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Evaluation of WRF performance for depicting orographically-induced gravity waves in the stratosphere 12 June 2007 Douglas C. Hahn Atmospheric Impacts Section Space Vehicles Directorate Hanscom AFB, MA Outline Introduction Case Study

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Evaluation of WRF performance for depicting orographically-induced gravity waves in the stratosphere

12 June 2007

Douglas C. Hahn

Atmospheric Impacts Section Space Vehicles Directorate Hanscom AFB, MA

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12 June 2007 2

Outline

  • Introduction
  • Case Study
  • Model Simulations
  • Results
  • Conclusions
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12 June 2007 3

Introduction

  • Internal Gravity (Buoyancy) Waves

– Means for transporting energy and momentum to upper atmosphere – Important in the formation of high altitude turbulence (Crooks, 1965)

  • Understanding Gravity Waves

– Boulder Windstorm, 11 January 1972 (Lilly & Zipser, 1972) – Several analytic and 2-D numerical simulations – Control of model dissipation and inclusion of an upper boundary

condition (Klemp & Lilly, 1978)

– Little effort beyond describing trapped lee waves and rotors (i.e. low

levels)

  • High Resolution Simulations using prognostic models

– Colorado Windstorm, 9 January 1989 (Clark, et al., 1994) – Intercomparison of several prognostic models by Doyle, et al. (2000) – Need for increased vertical and horizontal resolutions to capture

mountain generated gravity waves

– Applying WRF to T-Rex cases (Koch, et al., 2006)

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12 June 2007 4

Case Study

  • Field Campaign 22 November – 5 December 2004

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Observatoire de Haute Provence (OHP) , France (44º N, 5º 42’ E)

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Special Observation Period: 23-24 November 2004

  • “Light” Mistral Conditions

–

Measurements by Thermosonde (Brown, et al., 1982) and SCIDAR (Fuchs, et al., 1998)

  • Indicated turbulence occurring near 13 km around 0000 UTC 24 November

–

No convection or strong wind shear present to account for gravity wave activity present

Summary diagram depicting features of the mistral wind (from Jiang, et al., 2003)

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12 June 2007 5

Model Simulations

WRF ARW Core Version 2.1.1 (November 2005)

  • Model Set-up and Physical Parameterizations

– Air Force Weather Agency (AFWA) Joint Operational Testbed (July 2005)

  • AFWA Control version

– Horizontal: 45 km with nests of 15 and 5 km – Vertical: 42 Eta levels (model top @ 50 hPa)

  • Enhanced Resolution version

– Horizontal: 36 km with nests of 12, 4 and 1.3 km – Vertical: 82 Eta levels (model top @ 10 hPa) – Inclusion of gravity wave absorbing upper boundary condition (UBC)

  • Tested with different damping coefficients

– Tested without vertical velocity damping (w-damping)

  • Horizontal grid/nests centered on observation area (OHP)

– Runs initialized with 1º NCEP GFS data – 48 h simulation from 0000 UTC 23 November 2004

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12 June 2007 6

Model Simulations

Upper Boundary Condition

  • Gravity Wave Absorbing (Diffusion/Sponge) Layer (Skamarock, et al., 2005)

– Increase diffusion in horizontal/vertical by increasing eddy viscosities as

the top of the model is approached (Klemp & Lilly, 1978)

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − Δ Δ = 2 cos

2

π γ

d top g dh

z z z t x K ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − Δ Δ = 2 cos

2

π γ

d top g dv

z z z t z K 1 . 01 . ≤ ≤

g

γ

Horizontal: Vertical: Typically,

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12 June 2007 7

Model Simulations

  • Gravity wave absorbing layer tests for enhanced resolution

WRF-ARW simulations

– Damping layer depth (zd) constant, 5 km

  • Deeper layer would intrude on a greater part of the

domain in the stratosphere

– Damping coefficients (γg) tested for 0.01, 0.04 and 0.08

  • Horizontal examples :

γg=0.01, 0 ≤ Kdh ≤ 72000 m2 s-1 γg=0.04, 0 ≤ Kdh ≤ 288000 m2 s-1 γg=0.08, 0 ≤ Kdh ≤ 576000 m2 s-1

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12 June 2007 8

Model Simulations

Vertical Velocity Damping (w-damping)

– Improves model robustness for operational and semi-

  • perational applications

– Prevents strong updraft cores (when timesteps might be too

large)

– Decreasing timestep should allow runs without w-damping

  • Typically only horizontal grid is used to determine

timesteps (and avoid violating CFL criterion)

– WRF-ARW documentation: Δt = 6 * Δx (in km)

  • Must also recognize impacts from increased vertical

resolution

– Tested by Koch, et. al. (2006) – Smaller timesteps were chosen from beginning to test

simulations with and without w-damping

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12 June 2007 9

Results

Grid Point Verification Statistics

10 100 1000 2 4 6 8 10 12 14 16 18 Potential Temperature RMSE Pressure (hPa)

RMSE of Potential Temperature

  • 24 h Simulation vs. GFS Analysis valid 0000 UTC 24 November
  • AFWA Control ARW (blue) and Enhanced Resolution ARW (red)
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12 June 2007 10

Results

Grid Point Verification Statistics

RMSE (left) and Mean Error (right) of Total Wind

  • 24 h Simulation vs. GFS Analysis valid 0000 UTC 24 November
  • AFWA Control ARW (blue) and Enhanced Resolution ARW (red)

10 100 1000 1 2 3 4 5 6 7 8 9 Total Wind RMSE Pressure (hPa) 10 100 1000

  • 1.5
  • 1
  • 0.5

0.5 1 1.5 2 2.5 3 3.5 Total Wind Mean Error Pressure (hPa)

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12 June 2007 11

Results

Comparison with Radiosonde

Launch Time: 23 Nov 04, 2335 UTC (~24 hr forecast)

5 10 15 20 25 30 200 210 220 230 240 250 260 270 280 290 Temperature (K) Altitude (km)

Temperature and Wind Speed Profiles

  • Enhanced Resolution ARW model profiles (red) extracted from

36 km grid using balloon trajectories

Launch Time: 23 Nov 04, 2335 UTC (~24 hr forecast)

5 10 15 20 25 30 5 10 15 20 25 30 35 40 45 50 Wind Speed (m/s) Altitude (km)

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12 June 2007 12

Results

Enhanced Resolution ARW Cross Sections

  • Domain of 1.3 km inner nest of enhanced resolution ARW model (left)

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Line denotes vertical cross sections used for evaluation

–

Red dot marks location of OHP

  • Cross section from surface to 10 km (right) indicating the presence of orographically

generated gravity waves in the 24 h simulation valid 0000 UTC 24 November

–

Vertical line is location of OHP

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12 June 2007 13

Results

Enhanced Resolution ARW Cross Sections

  • Horizontal Cross Sections at 13 km for 24 h simulation valid

0000 UTC 24 November

– Dot indicates location of OHP

No UBC UBC, γg = 0.01

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12 June 2007 14

Results

Enhanced Resolution ARW Cross Sections

  • Comparison between gravity wave absorbing layer using

damping coefficients (γg) of 0.01 and 0.04

– Vertical line is location of OHP

γg = 0.01 γg = 0.04

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12 June 2007 15

Results

Enhanced Resolution ARW Cross Sections

  • Comparison between gravity wave absorbing layer using

damping coefficients (γg) of 0.04 and 0.08

– Vertical line is location of OHP

γg = 0.08 γg = 0.04

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12 June 2007 16

Conclusions

  • Mountain generated gravity (i.e. buoyancy) waves were simulated by

the enhanced resolution ARW version

– Not a particularly strong case over OHP where observations were

made

  • Unclear if increasing damping coefficient (γg) above 0.04 improves the

effectiveness of the gravity wave absorbing layer and simulated wave structure.

– Need more choices for UBC

  • Elimination of w-damping led to small differences in simulated vertical

velocity

– Continue without w-damping in order to eliminate one source of

model dissipation

  • Forecasts would be difficult to operationally implement

– Stratospheric Real-Time Turbulence Model (RTTM) (Kaplan, et al.,

2006)

– Dynamic Solution Adaptive Grid Algorithm (DSAGA) (Xiao, et al.,

2005)