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Philippe Drobinski (SA/IPSL) Contributors: P. Carlotti (CETU), J.L. Redelsperger (CNRM),

Near surface turbulence in neutrally stratified planetary boundary layer : processes and parametrisation. Philippe Drobinski (SA/IPSL) Contributors: P. Carlotti (CETU), J.L. Redelsperger (CNRM), T. Dubos (LMD/IPSL) , R.M. Banta (NOAA), R.C. Foster (UW/APL).

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Philippe Drobinski (SA/IPSL) Contributors: P. Carlotti (CETU), J.L. Redelsperger (CNRM),

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  1. Near surface turbulence in neutrally stratified planetary boundary layer:processes and parametrisation Philippe Drobinski (SA/IPSL) Contributors: P. Carlotti (CETU), J.L. Redelsperger (CNRM), T. Dubos (LMD/IPSL) , R.M. Banta (NOAA), R.C. Foster (UW/APL)

  2. The surface layer (SL):scientific issues • Turbulence: main process driving energy (momentum, sensible and latent heat) and matter transport between the surface and the planetary boundary layer (PBL) • Turbulent processes not explicitly computed in atmospheric models: subgrid scale parametrisation Objectives • Relation eddies/energetics in the SL • Turbulence length scales in the SL • Relation roughness & stratification/SL dynamics • Simulation of the SL (large eddy simulation, LES) • SL subgrid scale parametrisation

  3. Z = 15 m x = y = z = 6.25 m Shear-driven LES PBL flow with neutral stratification Drobinski et al. (2005)

  4. fE11 fE33 0 -2/3 -2/3 +1 1.5 m 0 -2/3 -2/3 +1 5 m 0 0 30 m -2/3 -2/3 Spectra in the near-neutral SL Drobinski et al. (2004) • Eddy Surface Layer (ESL) • Dynamical processes: shear and blocking • Shear Surface Layer (SSL) • Dynamical processes: shear 1/z ii=1, 2 1/z 1/z Hunt & Carlotti (2001) 1/z 1/z ii=1, 2, 3 1/z Kader & Yaglom (1989)

  5. Spectra in the near-neutral SL -1 -5/3

  6. Origin of impinging SL eddies • Near-surface eddy scales • Variances • Near surface organized eddies: first experimental evidence of streak existence in the near-neutral atmospheric SL (for LES streaks, see Drobinski & Foster, 2003) • Doppler lidar vs. LES

  7. u, v, w SL energetics • Variances u2, v2 and w2 • u2/u*2 ~ 5-6; v2/u*2 ~ 3; w2/u*2 ~ 1-2 in agreement with Panofsky (1974) • u2/u*2 and v2/u*2 decrease with height • v2/u2 and w2/u2 ~ 0.5 in agreement with LES studies (Moeng & Sullivan 1994) and observations (Nicholls and Readings 1979; Grant 1986; 1992) • Very close to the surface: • w2 ~ constant with height (Panofsky 1974 - Kansas experiment; Yaglom 1991) • w2 increases with heigth in Minessota experiments and wind-tunnel experiments (Mulhearn & Finnigan 1978) • w2 decreases with heigth in LES (Moeng & Sullivan 1994) because very subgrid-model dependent Hunt & Carlotti (2001)

  8. SL energetics u+w+, u-w-, u+w- (sweeps, splats), u-w+ (ejections, bursts, anti-splats) • Momentum transport • Sweeps events u+w- occur most of the time (35%) • Ejections have the strongest magnitude (2.5u*2) • Difficulty to distinguish between significant coherent structures of «long» duration and short sweeps and ejections embedded in these structures (Högström and Bergström 1996) • Sweeps+ejections almost entirely responsible for momentum flux and more than 75% is due to organized motion • At 1.5 m ejections=sweeps – above 10 m sweeps = 50% ejections

  9. Structure of the SL: new concept • From Hunt & Carlotti (2001)’ concept towards Drobinski et al. (2004) concept Shear induced near-surface streaks; SSL vertical extent: 80-100 m (Drobinski & Foster 2003) SSL ESL vertical extent: 10-30 m (Hunt & Carlotti 2001) blocked and sheared turbulent eddy - ‘Cat paws’ (Hunt & Morisson 2000) - Small scale elongated plumes (Wilczak & Tillman 1980; Shaw & Businger 1985)

  10. Free troposphere (FT) Wind profile E33 Mixed layer (ML) ~ 1000 m E11 Isotropic turbulence k1 km33 k1 km11 Upper surface layer (USL) ~ 300 m (Yaglom 1991) Shear surface layer (SSL) ~ 100 m E11 E33 Main process: shear (Yaglom 1991) Favourable to shear-instability induced eddies (Foster 1997; Drobinski and Foster 2003) km11 km33 1 1/ 1/ k1 Surface layer (SL) E33 E11 Eddy surface layer (ESL) ~ 10 m Main process: shear and blocking Favourable to distortion (Hunt and Carlotti 2001) k1 1/ km11 km33 k1 New layering concept in the SL Drobinski et al. (2004)

  11. E Hunt & Carlotti (2001) E Kolmogorov (1941) Direct transfer of energy towards dissipative scales Energy cascade towards dissipative scales   Impact on energy transfer processes • Anisotropy  modification of energy transfer processes from the large scales to the small scales • energy is injected at a scale L (typical eddy size within the PBL) and is transferred directly to the small scales • ‑1 power law spectral range energy deficit for a given dissipation with respect to a Kolmogorov spectrum

  12. Impact on subgrid models • SGS model suitable both for surface layer and free stream turbulence • Impact on z0 and flux calculation z0e~0.3z0 and u*e~0.4u* with L= z0e~1.15z0 and u*e~1.01u* with L=Az

  13. Publications Dubos T., Drobinski P., Carlotti P., 2005: EOF Analysis of a Neutral Surface Layer. J. Atmos. Sci., to be submitted Foster R.C., Vianey F., Drobinski P., Carlotti P., 2005: Near-Surface Sweeps and Ejections in a Neutrally-Stratified Large Eddy Simulation.Boundary-Layer Meteorol., submitted in february 2005 Drobinski P., Carlotti P., Redelsperger J.L., Newsom R.K., Banta R.M., 2005: Numerical and Experimental Investigation of the Neutral Atmospheric Surface Layer. J. Atmos. Sci., in revision Drobinski P., Carlotti P., Newsom R.K., Banta R.M., Foster R.C., Redelsperger J.L., 2004: The Structure of the Near-Neutral Atmospheric Surface Layer. J. Atmos. Sci., 61, 699-714. Carlotti P., Drobinski P., 2004: Length-Scales in Wall-Bounded High Reynolds number Turbulence. J. Fluid Mech., 516, 239-264 Drobinski P., Foster R.C., 2003: On the Origin of Near-Surface Streaks in the Neutrally-Stratified Planetary Boundary Layer. Boundary Layer Meteorol., 108, 247-256

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