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

2. 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 8 th Annual Meeting of the EMS, Amsterdam, 2008

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

4. 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):  x=  y=  z=6.25 m  Length scale correction turned off: =  =(  x  y  z) 1/3  c h =c m (c h,1 +c h,2,  ) = c m (c h,1 +c h,2 )  c m =0.12, c h,1 =1, c h,2 =2 8 th Annual Meeting of the EMS, Amsterdam, 2008

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

6. Solution close to Smagorinsky model's solution  Smagorinsky subgrid TKE solution:  LES subgrid constants: c f =2.5  c m =0.12, c  =0.76 corresponding Smagorinsky constant: c s =0.22

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

8. 8 Subgrid constants c m and c h c m more mixing of hor. winds c h more mixing of pot. temp.

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

10. Similarity relations: c f =2 (c m =0.096) DNS Van der Wiel et al. (2008)

11. High resolution:  x=  y=  z=1.5626m

12. 12 Conclusions 1.  =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  ) 2. High resolution simulation - smaller gradient for  m and  h compared to observations and DNS