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Atmospheric Structure Atmospheric Boundary Layer Diurnal Variation Atmospheric Boundary Layer Diurnal Variation Example Soundings for Homework Data Format (see ... – PowerPoint PPT presentation

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Title: USE


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e(T) es(TDew)
USE THESE VALUES.
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PRACTICE WITH STABILITY
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Atmospheric Structure
From Wallace and Hobbs, 2nd ed, Atmospheric
Science, chapter 9.
6
Atmospheric Boundary Layer Diurnal Variation
E.Z. refers to entrainment zone where energetic
parcels from the surface overshoot and entrain
and mix in air from the free atmosphere, as well
as to mix boundary layer air into the free
atmosphere above.
From Wallace and Hobbs, 2nd ed, Atmospheric
Science, chapter 9.
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Atmospheric Boundary Layer Diurnal Variation
Potential temperature in the U.S. standard
atmosphere, and its change near the Earths
surface due to turbulent mixing driven by
sunlight.
From Wallace and Hobbs, 2nd ed, Atmospheric
Science, chapter 9.
8
http//weather.uwyo.edu/upperair/sounding.html
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Example Soundings for Homework
500 am local time 3 September 2010
500 pm local time 3 September 2010
super adiabatic solar heated surface
top of boundary layer in afternoon. Mixing
height.
morning inversion
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5
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5
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Data Format (see spreadsheet online)
First set of columns.
Second set of columns
13
Equation for Boundary Layer Heightas a function
of time during the day,and by the day of year.
T0 surface temperature, P0 surface pressure,
?0 adiabatic lapse rate ?I environmental
lapse rate, typically of inversion and lt 0, ?
latitude, ? angle of tilt of Earths axis
23.5 degrees, tR time of sunrise in hour of
day, t time of day in hours, F is the
integrated solar irradiance (W m-2) over all
wavelengths at the surface, A is the surface
albedo, ?air is the thermal diffusivity of the
air, and ?ground thermal diffusivity of the
ground.
14
Observed Structure of the Atmospheric Boundary
Layer
  • Many thanks to Nolan Atkins, Chris Bretherton,
    Robin Hogan

Googled using presentation and Atmospheric
Boundary layer.
15
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
  • Longwave I?T4
  • Sensible heat flux Qh ? Cd Cp V (Tsurface -
    Tair)
  • Latent heat flux Qe ? Cd L V (qsurface - qair)
  • Bowen ratio B Qh/Qe Cp(Tsurface - Tair) /
    L(qsurface - qair) provides a simple way for
    estimating Qh and Qe when radiation measurements
    are available

16
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

17
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

18
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

19
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.

20
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)

21
Shear instability
  • Shear Change of wind in space

22
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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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
24
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

25
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)

26
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.

27
Boundary Layer depthEffects of diurnal forcing
over land
  • Daytime convective mixed layer clouds
    (sometimes)
  • Nocturnal stable boundary layer residual layer

28
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

29
Convective mixed layer (CML)Vertical profiles
of state variables
Strongly stable lapse rate
Nearly adiabatic
Super-adiabatic
  • Well-mixed (constant profile)

30
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
31
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

32
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
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