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Large Eddy Simulation of PBL turbulence and clouds

Large Eddy Simulation of PBL turbulence and clouds. Chin-Hoh Moeng National Center for Atmospheric Research. OUTLINE. The LES technique PBL turbulence and clouds Role of LES in PBL research Future direction. Numerical methods of studying turbulence. Reynolds-average modeling (RANS)

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Large Eddy Simulation of PBL turbulence and clouds

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  1. Large Eddy Simulation ofPBL turbulence and clouds Chin-Hoh Moeng National Center for Atmospheric Research

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

  3. 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

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

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

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

  7. LESEQUATIONS Apply filter G SFS

  8. 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

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

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

  11. PBL ~ meters

  12. 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

  13. PBL TURBULENCE • dispersion • transport • ground temperature • air-sea interaction • global radiation budget viamarine stratocumulus clouds

  14. ANNUAL STRATUS CLOUD AMOUNT

  15. < 10% ~ 100% transition

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

  17. from aircraft capped by a strong inversion

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

  19. 4% increase in area covered by PBL stratocumulus cloud 2-3 K cooling of global temperature (Randall et al 1984)

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

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

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

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

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

  25. 3. LES ofDIFFERENT PBL REGIMES • Domain setup • Large-scale forcing • Flow characteristics

  26. Clear convective PBL Convective updrafts ~ 2 km

  27. The stable PBL

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

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

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

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

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

  33. How to include longwave radiation in LES?

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

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

  36. How do we studyPBL turbulence and cloudswith LES?

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

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

  39. Entrainment

  40. Sullivan et al 1998 JAS

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

  42. Challenge of LESfor PBL Research Real-world PBLs: • complex terrain • complex land use • ocean waves • severe weather

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

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

  45. 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

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

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

  48. ? How to describe a turbulent inflow?

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

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