactions with Large Scales

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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

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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.

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The Simulations

• Numerical model
• Boussinesq (compressible, nonhydrostatic)
• f-plane
• Terrain-following coordinates
• Gravity-wave absorbing upper boundary
• Parameterized subgrid-scale mixing
• Multiply nested grids

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The Nested Grids
?z 150500 m
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Initial Condition Ingredients

• Square wave in streamfunction plus a mean flow
• Doubly periodic
• ?x ?y2700 km

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Initial Streamfunction

• Mean wind 7.5 m/s
• 0 lt local u lt 15 m/s
• Period 100 hrs
• (4 days)
• Constant N
• (2/3 lt Nh/u lt ?)

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w velocity vector _at_ 5km (x-y plane)
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w velocity vector _at_ 5km (x-y plane)
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w theta turbulent mixing coefficient (x-z
cross-section)
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w theta turbulent mixing coefficient (x-z
cross-section)
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relative vorticity velocity vector at surface
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relative vorticity velocity vector at surface
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PV _at_ 295K 2.5km at t 55 hrs
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PV _at_ 295K
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PV _at_ 295K
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PV _at_ 295K, t 0
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PV _at_ 295K, t 100hrs
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SummaryNear 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)

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SummaryScale 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!