Federal Department of Home Affairs FDHA Federal Office of Meteorology and Climatology MeteoSwiss TKE as a measure of turbulence COSMO General Meeting, Athens, 2007 Balázs Szintai, Pirmin Kaufmann MeteoSwiss

2 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Outline Dispersion modelling at MeteoSwiss Coupling strategies PBL height determination TKE profiles

3 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann 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 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann 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 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Test Case – COSMO_diag – LPDM_diag COSMO_prog – LPDM_diag COSMO_prog – LPDM_prog Concentration TKE Unrealistically high TKE values

6 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Test Case – TKE values Small changes COSMO model LPDM Significant changes Post-diagnosing diag. prog.

7 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann 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 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann 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 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann 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 (TKE c =0.08 m 2 s -2 ) TKE method with relative threshold (TKE c =0.1*TKE max )

10 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Verification using radiosonde data Convective case: PBL height from radiosonde profiles using the bulk Richardson number method with Ri c =0.22 (Vogelenzang 1996) RiTKE_fixedTKE_relSlabObs. Vienna Lindenberg Payerne 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 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Diurnal cycle Relative TKE method gives unrealistic results at sunrise PBL height [m AGL] on (Lindenberg) Default height

12 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann TKE profiles 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 Test case: Investigation of TKE profiles in the COSMO-7 model

13 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Test Case – UTC Pot. temperature Rel. humidity Stable stratification Dry conditions in the PBL

14 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Test Case – UTC Wind direction Wind speed Notable wind shear at level 41 (approx. 500 m AGL) Unrealistic features in the wind speed, above the strong wind shear

15 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Test Case – UTC TKE Oscillation of TKE over the strong wind shear

16 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann Test Case – UTC Mixing height determination with Richardson number Ri = TKE consumption by buoyancy / TKE production by wind shear Critical value: Ri c = 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 TKE as a measure of turbulence Balázs Szintai Pirmin Kaufmann 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