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Non-Universal Turbulence in Planetary Boundary Layers Igor N. Esau (igore@nersc.no) Nansen Environmental and Remote Sensing Centre Bergen, Norway

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Classical View Turbulent boundary layers consist of random eddies (Kolmogorov 1941) Small eddies produce the shear stress and transport heat, scalars and momentum, therefore - “active” (Townsend 1961) Large eddies do not produce the shear stress and do not transport heat, scalar and momentum, therefore - “inactive” (Townsand 1961)

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Universal Properties of Small Eddies After Chapman, 1979, AIAA papers Universal motions After Larson, 1986, RISOE report

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Universal Properties of Small Eddies Kolmogorov's law for the energy spectrum: Structure function for the turbulent stress: Smagorinsky-Lilly eddy-viscosity relation for the turbulent stress:

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Small eddies exert stress and carry momentum in classical boundary layers How do large eddies look like?

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Classical Large Eddies Top view Side view Horseshoe vortices Ejections of low speed fluid carry stress

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Turbulence in PBLs Real world turbulence is different: ● Rough surface ● Large scales ● Stratification ● Rotation

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New View Internal wave radiation from PBL top (Zilitinkevich, 2000 ) Eddy blocking and distruction in surface layer (Hunt, 2000)

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Fluxes of Turbulent Kinetic Energy Roughness Layer Classical view New view P> e P< e Surface Layer PBL Core Turbulence Free Atmosphere P= e P< e P= e=0 P> e P< e P=0 e>0

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Profiles of the Energy Flux Roughness layer Surface layer

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Maximum of Non-dimensional TKE Small eddiesLarge eddies Small stress Large stress Measurements in shallow near-neutral PBLs (Hogstrom, 1990) Measurements in deep near-neutral PBLs (Pennel, LeMone, 74) LES data

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Turbulent Stress Turbulent stress decreases with the eddy size Critical eddy size Turbulent stress does not change with the eddy size

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What determines the size of large eddies?

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Coherent Structures in Sheared Flow Typical size of the first characteristic eddy is close to the critical eddy size for the stress fall-off. L c ~ 600 meters in atmospheric boundary layer

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PBL Depth Imposed stability parameter accounts for the size of large eddies (Zilitinkevich, 2000)

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Instant View

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Why do we need this knowledge? Anthropogenic hazards Weather forecast Climate research Air pollution management Understanding of cloud structures

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Geostrophic Drag and Geostrophic Angle Larger eddiesSmaller eddies

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A and B Functions

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Conclusions Turbulent planetary boundary layer consists of large eddies Large eddies exert the most of the shear stress and transport the most of heat, scalar and momentum Large eddies are limited by (I) the PBL depth, which is the most important factor in real PBLs and (II) the characteristic size of coherent eddies Small eddies produce little shear stress and relate to large eddies

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Thank you for your attention Bergen, Norway

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