1. AOSC 634 Air Sampling and Analysis Lecture 6 Atmospheric Exposure and Siting Handbook: Chapters 4 & 5 Copyright Brock et al. 1984; Dickerson 2015 1

2. 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 2

3. Example of a good layout for a met site. Northern Hemisphere 3

4. 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 error Where Kr = sensor view  = Stephan Boltzmann const  = emissivity A r = area of radiating body T t = Temp of air at time t T r = Temp of radiatnig body H conductive heat transf coef A c = area for convection Fritschen and Gay, Environ. Ins 1979. 4

5. Traditional Weather Shelter 5

6. Inside Weather Shelter 6 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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8. A Class 1 WMO site has 1. Flat horizontal land surrounded by open space with slope <19 o. 2. Natural vegetation < 10 cm high. 3. More than 100 m standoff from heat source, building, water body, or projected shade. 8

9. 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. 9 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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11. Traditional wind speed estimates 11

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13. 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. 13

14. Example of signal averaging to reduce noise from Krotkov et al., JGR, 2008. The full benefit of spatial and temporal averaging is seldom realized. 14

15. References CIMO Guide to Meteorological Instruments and Methods of Observation, WMO-No. 8, 2008. Available on the internet www.wmo.int 15