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Large-eddy simulation of an observed evening transition boundary layer

Large-eddy simulation of an observed evening transition boundary layer. Dr Bob Beare John M Edwards and Alan Lapworth . Aims.

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Large-eddy simulation of an observed evening transition boundary layer

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  1. Large-eddy simulation of an observed evening transition boundary layer Dr Bob Beare John M Edwards and Alan Lapworth

  2. Aims • Establish if LES of the evening transition is possible. Nieuwstadt and Brost 86 and Sorbjan 97 look at decay of convective turbulence but not full transtion. Domain size for CBL (>2-4km in horizontal) and grid length requirements for SBL (1-10 m). • Comparison with surface and tethered balloon measurements. • Three types of run: full transition (CBL-SBL), nocturnal development (just SBL, smaller domain), degraded resolution (just SBL). • See Beare et al 2005, to appear in QJRMS.

  3. Cardington observations • Reasonably flat grassy site, z0m = z0h = 5 cm near Bedford UK. • Times series of 1.2 m temp., 4 m sensible heat flux and friction velocity. • Tethered balloon observations for transitions 7-8 April and 23-24 September 2003 (clear skies, moderate gradient winds ~ 7m/s, moderately stable: h/L~2-4), every hour after sunset. Sept case had significant nocturnal jet and inertial oscillation, thus focus of study. • See Lapworth and Mason (1988) and Lapworth (2003) for more info about site and obs method.

  4. Large-eddy simulation (LES) model • Full transition run: 12Z – 0Z; 2km x 2km x 3km domain; 10 m grid length. Mainly temperature boundary conditions (some flux). • Smagorinsky diffusion (blue) ; Stochastic backscatter (red), Brown et al. (1994). • Zero moisture; radiation code off (approximated by constant interior cooling forcing). • Temperature bottom boundary condition for other runs; grid length typically 5m; 500mx500mx1km domain.

  5. Full transition: surface parameters

  6. Full transition: flux profiles, every hour from 19Z. Buoyancy and TKE fluxes.

  7. Mean profiles. Observations thick dashed lines. Analytical initialiastion Full transition

  8. Comparison of LES and tethered balloon observations for Cardington, Bedford; nocturnal development

  9. Sensitivity to initial wind profile; nocturnal development

  10. Nocturnal development: sensitivity to forcings and model Model Forcings

  11. April 2003 case

  12. Cardington case: sensitivity to stability function;degraded resolution runs Grid length: 1000m in horizontal, 100m in vertical

  13. Vertical velocity cross-section and spectra Distinct peak Red positive, blue down

  14. Momentum budget (Sept 2003 case, 0230Z)

  15. Summary • LES of evening transition is possible (although robustness not fully explored due to computational burden) and qualitatively correct. Significant differences between model and obs in details: ageostrophic wind and collapse of the CBL stress (cf Mahrt 1981, for Wangara). • Good comparison for nocturnal development runs, initialised with obs in early evening. • Uncertainties in the geostrophic, surface and radiative forcings can make detailed comparison difficult and hard to say which model configuration is better. • Nesting may be helpful for full transition at higher resolution.

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