Ch2 structure

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  • 1. Observed Structure of the Atmospheric Boundary Layer Many thanks to: Nolan Atkins, Chris Bretherton, Robin Hogan

2. Review of the last lecture

  • Incoming shortwave + Incoming longwave = Reflected shortwave
  • + Emitted longwave + Latent heat flux + Sensible heat flux
  • Incoming solar radiation = (Solar constant) cos(Solar zenith angle)
  • Reflected solar radiation = (Incoming solar radiation) x Albedo
  • LongwaveI= T 4
  • Sensible heat flux Q h=C dC pV (T surface- T air )
  • Latent heat flux Q e=C dL V (q surface- q air )
  • Bowen ratioB= Q h /Q e= C p (T surface- T air ) / L(q surface- q air ) provides a simple way for estimating Q hand Q ewhen radiation measurements are available

3. Vertical Structure of the Atmosphere

  • Definition of theboundary layer : "that part of the troposphere that is directly influenced by the presence of the earth's surface and responds tosurface forcingswith a time scale of about an hour or less.
  • Scale: variable, typically between 100 m - 3 km deep

4. Difference between boundary layer and free atmosphere

  • The boundary layer is:
  • More turbulent
  • With stronger friction
  • With more rapid dispersion of pollutants
  • With non-geostrophic winds while the free atmosphere is often with geostrophic winds

5. Vertical structure of the boundary layer

  • From bottom up:
  • Interfacial layer (0-1 cm):molecular transport, no turbulence
  • Surface layer (0-100 m):strong gradient, very vigorous turbulence
  • Mixed layer (100 m - 1 km):well-mixed, vigorous turbulence
  • Entrainment layer:inversion, intermittent turbulence

6. Turbulence inside the boundary layer

  • Definition of Turbulence: The apparent chaotic nature of many flows, which is manifested in the form of irregular, almost random fluctuations in velocity, temperature and scalar concentrations around their mean values in time and space.

7. Generation of turbulence in the boundary layer: Hydrodynamic instability

  • Hydrodynamically unstable means that any small perturbation would grow rapidly to large perturbation
  • Shear instability : caused by change of mean wind in space (i.e. mechanical forcing)
  • Convective instability : caused by change of mean temperature in the vertical direction (i.e. thermal forcing)

8. Shear instability

  • Shear: Change of wind in space

9. Example: Kelvin-Helmholtz instability

  • Shear instability within a fluid or between two fluids with different density

Lab experiment Real world(K-H clouds) 10. Convective instability

  • Static stability refers to atmospheres susceptibility to being displaced
  • Stability related to buoyancyfunction of temperature
  • The rate of cooling of a parcel relative to its surrounds determines its stability of a parcel
  • For dry air (with no clouds), an easy way to determine its stability is to look at the vertical profile of virtual potential temperature
  • v =(1 + 0.61 r )
  • Where
  • = T (P 0 /P) 0.286is the potential temperature
  • ris the water vapor mixing ratio
  • Three cases:
  • (1) Stable (sub-adiabatic): v increases w/ height
  • (2) Neutral (adiabatic): v keeps constant w/ height
  • (3) Unstable (super-adiabatic): v decreases w/ height

Stable orsub-adiabatic Neutral or adiabatic Unstable or super-adiabatic 11. Forcings generatingtemperature gradience and wind shear,which affect the boundary layer depth

  • Heat flux at the surface and at the top of the boundary layer
  • Frictional drag at the surface and at the top of the boundary layer

12. Boundary layer depth: Effects of ocean and land

  • Over the oceans: varies more slowly in space and time because sea surface temperature varies slowly in space and time
  • Over the land: varies more rapidly in space and time because surface conditions vary more rapidly in space (topography, land cover) and time (diurnal variation, seasonal variation)

13. Boundary layer depth: Effect of highs and lows

  • Near a region of high pressure:
  • Over both land and oceans, the boundary layer tends to be shallower near the center of high pressure regions. This is due to the associated subsidence and divergence.
  • Boundary layer depth increases on the periphery of the high where the subsidence is weaker.
  • Near a region of low pressure:
  • The rising motion associated with the low transports boundary layer air up into the free troposphere.
  • Hence, it is often difficult to find the top of the boundary layer in this region. Cloud base is often used at the top of the boundary layer.

14. Boundary Layer depth: Effects of diurnal forcing over land

  • Daytime convective mixed layer + clouds (sometimes)
  • Nocturnal stable boundary layer + residual layer

15. 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

16. Convective mixed layer (CML): Vertical profiles of state variables

  • Well-mixed (constant profile)

Super-adiabatic Nearly adiabatic Strongly stable lapse rate 17. 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.

Strongly stable lapse rate Nearly adiabatic Weakly stable lapse rate 18. Boundary layer over land:Comparison between day and night

  • Subtle difference between convective mixed layer and residual layer: Turbulence is more vigorous in the former

Kaimal and Finnigan 1994 Super-adiabatic Nearly adiabatic Strongly stable lapse rate Strongly stable lapse rate Nearly adiabatic Weakly stable lapse rate 19. Summary

  • 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
  • Boundary layer over ocean
  • Boundary layer over land: diurnal variation
  • Please remember to bring your calculator on Friday