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
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.
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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