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Large-eddy simulation of an observed evening transition boundary layer

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Large-eddy simulation of an observed evening transition boundary layer

Dr Bob Beare

John M Edwards and Alan Lapworth

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.

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.

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.

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

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

Cardington case: sensitivity to stability function;degraded resolution runs

Grid length: 1000m in horizontal, 100m in vertical

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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