Surfacelayer-MOmixing

Surfacelayer-MOmixing - Ch. 2: Turbulence in the ABL and...

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Ch. 2: Turbulence in the ABL and transport from surface to the atmosphere Reading: Text, 3.1-3.3 (p40-42, p49-60) Foken 2006 Key questions: What control the transport of energy, water, momentum and chemical tracers between the surface and atmosphere? How can we formulate these transports?
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The planetary boundary layer (PBL): The lowest 10% of the troposphere where profiles of winds, temperature, humidity and other tracers tend to be distinctively different from the atmosphere above. Turbulence plays either a dominant or important role; Depth of the PBL varies from a few hundreds meters to a few
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Diurnal cycle of the PBL and its role in exchange between surface and free atmosphere:
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Vertical Structure of the PBL :
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Surface layer The lowest 10% of the ABL: Turbulent driven flux is constant with height, Wind is controlled solely by turbulent momentum transport One can apply the analogy of molecular diffusion to turbulent transport. For a neutral surface layer, turbulent flux is driven by wind shear. The momentum flux in the surface layer ~ that at the surface: K m r u z = t for a neutral surface layer, t = t o where t o = r u ' w ' ( ) o momentum transport (stress) at the ground wind stress at the surface can be represented by friction velocity, u * = u ' w ' ( ) o such that t o = r u * 2 , u * : a velocity scale, can be viewed as velocity losts at the surface due to downward turbulent momentum transport. It is controlled by both surface aerodynamic properties and wind speed.
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The logarithmic velocity law of a constant- flux layer: If we let eddy viscosity K m (m 2 /s) ~ u *o l where l = kz is a mixing length, where k : von Karman consstant then K m r u z = t o = r u * o 2 u * o kz u z = u * o 2 kz u * o u z =1 also k u * o du = dz z k u * o u = ln z + cons tan t wind profile is a logarithmic function of z. u also depends on surface frictional velocity, u
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Surfacelayer-MOmixing - Ch. 2: Turbulence in the ABL and...

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