Near surface turbulence in Near surface turbulence in neutrally stratified planetary neutrally stratified planetary

boundary layerboundary layer::processes and parametrisationprocesses 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)

The surface layer (SL):The surface layer (SL):scientific issuesscientific 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

ObjectivesObjectivesRelation eddies/energetics in the SLTurbulence length scales in the SLRelation roughness & stratification/SL dynamics Simulation of the SL (large eddy simulation, LES)SL subgrid scale parametrisation

Shear-driven LES PBL flow with Shear-driven LES PBL flow with neutral stratificationneutral stratification

Variable Meso-NH LES CASES’99

z0e (cm) 3-4 4

u* (m/s) 0.42 0.4Ug (m/s) 10 8-10

zi (m) 750 750

x = y = z = 6.25 m

Z = 15 m

Drobinski et al. (2005)

/1forEz/1/1forE

z/1forE

1011ii

11

11ii

13/5

11ii

Eddy Surface Layer (ESL) Dynamical processes: shear and

blocking

Shear Surface Layer (SSL) Dynamical processes: shear

Spectra in the near-neutral SLSpectra in the near-neutral SL

z/1forEz/1forE

101133

13/5

1133

ii=1, 2

Hunt & Carlotti (2001)

ii=1, 2, 3

Kader & Yaglom (1989)

/1forEz/1/1forE

z/1forE

1011ii

11

11ii

13/5

11ii

1/z

1/z

1/z1/z

1/z

1/z

1.5 m

5 m

30 m

fE11 fE33

+1-2/3

-2/3

-2/3

+1

0

-2/3

0

-2/3

0

-2/3

0

Drobinski et al. (2004)

Méso-NH CASES’99zESL ~20 m 20 mzSSL 270 m ---

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

– 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

Origin of impinging SL eddiesOrigin of impinging SL eddies

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

SL energeticsSL energetics

3/2e

2e

2*S

2 L/zuuw

Hunt & Carlotti (2001)

u, v, w

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

SL energeticsSL energeticsu+w+, u-w-, u+w- (sweeps, splats), u-w+ (ejections,

bursts, anti-splats)

From Hunt & Carlotti (2001)’ concept towards Drobinski et al. (2004) concept

Structure of the SL: new conceptStructure of the SL: new concept

Shear induced near-surface streaks; SSL

vertical extent: 80-100 m

(Drobinski & Foster 2003)ESL vertical extent:

10-30 m(Hunt & Carlotti

2001)

SSL

blocked and sheared turbulent eddy

- ‘Cat paws’ (Hunt & Morisson 2000)- Small scale elongated plumes (Wilczak & Tillman 1980; Shaw & Businger 1985)

New layering concept in the SLNew layering concept in the SL

Eddy surface layer (ESL) ~ 10 m

Shear surface layer (SSL) ~ 100 m

Mixed layer (ML) ~ 1000 m

Free troposphere (FT)

Main process: shear and blocking

Favourable to distortion(Hunt and Carlotti 2001)

Main process: shear (Yaglom 1991)

Favourable to shear-instability induced eddies(Foster 1997; Drobinski and Foster 2003)

E11

k1km11

E33

k1km33

E11

k1km11

E33

k1km33

Isotropic turbulence

Win

d pr

ofile

Sur

face

laye

r (S

L)

1/

E11

1km11

E33

k1km331/ 1/

Upper surface layer (USL) ~ 300 m (Yaglom 1991)

Drobinski et al. (2004)

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

Impact on energy transfer processesImpact on energy transfer processes

E

Direct transfer of energy towards

dissipative scales

Hunt & Carlotti (2001)E

Energy cascade towards

dissipative scales

Kolmogorov (1941)

– SGS model suitable both for surface layer and free stream turbulence

– Impact on z0 and flux calculation

Impact on subgrid modelsImpact on subgrid models

New SGS+surface layer similarity theory (Redelsperger et al. 2001)

AK= -1/2/CK~2.8A= 3/2C~2.8L=Az=AKz

dzdU

u3ln2/U2/3U

u3ln

zzln

**0

e0

4/14/3K

*

LLu

dzdU

LCC3ln

zzln 4/3

K

4/1

0

e0

z0e~0.3z0 and u*

e~0.4u* with L=

z0e~1.15z0 and u*

e~1.01u* with L=Az

PublicationsPublicationsDubos 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