Boundary Layer Notes 5

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Boundary Layer Notes 5

• Observational Techniques

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Observational Techniques

• Sources Kaimal Finnegan, Atmospheric Boundary
  Layer Flows their structure and measurement,
  Oxford University Press, 1994
• In situ techniques
• Important boundary layer measurements
• T, u, v, w, q, trace gases (e.g. CO2, CH4)

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Observational Techniques

• We need to measure both mean quantities and
  fluctuations.
• Why fluctuations? After all we can use
  flux-gradient relationships to determine fluxes
  using mean conditions. Answer because we
  dont know if we can trust the flux-gradient
  relationships especially in very turbulent,
  nearly well-mixed boundary layers.

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• For means, standard measurement techniques are
  acceptable (thermistors, wind vanes,
  cup-anemometers, propellers, hygristors,
  psychrometer, dew-point hygrometer)
• Sketch dew-point hygrometer, psychrometer.

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• For fluctuations, we need to get more clever.
• Why? Response time. We need independent
  measurements every 0.1 s to produce reliable flux
  measurements.
• Solution sonic anemometers, sonic thermometers,
  and TDL (and other radiometric) observations of
  trace gases.
• How do sonic anemometers work?
• Derive speed of sound c vgamma RTv/m
• Note approximateness of Tv dependence derive (1
  0.38e/p) factor.
• Sketch sonic anemometer layout, to justify Vd
  c2/2d(t2-t1),
• (old fashioned measurement, when anemometer only
  measures the difference between the times).
• Vd d/2(1/t1 1/t2)
• Mast issues have to place instruments far away,
  and not down-wind of towers.

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• Remote sensingfine for mean state of boundary
  layer, not for direct flux measurements.
• Describe
• radar wind profiles
• Sodar
• Lidar
• Radio Acoustic Sounding System (bouncing radar
  off of density gradients caused by sound wave
  emissions).(lidar very expensive as of 94
  anyway).
• Old wind profilers werent useful for boundary
  layer studies because their minimum ranges were
  1 km. Newer ones at 915 Mhz can measure from
  100 m to 1.5 km with 50 m resolution. But
  time constant is still a few minutes.

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Sonic Anemometer
Derive speed of sound
Sketch anemometer layout, and derive
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Sonic Anemometer, contd.

• We can now make our measurement of the
  temperature more accurate, by adding together the
  reciprocals of the separate times, and if were
  using a three-dimensional sonic anemometer, we
  can get t1 and t2 from the vertical anemometer
  axis, and Vn from the magnitude of the horizontal
  wind.

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A bunch of sonic anemometers from Applied
Technologies. Data rates range from 10/s to
1/s.f
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Li-Cor (brand) IR absorbtion Gas Analyzers.
Funny-looking one is an open cell, for
eddy-correlation measurements.
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