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Large Eddy Simulation of Stable Boundary Layers with a prognostic subgrid TKE equation

Large Eddy Simulation of Stable Boundary Layers with a prognostic subgrid TKE equation. Stephan R. de Roode and Vincent Perrin Clouds, Climate and Air Quality, Dept. of Applied Sciences , Delft University of Technology, Delft, Netherlands. 8 th Annual Meeting of the EMS, Amsterdam, 2008.

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Large Eddy Simulation of Stable Boundary Layers with a prognostic subgrid TKE equation

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  1. Large Eddy Simulation of Stable Boundary Layers with a prognostic subgrid TKE equation Stephan R. de Roode and Vincent Perrin Clouds, Climate and Air Quality, Dept. of Applied Sciences, Delft University of Technology, Delft, Netherlands 8th Annual Meeting of the EMS, Amsterdam, 2008

  2. Contents Problem/question - Dutch LES model: Stable boundary layer simulation dominated by subgrid contributions Strategy - Analysis of subgrid prognostic TKE model LES results - subgrid vs resolved - similarity relations - high resolution results Conclusions 8th Annual Meeting of the EMS, Amsterdam, 2008

  3. Prognostic subgrid TKE equation (Deardorff 1980)  subgrid fluxes ,  eddy diffusivity  length scale  subgrid TKE 8th Annual Meeting of the EMS, Amsterdam, 2008

  4. GABLS SBL intercomparison case  Neutral layer becomes stable due to a prescribed surface cooling (-0.25 K/h)  Original set up according to Beare et al. (2003): Dx=Dy=Dz=6.25 m  Length scale correction turned off: l=D=(Dx Dy Dz)1/3  ch=cm(ch,1+ch,2,l/D) = cm(ch,1+ch,2)  cm=0.12, ch,1=1, ch,2=2 8th Annual Meeting of the EMS, Amsterdam, 2008

  5. LES results: Examples taken from the 5th hour Turbulent fluxes dominated by subgrid contribution

  6.  Smagorinsky subgrid TKE solution:  LES subgrid constants: cf=2.5  cm=0.12, ce=0.76 corresponding Smagorinsky constant: cs=0.22 Solution close to Smagorinsky model's solution

  7. Changing the filter constant cf=2.52 Less filtering  more resolved motions

  8. Subgrid constants cm and ch lowerRig, more resolved ch more mixing of pot. temp. cm more mixing of hor. winds

  9. Solution if solution is 100% subgrid (Baas et al., 2008) Similarity relations

  10. DNS Van der Wiel et al. (2008) Similarity relations: cf=2 (cm=0.096)

  11. High resolution: Dx=Dy=Dz=1.5626m

  12. Conclusions 1. D=6.25 m resolution not enough - Solution dictated by Smagorinsky subgrid TKE solution - too much dependency on subgrid constants: bad simulation - recommendation: refine grid resolution (smaller D) 2. High resolution simulation - smaller gradient for fm and fh compared to observations and DNS

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