2. Large Eddy Simulation of PBL turbulence and clouds Chin-Hoh Moeng National Center for Atmospheric Research

3. OUTLINE 1.The LES technique 2.PBL turbulence and clouds 3.Role of LES in PBL research 4.Future direction

4. Numerical methods of studying turbulence Reynolds-average modeling (RANS) model just ensemble statistics Direct numerical simulation (DNS) resolve for all eddies Large eddy simulation (LES) intermediate approach

5. 1. LES turbulent flow Energy-containing eddies Subfilter scale eddies (not so important) (important eddies)

6. Example: An 1-D flow field f Apply filter 

7. Reynolds average model (RANS) f Apply ensemble avg  non-turbulent

8. LES EQUATIONS SFS Apply filter G

9. The premise of LES Large eddies, most energy and fluxes, explicitly calculated Small eddies, little energy and fluxes, parameterized, SFS model LES solution is supposed to be insensitive to SFS model

10. Caution near walls: eddies small, unresolved very stable region: eddies intermittent cloud, radiation, chemistry… introduce more uncertainties

11. Major differences between geophysical and engineer flows inertial (vs. viscous) layer near walls (molecular term is always neglected) entrainment-into-inversion (vs. rigid top) buoyancy effect cloud processes

12. PBL ~ meters

13. 2. WHAT IS THE PBL? turbulent layer –lowest ~km on the Earth surface directly affected by surface –heating, moisture, pollution, sfc drag diurnal cycle over land –convective and stable PBLs

14. PBL TURBULENCE dispersion transport ground temperature air-sea interaction global radiation budget via marine stratocumulus clouds

15. ANNUAL STRATUS CLOUD AMOUNT

16. ~ 100% < 10% transition

17. marine stratocumulus off California coast persistent all NH summer!

18. from aircraft capped by a strong inversion

19. Stratocumulus-topped PBL ~ 50% < 10% ocean PBL

20. 4% increase in area covered by PBL stratocumulus cloud 2-3 K cooling of global temperature (Randall et al 1984)

21. Stratocumulus-topped PBL cold ocean water PBL entrainment radiative cooling evaporation drizzle condensation Warm and dry aloft

22. two cloud-top processes radiation evaporation entrainment PBL cold ocean surface

23. cloud-top mixing process fluid a fluid b saturation point 1

24. ISSUES on marine stratocumulus PBL formation and dissipation processes? parameterization in climate model? cloud albedo? cloud amount or if global warming occurs?

25. Different PBL Regimes convective PBL stable PBL oceanic boundary layer shallow cumulus-topped stratocumulus-topped PBL over wavy surface …

26. 3. LES of DIFFERENT PBL REGIMES Domain setup Large-scale forcing Flow characteristics

27. Clear convective PBL Convective updrafts ~ 2 km

28. The stable PBL

29. Oceanic boundary layer Add vortex force for Langmuir flows McWilliam et al 1997

30. Shallow cumulus clouds Add phase change---condensation/evaporation ~ 6 km ~3 km ~ 12 hr

31. How to include condensation/evaporation in LES? conserved variables

32. Stratocumulus-topped PBL Add latent heat and longwave radiation ~ 5 km ~1 km rad cooling cloud layer thin rad cooling layer >10K

33. F F height 0 Q_rad IR radiative fluxes O(100K/day)

34. How to include longwave radiation in LES?

35. LES vs. observation mean thermodynamic properties time evolution of cloud top, bottom w-variance and skewness

36. heat fluxes moisture flux buoyancy flux Z (m) cld top cld base

37. How do we study PBL turbulence and clouds with LES?

38. Study turbulence behavior and processes responsible for transport (creative thinking; flow vis.) Develop or calibrate ensemble- mean models (RAN models) (large database)

39. CLASSICAL EXAMPLES Deardorff (1972; JAS) - mixed layer scaling Lamb (1978; atmos. env) - plume dispersion property

40. Entrainment

41. Sullivan et al 1998 JAS

42. So far, idealized PBLs: Flat surface Periodic B.C. in horizontal Shallow cloud regimes

43. Challenge of LES for PBL Research Real-world PBLs: – complex terrain – complex land use – ocean waves – severe weather

44. 4. FUTURE RESEARCH Extending LES applications to real-world PBL problems

45. Use a state-of-the-art weather model

46. Why Weather Research and Forecast (WRF) model? Available input data: –Terrain, land properties, meteorol conditions Higher-order numerical schemes Terrain-following coordinate Design for massive parallel computers – partition in vertical columns

47. 500 km 20 km nest an LES inside the WRF model

48. Technical Issues Inflow boundary conditions SFS representation near irregular surfaces Proper scaling; how to represent ensemble statistics

49. ? How to describe a turbulent inflow?

50. SUMMARY LES in advancing PBL research Marine stratocumulus in climate models Technical issues in extending LES to real PBLs