zentralanstalt für meteorologie und geodynamik atmospheric stability in urban areas detected by...

Zentralanstalt für Meteorologie und Geodynamik

Atmospheric stability in urban areasdetected by ultrasonic anemometers

Martin Piringer, August Kaiser


04/10/23

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The problem – and a solution?

Commercially available dispersion models widely used in air pollution assessment

They are usually fed with conventional meteorological

data (wind and stability – discrete stability categories)

Meteorology measured at air pollution monitoring stations run by local governments – instrumentation sometimes not properly placed

Aim: Improvement by 3D ultrasonic anemometersboth with respect to placing as well as data


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Outline

Site and instrumentation

Comparison of wind data: air pollution monitoring station against ultrasonic anemometer

Advanced determination of atmospheric stability by 3D sonics against discrete stability classes

Results of dispersion modelling for an industrial complex in an Alpine valley – conventional vs. 3D sonics input data


Area of investigation

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U: ultrasonic anemometerR: RASSD, L, G: Air Pollution Monit. Stations of Local GovernmentP: Representative up-valley site (no pollution monitoring)


Measurement locations

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Air pollution monitoring station D Ultrasonic anemometer at U


Comparison of wind speed

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Site D: High frequency of weakwinds, especially at night; main wind directions deflected fromvalley axis

Site U: valley wind system; along-valley wind directions show largest average speeds


Comparison of wind direction

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Site D, all winds

Site D,winds > 0,7 ms-1

Site U, all winds


Atmospheric stability by 3D ultrasonic anemometers

Atmospheric stability characterized by MOS (= 1/L) derived via algorithm based on Stull (1988) made available by the manufacturer of the instrument

Necessary because uncorrected MOS shows large variations in time

u* from co-variances xz and yz (from wind measurements) and

smoothed over 1,5 hours From Hf (smoothed over 1,5 hours) and Tsonic,corr MOS is derived:

MOS = (-0,37 * 9,81 * Hf)/(1,292 * 1005,0 * (Tsonic,corr + 273,16) * u*3

MOS not defined for very low values of u*

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Sensible heat flux and MOS

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Daytime: heat flux larger in summer

Nighttime: larger heat flux in winter Explanation: influence of external heat sources

Daytime: positive MOS due toshading by valley sidewalls

Nighttime: External heat sourceslead to more unstable situations in winter


Example: time series of MOS

Black dots: uncorrected MOS; green dots: corrected MOS

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Discrete stability classes

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Wind from site D, cloudinessfrom nearest airport:No unstable cases at night(per definition)

MOS and wind at site U:Frequent occurrence of unstablesituations, also at nightUrban effect?


Features of the dispersion model LASAT

LASAT is a Lagrangian particle dispersion model(Janicke Consultants, Germany)

LASAT includes a diagostic wind field model, realistic flow simulation in complex terrain

LASAT includes also a building module to simulate flow around and behind buildings (not used here)

Point, area, and line sources included Example:

Yearly averaged concentration fields of PM10 and SO2caused by over 40 different sources within the industrial complex; one-year time series of met. data from sites D(with cloudiness from nearest airport) and U

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Annually averaged PM10 concentrations

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Annually averaged SO2 concentrations

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Conclusions

If properly placed, 3D sonics measure strength and direction of valley wind system without immediate influence by obstacles

In built-up areas, measurements have to be taken above approx. twice the average building height to be representative

3D ultrasonic anemometers enable on-site determination of wind and stability and are therefore to be preferred over conventional sensors, but:• Extremely sensitive to local conditions at the site:

Influence of heated/cooled roofs and external heat sources

• „Ideal“ position of sensor not easy to find

• Uncertainty in the determination of L/MOS in conditions of low wind speeds

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zentralanstalt für meteorologie und geodynamik atmospheric stability in urban areas detected by...

Documents

martin piringer folie

u slide

winds site d

winter slide

pollution monitoring

valley site

d sonics input data

wind measurements