Atmospheric stability in urban areas detected by ultrasonic anemometers

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Atmospheric stability in urban areasdetected by
ultrasonic anemometers

• Martin Piringer, August Kaiser

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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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Martin Piringer
24.11.2020
Folie 3
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

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Area of investigation
U ultrasonic anemometer R RASS D, L, G
Air Pollution Monit. Stations of
Local Government P Representative up-valley
site (no pollution monitoring)
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Measurement locations
Air pollution monitoring station D
Ultrasonic anemometer at U
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Comparison of wind speed
Site D High frequency of weak winds, especially
at night main wind directions deflected
from valley axis
Site U valley wind system along-valley wind
directions show largest average speeds
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Comparison of wind direction
Site U, all winds
Site D, all winds
Site D, winds gt 0,7 ms-1
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Atmospheric stability by 3D ultrasonic anemometers
Martin Piringer
24.11.2020
Folie 8

• 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) u3
• MOS not defined for very low values of u

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Sensible heat flux and MOS
Daytime positive MOS due to shading by valley
sidewalls Nighttime External heat sources lead
to more unstable situations in winter
Daytime heat flux larger in summer Nighttime
larger heat flux in winter Explanation
influence of external heat sources
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Example time series of MOS
Martin Piringer
24.11.2020
Folie 10
Black dots uncorrected MOS green dots
corrected MOS
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Discrete stability classes
MOS and wind at site U Frequent occurrence of
unstable situations, also at night Urban effect?
Wind from site D, cloudiness from nearest
airport No unstable cases at night (per
definition)
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Features of the dispersion model LASAT
Martin Piringer
24.11.2020
Folie 12

• 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
• ExampleYearly 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 NOx concentrations
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Annually averaged PM10 concentrations
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Annually averaged SO2 concentrations
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Conclusions
Martin Piringer
24.11.2020
Folie 16

• 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