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TKE as a measure of turbulence

TKE as a measure of turbulence. Balázs Szintai, Pirmin Kaufmann MeteoSwiss. COSMO General Meeting, Athens, 2007. Outline. Dispersion modelling at MeteoSwiss Coupling strategies PBL height determination TKE profiles. Dispersion modelling at MeteoSwiss.

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TKE as a measure of turbulence

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  1. TKE as a measure of turbulence Balázs Szintai, Pirmin Kaufmann MeteoSwiss COSMO General Meeting, Athens, 2007

  2. Outline • Dispersion modelling at MeteoSwiss • Coupling strategies • PBL height determination • TKE profiles

  3. Dispersion modelling at MeteoSwiss • Lagrangian Particle Dispersion Model (LPDM) of DWD • Emergency concentration calculations • Input: Operational COSMO output • Output: Concentration fields • Off-line coupling: dispersion model is run after the COSMO integration • Turbulence fields: • Post-diagnose from mean meteorological fields • Direct usage of TKE

  4. Coupling • Until December 2005: • COSMO runs with diagnostic TKE (Mellor&Yamada level 2) • Post-diagnosed by LPDM • From December 2005 until February 2007 • COSMO runs with prognostic TKE (Mellor&Yamada level 2.5) • Post-diagnosed by LPDM • From February 2007 • COSMO runs with prognostic TKE (Mellor&Yamada level 2.5) • Direct usage of TKE by LPDM Highly different concentration results

  5. Test Case – 2006-10-23 COSMO_diag – LPDM_diag COSMO_prog – LPDM_diag COSMO_prog – LPDM_prog Concentration Unrealistically high TKE values TKE

  6. Test Case – TKE values COSMO model LPDM Small changes diag. Post-diagnosing Significant changes prog. Post-diagnosing

  7. Coupling - Consequences • The mean meteorological fields of the COSMO model are dependent on the turbulence closure used, affecting derived fields more than expected • For the reconstruction of the turbulence fields, always the same type of closure should be used in the dispersion model as in the NWP model

  8. Turbulence in dispersion models • Drawback of coupled NWP – Dispersion model systems: • Turbulence parametrizations in the NWP model are “tuned” with an aim to get better meteorological fields • Turbulence fields of NWP models are not verified From the viewpoint of dispersion modelling a verification and inter-comparison of the different coupling strategies would be desirable

  9. Plans at MeteoSwiss • Inter-comparison of closure-type coupling with similarity theory approaches • Calculation of boundary layer height is inevitable • Methods applied to model outputs: • Gradient Richardson number (B. Fay) • Slab model (Batchvarova and Gryning 1991) • TKE method with fixed threshold (TKEc=0.08 m2s-2) • TKE method with relative threshold (TKEc=0.1*TKEmax)

  10. Verification using radiosonde data • Convective case: 2006-07-18 • PBL height from radiosonde profiles using the bulk Richardson number method with Ric=0.22 (Vogelenzang 1996) PBL heights [m AGL] • Over highly convective grid points the slab model is closest to observations • In case of shallow PBL the relative TKE method gives the best result

  11. Diurnal cycle PBL height [m AGL] on 2006-07-18 (Lindenberg) • Relative TKE method gives unrealistic results at sunrise Defaultheight

  12. Strong cyclonic activity Investigated grid point over the Swiss Plateau During PBL height verifications, the Richardson number method turned out to be very sensitive to TKE oscillations TKE profiles Test case: 2006-10-23 • Investigation of TKE profiles in the COSMO-7 model

  13. Test Case – 2006-10-23 12 UTC Pot. temperature Rel. humidity Dry conditions in the PBL Stable stratification

  14. Test Case – 2006-10-23 12 UTC Wind direction Wind speed Unrealistic features in the wind speed, above the strong wind shear Notable wind shear at level 41 (approx. 500 m AGL)

  15. Test Case – 2006-10-23 12 UTC TKE Oscillation of TKE over the strong wind shear

  16. Test Case – 2006-10-23 12 UTC Mixing height determination with Richardson number • Ri = TKE consumption by buoyancy / TKE production by wind shear • Critical value: Ric = 0.38 • Mixing height: Ri exceeds the critical value Ri Due to the unrealistic oscillation of TKE problems could arise by the determination of the mixing height

  17. Conclusions • The mean meteorological fields, depending on the turbulence closure used, affect derived fields in an unexpected way • Unrealistic oscillations occur in TKE and Ri • To use either method, TKE or Ri, for PBL height determination, the model output needs to be smoothed vertically, diminishing the effective resolution

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