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Observed Structure of the Atmospheric Boundary Layer (2)

Observed Structure of the Atmospheric Boundary Layer (2). Review of last lecture. Vertical structure of the atmosphere and definition of the boundary layer Vertical structure of the boundary layer Definition of turbulence and forcings generating turbulence

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Observed Structure of the Atmospheric Boundary Layer (2)

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  1. Observed Structure of the Atmospheric Boundary Layer (2)

  2. Review of last lecture • Vertical structure of the atmosphere and definition of the boundary layer • Vertical structure of the boundary layer • Definition of turbulence and forcings generating turbulence • Static stability and vertical profile of virtual potential temperature: 3 cases. Richardson number • Boundary layer over ocean

  3. Boundary Layer depth:Lidar

  4. Boundary Layer depth:Lidar

  5. Turbulence inside the boundary layer

  6. Boundary Layer depth:Effects of diurnal forcing over land • Daytime convective mixed layer + clouds (sometimes) • Nocturnal stable boundary layer + residual layer

  7. Convective mixed layer (CML):Growth The turbulence (largely the convectively driven thermals) mixes (entrains) down potentially warmer, usually drier, less turbulent air down into the mixed layer

  8. Convective mixed layer (CML):Vertical profiles of state variables Well-mixed (constant profile) Strongly stable lapse rate Nearly adiabatic Super-adiabatic

  9. Nocturnal boundary layer over land: Vertical structure • The residual layer is the left-over of CML, and has all the properties of the recently decayed CML. It has neutral stability. • The stable boundary layer has stable stability, weaker turbulence, and low-level (nocturnal) jet. Weakly stable lapse rate Nearly adiabatic Strongly stable lapse rate

  10. Comparison between day and night Subtle difference between convective mixed layer and residual layer: Turbulence is more vigorous in the former Strongly stable lapse rate Nearly adiabatic Super-adiabatic Kaimal and Finnigan 1994 Weakly stable lapse rate Nearly adiabatic Strongly stable lapse rate

  11. Summary • Daytime: convective mixed layer + clouds (sometimes) • Nocturnal: stable boundary layer + residual layer

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