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AOSC 634Air Sampling and Analysis

Lecture 6Atmospheric Exposure and Siting

Handbook: Chapters 4 & 5

Copyright Brock et al. 1984; Dickerson 2015

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Atmospheric Exposure: General

• Representativeness– Synoptic ~100 km– Microscale ~10 km

• Outdoor instruments should be installed on a level piece of ground, preferably no smaller than 25 m x 25 m.

• For climate studies, no changes in environment – was a climate station in our parking lot but then CSS was expanded.

• Level ground. http://www.wmo.int/pages/prog/www/IMOP/CIMO-

Guide.html

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Example of a good layout

for a met site.Northern

Hemisphere

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Atmospheric Exposure: Temperature

• Minimize radiative heating• Maximize ventilation (without compressive heating)• Air is a poor conductor• Screened shelters induce ~1 m/s wind• WMO suggests > 2.5 m/s ventilation

Radiation errorWhere Kr = sensor views = Stephan Boltzmann conste = emissivityAr = area of radiating body

Tt = Temp of air at time t

Tr = Temp of radiatnig body

H conductive heat transf coefAc = area for convection

Fritschen and Gay, Environ. Ins 1979.

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Traditional Weather Shelter

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Inside Weather Shelter

WMO guidelines call for accuracy of +/- 0.1 °C and a 20 s time constant.

Is this achievable with our -20 to 100 °C thermometers?

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A Class 1 WMO site has

1. Flat horizontal land surrounded by open space with slope <19o.2. Natural vegetation < 10 cm high.3. More than 100 m standoff from heat source, building, water body, or projected shade.

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Atmospheric Exposure: Anemometers

• From the UK Met office Handbook

• Standard exposure = 10 m with no obstructions within 300 m.

• For small obstructions of height h, the anemometer should be h+10 m.

• If obstruction height h >12m and lies within h in the horizontal (X ≤ h), then the sensor should be 1.75 to 2.25 h above ground level.

• If the distance to the obstruction is greater than 30 h then the sensor need

only be h m agl.

X

If h > 12 m and X ≤ h then site sensor ~2h agl. If X ≥ 30h then site senor at h.

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Traditional wind speed estimates

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Atmospheric Exposure: Radiation

• Obstructions– Absorb solar radiation– Reflect solar radiation– Emit IR radiation

See: Latimer Radiation Measurement For an essay on correction for obstructions to diffuse radiation.

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Example of signal averaging to reduce noise from Krotkov et al., JGR, 2008.

The full benefit of spatial and temporal averaging is seldom realized.

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References

• CIMO Guide to Meteorological Instruments and Methods of Observation, WMO-No. 8, 2008. Available on the internet www.wmo.int

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