Large Eddy Simulation of Atmospheric Boundary Layer Flow over Heterogeneous Surfaces

E. Bou-Zeid
PhD Thesis, The Johns Hopkins University
April 2005
Baltimore MD

ABSTRACT:

The Large-Eddy Simulation (LES) approach originated in meteorological modeling but its scope has broadened and it is currently a very appealing technique for a wide range of engineering and environmental fluid mechanics applications. The surge in computational power available for simulations and the improvements in the modeling of the sub-grid scale (SGS) term have enabled the increase and diversification of LES applications in recent years. A new-generation subgrid-scale turbulence model that is faithful to the physics of ABL flows over complex surfaces is proposed in this work. The model: 1) does not require any a-priori specification of coefficients, 2) is fully local, 3) recognizes the scale-dependence of SGS fluxes in the atmosphere. The new SGS model is validated against classic results for flow over homogeneous terrain and against field experimental results (Bradley, 1968) for flow over an abrupt change in surface roughness.

The LES code with the new SGS model is then used to simulate ABL flows over heterogeneous surfaces (representing for example regions with a mixed cover of grass-lands, forests and urban areas). The LES dynamically captures the effect of heterogeneity and change in land-use on land-atmosphere interaction. This allows many open scientific questions to be answered such as: (i) How are internal boundary layers formed over patches blended and what is the mixing height above which turbulence blends out the effects of surface heterogeneity? (ii) Can the Monin-Obukhov similarity (MOS) (Monin and Obukhov, 1954) theory be used to parameterize land-atmosphere interaction over heterogeneous surfaces? (iii) If so, how can the parameters used in MOS to describe the earth surface, such as surface roughness length zo, be determined for heterogeneous terrain.

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