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Observed Structure of the Atmospheric Boundary Layer

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Title: Observed Structure of the Atmospheric Boundary Layer


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

2
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)
3
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

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 speed and/or direction 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
Video Kelvin-Helmholtz instability
  • https//www.youtube.com/watch?vHZII9-OUJrE

11
Video Kelvin-Helmholtz Clouds Over the Alps
  • https//www.youtube.com/watch?vQVNO6lyAcZc

12
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
13
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.
14
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

15
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)

16
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.

17
Boundary Layer depthEffects of diurnal forcing
over land
  • Daytime convective mixed layer clouds
    (sometimes)
  • Nocturnal stable boundary layer residual layer

18
Video Convective boundary layer observed by lidar
  • https//www.youtube.com/watch?v_lvQ-uyttuo

19
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

20
Convective mixed layer (CML)Vertical profiles
of state variables
Strongly stable lapse rate
Nearly adiabatic
Super-adiabatic
  • Well-mixed (constant profile)

21
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
22
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

23
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
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