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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