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COSMO General Meeting 7 September 200 9 , Offenbach, Germany

Analyzing the TKE budget of the COSMO model for the LITFASS-2003 convective case: comparison with turbulence measurements and LES data. Balázs Szintai MeteoSwiss Daniel Nadeau EPFL, Switzerland. COSMO General Meeting 7 September 200 9 , Offenbach, Germany. Outline. Goal of the work

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COSMO General Meeting 7 September 200 9 , Offenbach, Germany

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  1. Analyzing the TKE budget of the COSMO model for the LITFASS-2003 convective case: comparison with turbulence measurements and LES data Balázs Szintai MeteoSwiss Daniel Nadeau EPFL, Switzerland COSMO General Meeting 7September 2009, Offenbach, Germany

  2. Outline • Goal of the work • LITFASS-2003 campaign • Large Eddy Simulation • COSMO-3D simulations • Mean variables (temperature, wind) • TKE • TKE budget terms • Summary and Outlook

  3. Goal of the work • Evaluate the current one-equation turbulence scheme of the COSMO model • Investigation of both unstable and stable cases • Comparison with measurements and LES data • Eventually further tuning

  4. LITFASS-2003 • Measurement campaign conducted in the area of Lindenberg • Main goal: measurement of turbulent fluxes in an inhomogeneous terrain • Case study: 2003-05-30 : convective day, calm winds, no clouds • Simulation of the whole diurnal cycle • Measurementsused: • Radiosoundings • Micrometeorological stations  all land cover types • Tower turbulence measurements (at 50 and 90 m) 20 km 20 km

  5. Large Eddy Simulation • Large Eddy Simulation runs were performed at the Environmental Fluid Mechanics Laboratory (EFLUM) at EPFL, Switzerland • Model was developed at the Johs Hopkins University and was used several times previously to simulate the atmospheric boundary layer • LES details: • Solves the filtered incompressible N-S equations • Lagrangian scale-dependent dynamic subgrid scale model • Spectral representation in the horizontal directions • Finite differences in the vertical direction • Periodic lateral boundary conditions • Stress-free lid as a top BC for unstable conditions • Surface temperature is imposed • Heat exchange using MOS

  6. LES – ideal cases Neutral Stable Unstable Velocity variances Velocity spectra Daniel Nadeau, EPFL

  7. LES - LITFASS-2003 case • Domain size • 6 x 6 km horizontal (Δx = Δy = 94 m) • 3 km vertical (Δz = 47 m) • Time step: 0.2 s • 3 hours of simulation from 9 to 12 UTC • Geostrophic forcing calculated from COSMO-7 analysis (constant in time, vertically changing) • Changing lower BC from measurements for each land cover type • 10 minute averaging in output • No space averaging • Work in progress, preliminary results

  8. COSMO-3D at 1 km COSMO-1 COSMO-7

  9. COSMO-3D runs • Horizontal resolution: 0.0089° ~ 1 km • Timestep : 10 s • Operational external parameters (soil type) are not accurate in the area  improved dataset from Felix Ament is used • Soil analysis was produced with a one-month TERRA standalone run forced with COSMO-7 analyses • Investigated: • Numerical horiz. diffusion • Implicit TKE diffusion • Finer vertical resolution • In the following the control run is presented: • no horizontal diffusion • explicit TKE diffusion • oper. 60 levels

  10. Wind COSMO LES Wind speed [m/s] Wind dir. [deg]

  11. Wind profiles at 12 UTC Wind direction Wind speed • Wind direction is simulated accurately • Wind speed is overestimated by LES • Problem in LES above the PBL – wave pattern

  12. Potential temperature LES COSMO • COSMO simulates a too warm and too shallow PBL • LES background stratification is too stable 12 UTC

  13. TKE LES COSMO • COSMO gives higher values than LES in the lower half of the PBL • Height of TKE maximum is too high in LES • Double peak in the profiles of horizontal velocity variances in LES (not shown) 12 UTC

  14. Component testing Prognostic equation for TKE: • Budget terms: • I. : Local tendency • II. : Buoyancy production term • III. : Shear production term • IV. : Turbulent transport of TKE and pressure correlation term • V. : Dissipation I. II. III. IV. V.

  15. TKE budget terms I. LES COSMO Buoyancy Shear

  16. TKE budget terms II. LES COSMO Transport Dissipation Budget imbalance

  17. TKE budget - measurements • Average values computed from two sonic anemometers at 50 and 90 m • Turbulent transport: triple correlations (M. Mauder, Research Centre Karlsruhe) • Dissipation: turbulence spectrum (D. Michel, University Basel) • Positive values of turbulent transport in COSMO • Large negative imbalance in measurements Buoyancy Transport Shear TKE Dissipation Budget imbalance

  18. Summary and Outlook • The diurnal cycle of a dry convective case has been simulated with the COSMO model at 1 km resolution • TKE budget terms have been compared to preliminary LES data and measurements • The shear production term shows vertical oscillations (in stable regimes) • The transport term near the surface have different sign as compared to measurements and LES • The TKE budget from turbulence measurements shows a large negative imbalance • Outlook: • More reliable LES data • Stable case: GABLS-3

  19. Thank you for your attention!

  20. Reserve slides

  21. TERRA standalone run • Upper levels of the produced soil analysis are colder and moister than measurements

  22. COSMO-3D: Mean Vertical Velocity 2003-05-30 12 UTC, 800 m AGL Without horiz. Diffusion CTRL-60 With horiz. Diffusion HDIFF-60 300 km 100 km

  23. COSMO-3D: Turbulent Kinetic Energy2003-05-30 12 UTC, 800 m AGL Without horiz. Diffusion CTRL-60 With horiz. Diffusion HDIFF-60 • Without horizontal diffusion the results are not very realistic   wavelength is on the order of the mesh size • Horizontal diffusion reduces the amplitude of waves and increases the wavelength • Cellular updrafts on areas with low winds vanish with horizontal diffusion

  24. Validation of COSMO-3D – 10 m wind CTRL-60 IMPL-60 -3D -3D [m/s] [m/s] • Too strong wind in analysis – vanishes after 2 hours • Small maximum in the morning hours not present in measurement HDIFF-60 -3D IMPL-74 [m/s] -3D [m/s]

  25. Validation of COSMO-3D – Surface sens. heat flux -3D -3D • No significant differences between the experiments • Surface sensible heat flux is overestimated during daytime • Surface latent heat flux is underestimated, large horizontal variability (not shown) CTRL-60 IMPL-60 -3D -3D HDIFF-60 IMPL-74

  26. Validation of COSMO-3D – Temperature and humidity profiles 2003-05-30 12 UTC • 60 level runs are very similar  to diffusive in the entrainment zone • 74 levels bring improvement in the PBL CTRL-60 IMPL-60 Specific hum. [kg/kg] Potential temp. [K] Potential temp. [K] Specific hum. [kg/kg] HDIFF-60 IMPL-74

  27. Validation of COSMO-3D – TKE timeseries at 90 m CTRL-60 IMPL-60 • Experiments give similar results • Maximum of TKE is earlier in the model • Night time TKE is overestimated by the model • Fairly accurate results for TKE with „wrong” surface heat flux? IMPL-74 HDIFF-60

  28. COSMO-3D: TKE budget terms • Similar results to the idealized convective case • Turbulent transport of TKE is too weak • Negative buoyancy flux at PBL top is missing • Higher vertical resolution in the entrainment zone and implicit formulation are improving the results IMPL-60 CTRL-60 IMPL-74 HDIFF-60

  29. LES – TKE budget profile

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