Boundary Layer Notes 5

• Observational Techniques

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)

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 don’t know if we can trust the flux-gradient relationships; especially in very turbulent, nearly well-mixed boundary layers.

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

•Sketch dew-point hygrometer, psychrometer.

•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 = √gamma 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.

•Remote sensing—fine 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 weren’t 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.

Sonic Anemometer

€

α =sin−1 Vn /c( )

t1 =d

ccosα −Vd,

t2 =d

ccosα +Vd,

€

c = γRTv /MDerive speed of sound:

Sketch anemometer layout, and derive:

€

Vd =d2

1t1

−1t2

⎛

⎝ ⎜

⎞

⎠ ⎟

Sonic Anemometer, cont’d.

• We can now make our measurement of the temperature more accurate, by adding together the reciprocals of the separate times, and if we’re 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.

€

1

t1+

1

t2=

2

dccosγ

=2

dc2 −Vn

2( )

1/2

c2 =d2

4

1

t1+

1

t2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

+Vn2

403Tv =d2

4

1

t1+

1

t2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

+Vn2

Tv =d2

1612

1

t1+

1

t2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

+1

403Vn

2

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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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