Zentralanstalt für Meteorologie und Geodynamik Atmospheric stability in urban areas detected by ultrasonic anemometers Martin Piringer, August Kaiser

Zentralanstalt für Meteorologie und Geodynamik Martin Piringer Folie 2 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 anemometers both with respect to placing as well as data

Zentralanstalt für Meteorologie und Geodynamik Martin Piringer 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

Zentralanstalt für Meteorologie und Geodynamik Area of investigation Martin Piringer Folie 4 U: ultrasonic anemometer R: RASS D, L, G: Air Pollution Monit. Stations of Local Government P: Representative up-valley site (no pollution monitoring)

Zentralanstalt für Meteorologie und Geodynamik Measurement locations Martin Piringer Folie 5 Air pollution monitoring station D Ultrasonic anemometer at U

Zentralanstalt für Meteorologie und Geodynamik Comparison of wind speed Martin Piringer Folie 6 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

Zentralanstalt für Meteorologie und Geodynamik Comparison of wind direction Martin Piringer Folie 7 Site D, all winds Site D, winds > 0,7 ms -1 Site U, all winds

Zentralanstalt für Meteorologie und Geodynamik 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 H f (smoothed over 1,5 hours) and T sonic,corr MOS is derived: MOS = (-0,37 * 9,81 * H f )/(1,292 * 1005,0 * (T sonic,corr + 273,16) * u * 3 MOS not defined for very low values of u * Martin Piringer Folie 8

Zentralanstalt für Meteorologie und Geodynamik Sensible heat flux and MOS Martin Piringer Folie 9 Daytime: heat flux larger in summer Nighttime: larger heat flux in winter Explanation: influence of external heat sources Daytime: positive MOS due to shading by valley sidewalls Nighttime: External heat sources lead to more unstable situations in winter

Zentralanstalt für Meteorologie und Geodynamik Example: time series of MOS Black dots: uncorrected MOS; green dots: corrected MOS Martin Piringer Folie 10

Zentralanstalt für Meteorologie und Geodynamik Discrete stability classes Martin Piringer Folie 11 Wind from site D, cloudiness from nearest airport: No unstable cases at night (per definition) MOS and wind at site U: Frequent occurrence of unstable situations, also at night Urban effect?

Zentralanstalt für Meteorologie und Geodynamik 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 SO2 caused 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 Martin Piringer Folie 12

Zentralanstalt für Meteorologie und Geodynamik Annually averaged NOx concentrations Martin Piringer Folie 13

Zentralanstalt für Meteorologie und Geodynamik Annually averaged PM10 concentrations Martin Piringer Folie 14

Zentralanstalt für Meteorologie und Geodynamik Annually averaged SO2 concentrations Martin Piringer Folie 15

Zentralanstalt für Meteorologie und Geodynamik 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 Martin Piringer Folie 16