Mesoscale Modeling with a 3D Turbulence SchemeJocelyn Mailhot and Yufei Zhu(Claude Pelletier)

Environment Canada

MSC / MRB

3rd Annual Meeting on CRTI Urban Canyon Project – Dorval QC – 22-23 August 2006

Implementing 3D Turbulence

• Current 1D (vertical) turbulent diffusion scheme parametrizes effects of large eddies in PBL

• High-resolution models (< 1km) partly resolve large eddies

• Adjustments needed to avoid “double-counting” of diffusion processes

• Must also include XY contributions as grid resolution increases and move toward LES (quasi-isotropic 3D diffusion)

• Cascade to LES-type model resolution (Large-eddy simulation - i.e. 10-50m) with Smagorinsky-Lilly approach

• Smooth transition of diffusion intensity as function of model resolution

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• Implemented in MC2

• Included all XY components of the dynamic Reynolds stress tensor• Added TKE gradient terms• Horizontal corrections introduced in all remaining transport equations• Finite difference discretization on Arakawa-C grid and Charney-Phillips vertical staggering (this has yet to be done in GEM)• Modified operator splitting technique used by TKE solver• Modified appropriate scale-dependent mixing length

Implementing 3D Turbulence

Ratio of H/V motion components (OKC 16:00 CDT, 40 m mesh size)

Z = 1500 m Z = 2500 m >10

)V()H()(u)(u 1n1nn1n tttt

Implementing 3D Turbulence …Implementing 3D Turbulence …

Validation step:• Validation of 3D turbulence code against published LES results • 3D LES-type turbulent diffusion scheme with 40-m runs.

Study of impacts:• Impact on structure of urban boundary layer• Comparison of 1D versus 3D turbulence code with 250-m runs in

Oklahoma City in July 2003• Comparison and assessment on 1-km and 250-m runs.

Vertical heat flux: published LES results

Moeng et al., J. Atmos.,Sci., 1994

• 30 m resolution

• SB1 and SB2: strong shear + moderate convection

• SGS (sub-grid-scale parameterization) model mostly active:- lower levels (near surface) -top of PBL (entrainment zone)

Vertical heat flux: resolved and subgrid scales (OKC 16:00 CDT)

40 m 200 m

3D_LES_KC option3D_MESO option

Model : MC2 v4.9.8 Phy. v4.3.1Resolution : 40 m

40 m 40 m

Vertical heat flux: resolved and subgrid scales (OKC 16:00 CDT)

3D_LES_KC option3D_MESO option

Model : MC2 v4.9.8 Phy. v4.3.1Resolution : 40 m

40 m 40 m

Vertical heat flux: resolved and subgrid scales (OKC 16:00 CDT)- “zoom” on bottom 300 m -

3D_LES_KC option3D_MESO option

Model : MC2 v4.9.8 Phy. v4.3.1Resolution : 40 m

TKE resolved and subgrid scales (OKC 16:00 CDT)

40 m 40 m

… … Implementing 3D TurbulenceImplementing 3D Turbulence

Validation step:• Validation of 3D turbulence code against published LES results • 3D LES-type turbulent diffusion scheme with 40-m runs.

Study of impacts:• Impact on structure of urban boundary layer• Comparison of 1D versus 3D turbulence code with 250-m runs in

Oklahoma City in July 2003• Comparison and assessment on 1-km and 250-m runs.