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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Review of last lecture Surface water balance
The changing rate of soil moisture S dS/dt
P - E - Rs - Rg I
Precipitation (P)
Evaportranspiration (EEbEiEsTR)
Irrigation (I)
Runoff (Rs)
dS/dt
(PDSI, desertification)
Infiltration (Rg)
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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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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)

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Shear instability

• Shear Change of wind speed and/or direction in
  space

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Example Kelvin-Helmholtz instability

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

Lab experiment
Real world (K-H clouds)
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Video Kelvin-Helmholtz instability

• https//www.youtube.com/watch?vHZII9-OUJrE

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Video Kelvin-Helmholtz Clouds Over the Alps

• https//www.youtube.com/watch?vQVNO6lyAcZc

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Convective instability

• 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 stability conditions - Richardson
number

• The Richardson number is a convenient means of
  categorizing atmospheric stability in the
  boundary layer
• Where g is acceleration due to gravity, ? is mean
  temperature, U is mean wind speed, z is height,
  and Ri is a dimensionless number.
• Ri gt0 stable
• 0 neutral
• lt0 unstable

Lewis Fry Richardson (11 October 1881 - 30
September 1953) was an English mathematician,
physicist, meteorologist, psychologist and
pacifist who pioneered modern mathematical
techniques of weather forecasting, and the
application of similar techniques to studying the
causes of wars and how to prevent them.
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Forcings generating temperature 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

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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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Video Convective boundary layer observed by lidar

• https//www.youtube.com/watch?v_lvQ-uyttuo

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

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

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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. Richardson number
• Boundary layer over ocean
• Boundary layer over land diurnal variation