Observed Structure of the Atmospheric Boundary Layer

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Observed Structure of the Atmospheric Boundary
Layer

• Many thanks to Nolan Atkins, Chris Bretherton,
  Robin Hogan

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Before we start

• In-class assignment I grades have been posted on
  Carmen
• In-class presentation schedule has been posted
  on the course website. Please start to read and
  understand the paper and search for the
  background materials.

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Review of last lecture

• The modern climatology (meteorology) was born in
  the 1940s (a very young science!), but has been
  growing very fast! Now we have a global
  observational network with many satellites,
  ships, radars and surface stations, as well as
  very comprehensive prediction models running on
  the worlds largest computers.
• The current status of weather and climate
  predictions (1) weather prediction good to 10
  days, (2) tropical cyclone prediction good in
  track but not in intensity, (3) climate
  prediction good to two seasons, (4) climate
  change projections have a 3-fold difference in
  magnitude.
• The main reasons of the difficulties (1)
  Teleconnection problem, (2) Feedback problem, and
  (3) Subgrid-scale problem. Boundary layer
  processes are important for both the feedback
  problem and the subgrid-scale problem.

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Vertical Structure of the Atmosphere

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

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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

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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

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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.

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Forcings generating turbulence and affecting the
boundary layer depth

• Heat transfer from/to the ground
• Heat transfer from/to the top of the boundary
  layer
• Frictional drag from the ground
• Frictional drag from the top of the boundary
  layer

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Static Stability

• Static stability refers to atmospheres
  susceptibility to being displaced
• Stability related to buoyancy ? function 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 (P0/P)0.286 is the potential
  temperature
• r is 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 or sub-adiabatic
Neutral or adiabatic
Unstable or super-adiabatic
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Boundary layer depthEffects 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)

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Boundary layer depthEffect 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.

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Boundary Layer depthEffects of diurnal forcing
over land

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

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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

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Convective mixed layer (CML)Vertical profiles
of state variables
Strongly stable lapse rate
Nearly adiabatic
Super-adiabatic

• Well-mixed (constant profile)

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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
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Boundary layer over land Comparison between day
and night
Kaimal and Finnigan 1994
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Boundary layer clouds Closed cells and open
cells
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Boundary layer clouds Distribution
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Boundary layer clouds Horizontal convective
rolls
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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
• Boundary layer clouds