canopy-urban-CBL - • Scaling law for mixing layer over...

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Unformatted text preview: • Scaling law for mixing layer over canopy and convective mixing layer (Garret’s Ch. 3.4). • Slab model for well mixed convective boundary layer Mixing layer over canopies and convective mixing layer: Over flat surface Changes in TKE with height in the PBL: • z ≤ 0.1h, Shear production S and viscous dissipation D dominate below 0.1h (surface layer). Turbulent transport T is downward. ProFles are controlled by MOS, sensitive to z and u *o . • 0.2h ≤ z~h: buoyancy B dominates and S~0, T>0, net transport of TKE from lower PBL to upper PBL D Dt e = " u ' w ' # # z u Shear " # # z w ' e turbulent transport " 1 $ # # z w ' p ' pressure transport + g % vo w ' % w ' ( ) buoyancy " & vis dis T S B D Why turbulence behavior differently below and above 0.1h? • Eddies generated by cumuli dominates the vertical transport of TKE, and the height of the PBL, h. • Eddies <<h, viscous dissipation of TKE Eject, W’>0,u’>0 Sweep, w’<0,u’<0 Eddies break down At the scale ~ branches, leaves How does canopy infuence the fow? – Eddies comparable to height oF the canopy or boundary layer convection dominate the TKE and vertical transport and determine h. – The eddies << h act as viscous Friction and dissipating TKE. – Larger eddies (>>h) also contribute little to vertical transFer, except to make active canopy-scale eddies intermittent. Similar to a convective mixed layer. • Wake/waving extract energy from large eddies produced by shear, directly convert the energy to small eddies comparable to leaf size. dissipate the energy at the scale of leaves or stem (Kolmogorov η ~1 mm in canopy). Viscosity in canopy is much higher. Flat homogeneous surface: TKE cascades from large to small eddies. Λ ~h c • Infuence oF the canopy on momentum, heat and moisture oF the surFace layer: D Dt u = = " # # z u ' w ' " D ( z ) $ aerodynamic drag on the canopy D Dt % = = " # # z u ' % ' " F h transport of heat across the surface of the plants D Dt q = = " # # z u ' q ' " F q transport of humidity across the surface of the plants D Dt e = = " u ' w ' # # z u Shear " # # z w ' e turbulent transport " 1 $ # # z w ' p ' pressure transport + g % vo w ' % w ' ( ) buoyancy " & vis dis " dispersive transport + wake production + waving production Over a canopy surface • Below d~0.7hc, eddies<<h dissipate TKE, • Downward transport of TKE (T) and shear production S peak near hc (canopy top) • wake production cause intermittent downward transport of TKE....
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This note was uploaded on 11/07/2011 for the course EAS 8803 taught by Professor Staff during the Spring '08 term at Georgia Institute of Technology.

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canopy-urban-CBL - • Scaling law for mixing layer over...

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